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Home , Cottage garden, Gardening, Greenhouse

in the Global The Impacts of Climate Change on in the UK

Gardening in the Global Greenhouse was funded by the following organisations:

Research was carried out by:

The UK Climate Impacts Programme The UK Climate Impacts Programme (UKCIP) is based at the University of Oxford and funded by DEFRA to coordinate an assessment of how climate change will affect the UK, and help organisations assess how they might be affected. UKCIP, Union House, 12–16 St. Michael’s Street, Oxford, OX1 2DU, UK; [email protected]. Tel: +44(0)1865 432076; Fax: +44(0)1865 432077. Summary report written by Dr Phil Gates, University of Durham. Cover images (from top): Protea longiflora (NT); Hardy’s Cottage (Eric Chrichton, NTPL); Nymans (Patsy Fagan, NTPL). Printed on 50% recycled paper. Published in November 2002 under the UK Climate Impacts Programme. Summary Report Gardening in the Global Greenhouse: Summary Report Introduction

Introduction: The British and their gardens The British are noted for their love of gardens. As an art form, gardening is an essential part of our culture. It is one of the leading in Britain, with an estimated 27 million (41 per cent of the population) participating in some way. Gardeners sustain a multi-million pound horticultural retail industry. Our heritage gardens and their collections, representing 500 years of history, attract 24 million visitors each year, contributing an estimated £300 million to the industry.

A wake up call from the weather

Gardeners are more aware than many how organisations, commercial horticulturists, the weather affects them and their interests. landscape consultants, the horticultural press, Over the last four decades, extreme weather and representatives from local and national events have severely damaged many gardens government, universities and research and resulted in major economic losses. These organisations, to consider how gardens would events include: be affected by climate change.

• severe winter weather in 1962/63 that The primary outcome of the meeting was killed many hardy ; the commissioning of a technical report, entitled ‘The Impacts of Climate Change • drought in 1976, which weakened Drought dried out lakes and reservoirs in 1976 on Gardens in the UK’, by Richard Bisgrove and dried out lakes; (Mike Sleigh, RHS) and Professor Paul Hadley, of the University • storms in 1987 and 1990 that felled of Reading (2002). The authors assembled millions of trees; and evaluated evidence on how plants and garden components respond to altered • torrential and prolonged rain leading to climatic conditions, and assessed the likely soil erosion, flooding and drowning of impact of the UKCIP1 climate change plant roots in 2000 and 2001. scenarios on gardens in the United Kingdom. These events highlight the vulnerability of This document summarises their report. It gardens to the vagaries of weather and describes effects on garden plants, domestic climate. A workshop was held in in gardens, nationally important heritage April 2000, attended by gardening gardens, the landscape industry and the retail

Trees at Chartwell, Kent felled by the storm in 1987 horticultural industry. It discusses aspects of (Peter Baistow, NTPL) gardening that might benefit from climate change and identifies techniques and practices that might reduce undesirable effects of climate change on gardens. It also highlights the important role that gardens can 1 The UKCIP02 scenarios describe how the climate of play in raising awareness of environmentally the UK is likely to change over the course of the 21st century. They were produced for the UK Climate sustainable practices which can minimise the Impacts Programme by the Hadley Centre at the Met. effects of climate change, and identifies areas Office and by the Tyndall Centre for Climate Change Research with funding from the Government’s for further research. Department for Environment, Food and Rural Affairs. Flooding at Westbury Court, Glos. in 2000/01 (NT)

1 Gardening in the Global Greenhouse: Summary Report What is happening to our climate? Gardening in the Global Greenhouse: Summary Report What is happening to our climate?

What is happening to our climate?

The current phase of climate change is part of a continuous warming of the climate Change in average winter and summer temperature for the low and high emissions scenarios. recorded over the last 150 years and is being caused primarily by industrial activities that have raised levels of greenhouse gases which retain heat within the atmosphere. Winter Summer 2020s 2050s 2080s 2020s 2050s 2080s There is overwhelming evidence that the rate In the low emissions scenario, at the current • mean annual rainfall may decrease by deg C of warming is increasing, although rate of 0.1°- 0.3°C rise per decade annual 10-20 per cent, but with 10-30 per cent 4.5 projections for the rate and extent of average temperatures will increase by more falling in winter and 20-50 per cent 4 Low Emissions warming vary, depending on predictions of 1° - 2.5°C by the 2080s. This lower rate of less in summer by the 2080s. Rain will 3.5 economic growth, human population levels increase would depend on the use of clean tend to fall with greater intensity; 3 and compliance with international and efficient technologies, reduced use of • by the 2080s summer droughts will be 2.5 agreements to regulate emissions of carbon natural resources that generate carbon more frequent, as will very wet winters, 2 dioxide, the most important . dioxide, and global solutions to problems but autumns will be drier. Higher 1.5 of social and economic stability and In the UKCIP02 high emissions scenario, with temperatures and less cloud cover in 1 equity between developed and High rapid economic growth in developed and less summer will lead to greater evaporative Emissions 0.5 developing countries. developed countries and a market driven loss from soils and , reliance on fossil fuels, by the 2080s Climate change is already occurring and worsening drought conditions; average temperatures are likely to rise by further change is inevitable, with common • although predictions for extreme weather 2°- 4.5°C, at a rate of between 0.3°- 0.5°C consequences for climate in the UK. These are: events are less certain than for per decade. This is between two and three • higher mean annual temperatures, which temperature, weather patterns are likely to times faster than the current rate of will increase the length of the growing become more erratic, with greater temperature increase. season for many plants. A 1°C increase in frequency of torrential rain, temperature mean temperature will increase the extremes and storms. Percentage change in average winter and summer precipitation for the 2020s, 2050s and 2080s growing season by three weeks in south Within these overall trends there will be east and by 10 days in the for the low and high emissions scenarios pronounced regional differences, with the north west; lowest rainfall and highest temperatures • greater warming in summer and autumn occurring in south east England. The reduction Winter Summer than in winter, with summer maximums of soil moisture will therefore be greatest in 2020s 2050s 2080s 2020s 2050s 2080s per cent rising faster than summer minimums, areas where water supplies are already low. 30 leading to increased frequency of hot By the 2050s sea levels are expected to rise 25 summer days. By the 2080s temperatures by 14-18 cm, and by the 2080s by 23-36 cm. 20 may exceed 42°C about once per decade Low 15 The effects will be greatest in the south east, Emissions Traditional scenes such as this at Bodnant, Gwynedd in lowland England; 10 where the land mass is naturally subsiding. will become a rarity (Christopher Gallagher, NTPL) 0 • winter minimum temperatures rising faster Coastal flooding could damage some -10

than winter maximums, leading to milder domestic and major heritage gardens. -20 winters with a reduced temperature range These climate changes will have major -30 and fewer frosts. In many parts of the UK, -40 effects on plant growth and on the especially in the south west, frosts will be High -50 ways in which we design, plant and Emissions rare and might occur in only maintain gardens. Changes within about once every 10 years by the 2080s, 'natural' variability although local variations will occur. Snowfall will decrease everywhere in lowland and coastal regions by as much as Source: UKCIP02 Climate Change Scenarios 90 per cent by the 2080s, and by 60 per cent in high snowfall areas such as the Droughts will become more frequent (NT) Scottish Highlands;

2 3 Gardening in the Global Greenhouse: Summary Report Plants in a warmer world Gardening in the Global Greenhouse: Summary Report Plants in a warmer world

Plants in a warmer world: how climate affects plant growth and development

Plants are influenced by extreme weather events and by gradual climate changes, the effects of “Grape cultivation could extend Failure of plants to shut down metabolic as far north as Scotland during activity in winter might increase frost damage, which are more insidious but potentially dramatic. A small change in average annual temperature the second half of the but higher sugar levels generated in tissues of, say, 0.1°C has a cumulative effect through the growing season, with a large net impact. An of evergreen species by winter growth might 21st century.“ have the reverse effect. Species-specific effects analogy would be a small change in bank interest rate, which might have little immediate effect Bisgrove and Hadley (2002) are likely, and further research is needed. on the daily economy but would have a profound impact on annual accounts. Interactions between higher levels and higher temperatures Grapes could be grown in Scotland in future (RHS) Effects of individual elements of climate so although plants will grow more rapidly, As temperatures rise, plants respond more change have been tested in controlled fertiliser requirements do not necessarily flowerformosa) earlier, but others such as honesty vigorously to higher carbon dioxide levels. environments for some plant species of increase. Increases in carbon dioxide levels (Lunaria annua) and bleeding heart (Dicentra Doubling atmospheric carbon dioxide levels agricultural importance and there is already can hasten bud burst, flowering and fruiting will be largely unaffected. and raising temperature by 3°C can increase evidence that rises in temperature and and lead to earlier cessation of growth, growth by over 50 per cent, although this Temperature increase is likely to extend the atmospheric carbon dioxide levels have depending on species investigated. In , may not be reflected in higher yields. The northern limit for growing some increased growth of forest trees. Further higher carbon dioxide levels increase the benefits are greater in vegetables harvested and . Grape bud burst and maturity research is required for the majority of number of buds, which bloom earlier early in development, such as carrots, than in dates may advance by up to 50 days and garden plants. and accelerate flowering. those that are cropped closer to maturity, yield may increase by up to 25 per cent by such as onion and cauliflower. Warmer Higher carbon dioxide levels the 2050s. temperatures shorten the growing period of Atmospheric carbon dioxide, the primary Temperature levels play a key role in vegetables like onions, lowering ultimate agent of climate change, is used by plants in regulating the annual growth cycle of yields. In , higher carbon dioxide levels photosynthesis to produce chemical energy, perennial plants. Normal bud burst often increase rates of photosynthesis and so sugars and other organic molecules essential depends on accumulated experience of slightly increase overall yields, but higher for plant growth and development. Increases winter chilling, which breaks dormancy and temperatures hasten foliage death. in carbon dioxide levels, which are expected prepares buds to open as soon as favourable to rise from the current 350 parts per million spring weather arrives. At the end of the (ppm) to between 525 and 810 ppm by the growing season, falling temperatures halt “.. doubling carbon dioxide 2080s under the low and high emissions Higher carbon dioxide levels can hasten bud burst growth and trigger fall. The intervening (Stephen Robson, NTPL) levels can increase plant growth scenarios respectively, will increase the rate period constitutes the growing season, which by as much as 50 per cent ..“ of photosynthesis and so enhance plant will extend as climate changes. Currently growth. Experiments show that doubling spring is arriving 2 - 6 days earlier per Bisgrove and Hadley (2002) Higher temperatures carbon dioxide levels can increase plant decade and autumn two days later each growth by as much as 50 per cent, although Higher temperatures increase growth rates, decade. Precocious spring flowering, delayed this varies depending on plant species, provided that no other factors are limiting. It leaf fall, extended growth and temperature, water supply and mineral is unlikely that the higher temperatures unseasonal winter flowering are already nutrient availability. expected by the 2080s will physically commonplace and will increase in extent and damage plants in the open ground, except frequency as climate changes. The way in which plants use the additional for brief, exceptional periods, but damage products of photosynthesis varies between Earlier springs lengthen the growing season may occur in glasshouses unless these are species and is affected by other and allow earlier planting dates. Bud burst of well shaded and ventilated. environmental factors. Under conditions of most plants will be advanced, but the likelihood Autumn is arriving later and later (Michael Caldwell, NTPL) water stress, some plants divert resources Higher temperatures produce faster of frost damage to precocious growth will be into root production, improving water , bud burst, leaf expansion and no greater than at present, because frosts will uptake. Plants use water more efficiently flowering. Advances in flowering date can become less frequent. Frost damage in autumn as carbon dioxide levels rise, because a be predicted for species whose temperature may become more serious, as higher average reduction in the numbers and opening of response characteristics are known. Plants temperatures delay dormancy and clearer skies stomatal pores on leaf surfaces slows water like Mexican orange (Choisya ternata) and increase the possibility of frost. loss. Nitrogen is also used more efficiently, Rhododendron ‘Praecox’ are likely to

4 5 Gardening in the Global Greenhouse: Summary Report Soils Gardening in the Global Greenhouse: Summary Report Pandora’s greenhouse

Soils: winter waterlogging, summer drought Pandora’s greenhouse: more pests, more diseases, more

The UKCIP02 scenarios point to major changes in soil water content and hence nutrient Garden pests, diseases and weeds tend to have short, rapid life cycles and reproduce in large availability. Managing these changes represents one of the biggest challenges for gardeners numbers – ideal credentials for organisms that can benefit from the predicted changes in in the 21st century. climate. Increased temperatures shorten life cycles so that populations increase faster, while longer growing seasons allow additional reproductive cycles at a time of the year when Seasonal temperature increases will enhance Sudden heavy downpours, coupled with soil pest levels are already high. biological activity of soil micro-organisms, compaction, poor drainage and leading to more rapid breakdown of soil with impervious materials, will lead to rapid organic matter and faster nutrient release. surface run-off and localised flooding. Pests Beneficial or benign will be equally Warmer temperatures in the UK are also affected by temperature change, so likely to favour a northwards advance of Increased availability of soluble soil nutrients Prolonged waterlogging weakens and kills Pests that undergo many breeding cycles in increased natural predation of pests is likely. native pest species and an influx of pests will bring about faster and more vigorous roots and will increase the risk of a growing season, such as thrips, and The nematode currently used as a biological from continental , either by natural plant growth, but will also increase the loss damage to property from large moribund spider mites, will complete their life cycles control for vine weevil could become more migration or accidental introduction. of soil nutrients through leaching. Heavier, trees growing close to houses. Cedars at the faster. Milder winters will benefit many widely used in future, as warmer conditions Termites and a distinct strain of the Asian intense rain, especially in winter when there National Trust garden at Osterley Park in pests that already overwinter as adults, will enable it to be effective for a longer gypsy moth (Lymantria dispar ssp.) have is no vegetation cover, may wash away soil Middlesex and at Westbury Court in while others that currently overwinter as season. There is already evidence that already been recorded in southern Britain. minerals, leading to pollution of streams, Gloucestershire have already been affected eggs may also adopt this strategy. Earlier butterflies are emerging earlier and that ponds and lakes. by flooding. This can be avoided by improving springs will allow them to disperse onto The heap: addition of organic matter will drainage, using gullies, soakaways and their ranges are extending, but increased become increasingly important (Stephen Robson, NTPL) Strategies for conserving soil fertility and garden plants sooner, provided that these ditches in large gardens but is much more activity of in mild winters reduces their moisture will depend largely on soil plant hosts also begin growth earlier. The difficult to control in closely spaced, small chances of survival. conditions and local effects of climate cabbage (Brevicoryne brassicae) suburban gardens. “Evidence suggests that nitrate change. Peak garden demand already benefits from mild winters and for Many insect pests transmit virus diseases, losses from soils may double. for water will be greatest in the densely every 1°C increase in temperature, aphid which will become more prevalent as pest Maintaining soil fertility, by populated south east where it is least attacks begin two weeks earlier. An increase infestations intensify and occur earlier in the replacing organic matter, will available. Free draining sandy soils will lose in spring temperature of 2°C will allow year, when plants are more severely be essential.” organic matter faster, further reducing cabbage root fly (Delia radicum) to become vulnerable. Insects that are currently water-holding capacity during summer active a month earlier than at present. glasshouse pests, such as western flower Bisgrove and Hadley (2002) droughts, and will be more prone to water Faster plant growth reduces leaf nutritive thrips, may move into the open garden, erosion in winter. Addition of organic matter value and can increase pest feeding activity carrying virus diseases such as and mulching will be required. Heavy soils by 20-40 per cent. spotted wilt virus and Impatiens necrotic Warmer conditions will allow greater use of biological should retain sufficient water for plant spot viruses, which can infect a range of control of the vine weevil (Chris Prior, RHS) growth in summer but winter waterlogging vegetables. There is good reason to suppose may kill plants unless grit is incorporated or that other pest species currently confined to Hedges at Westbury Court, Gloucestershire, have raised beds are constructed. been affected by flooding (John Millar, NTPL) the sheltered environment of glasshouses will move into the open garden. This has Higher summer temperatures will increase “Managing rainwater run-off .. already happened with red spider mite. will be of crucial importance.” water evaporation from leaves and the soil, lowering soil moisture content. A 3°C Bisgrove and Hadley (2002) increase in soil temperature, of the kind that might be experienced by the 2080s under the high scenario, can increase such water loss by 30 per cent and decrease soil moisture by 25 per cent. When this effect is combined with substantially lower summer rainfall, plants will suffer from severe drought stress more frequently.

6 7 Gardening in the Global Greenhouse: Summary Report Pandora’s greenhouse Gardening in the Global Greenhouse: Summary Report Pandora’s greenhouse

ohridella) aquilinum) Horse chestnut leaf miner (Cameraria As with insect pests, climate change can be There is great potential for severe fungal There is evidence that bracken (Pteridium has devastated trees in , expected to bring an influx of new diseases damage to plants that form key structural , which is a serious on large become established in Northern Italy and from overseas, posing major threats to elements in the formal planting of heritage estates, will benefit significantly from climate rolfsii recently appeared in Wimbledon. An influx garden plants. Soil borne Athelia (Corticium) gardens, including yew, box and . Box change, colonising at higher altitudes of exotic pests into British gardens is a likely , which would be a serious threat to blight (Cylindrocladium buxicola) is spread and penetrating further into lightly consequence of climate change. garden plants, has already been introduced by water splash and will be favoured by shaded woodlands. into Britain and could become established warmer, wetter conditions. Yew root rot Larger pests, like roe deer and grey squirrels, Fears have been expressed about the here if warmer winters enable it to survive. (Phytophthora cinnamomi) will become a will be favoured by warmer winters, which possibility of the uncontrolled spread of Olive scab (Spilocaea oleagina) has also greater problem in wetter, warmer winters will increase their survival rates. Grey squirrels alien plant introductions. Himalayan balsam been introduced and is an example of how and will spread northwards and eastwards. ponticum have a predilection for (Fagus sylvatica), (Impatiensjaponica) glandulifera), Rhododendron gardeners may inadvertently introduce new Holly leaf blight (Phytophthora ilicis) arrived which is also very sensitive to drought, and and Japanese knotweed (Fallopia diseases as they exploit climate changes to from the USA in the 1980s and its recent the combined effects of climate and squirrels have all escaped from gardens grow new plants. spread may be due to warmer, wetter on this species could be particularly severe. and pose threats to natural ecosystems. As winters. It has already caused damage in the Quarantine regulations, currently designed to the UK climate continues to warm it will be Carmeraria ohridella is now affecting horse Diseases Camellia petal blight will spread from Devon garden at Nymans in Sussex. Other species protect plants of major economic importance, necessary to monitor carefully the potential chesnuts in England (J. Metzer, LWF) and Cornwall (Chris Prior, RHS) at increasing risk from Phytophthora species Plant diseases often have complex life may need to be revised and extended and observed threats from invading exotic include Lawson cypress and alders. cycles and a range of hosts, so predicting to garden plants. Camellia petal blight plants, but the threat that they represent consequences of climate change for disease (Ciborinia camelliae), originated in Japan and Weeds should not be overstated. infestations is difficult. Wetter, warmer has spread around the world, unimpeded by More favourable conditions for germination winters will favour the spread of water- plant health authorities that did not consider and growth will increase the need for transmitted diseases such as Phytophthora, it necessary to impose import restrictions. weeding, while slow growth in dry summers or species with higher temperature The disease has become established in fuciformis) may reduce the effectiveness of glyphosate requirements like red-thread (Laetisaria Devon and Cornwall, at the northern limit of and hormone-based , which work disease of . Drier, warmer its climatic tolerance, but warmer conditions best on plants that are growing rapidly. summers will increase the incidence of in future will favour its spread. spraying will need to be aerially dispersed spores of diseases like Oak wilt (Ceratocystis fagacearum), a major conducted earlier in the year. powdery mildew. disease in North America, is transmitted by Continued plant growth during mild winters an insect that cannot survive under current will enhance winter survival of diseases and UK climate conditions. When these change, lead to their rapid re-establishment in earlier accidental introduction of this insect could springs. Rates of disease will depend on be a serious threat to oaks in Britain. their tolerance of drier, warmer summers but in general higher temperatures would be expected to increase the severity of disease attacks. Drought also tends to increase plants’ susceptibility to diseases, as in the corticale) case of sooty bark disease (Cryptostroma of sycamore. Warmer winters will favour bark and wood invading fungi such as honey (Armillaria spp.) and apple canker (Nectria galligena). Warmer conditions will favour honey fungus Holly leaf blight is already causing damage in the UK (Chris Prior, RHS) (Béatrice Henricot)

8 9 Gardening in the Global Greenhouse: Summary Report Garden management in a warmer world Gardening in the Global Greenhouse: Summary Report Garden management in a warmer world

Garden management in a warmer world

Climate change and the character The retail horticultural trade can respond to The character of our gardens and the plant collections found in them are influenced by the “.. in the contemporary garden, climate change offers exciting of gardens these changes in gardening fashion, and is already meeting consumer demand for such physical constraints of what may be grown, plant , temperature and water requirements, opportunities ..” The extent to which the overall style and exotic species as phormiums, bamboos, and what is desirable from the perspective of historical precedent, culture or fashion. character of gardens is affected by climate Bisgrove and Hadley (2002) cannas, palms, tree ferns and bananas which depends on how much we wish to preserve were, until recently, only grown in large, their character for cultural reasons. “Tresco has spread to Tunbridge The craft and science of allows Temperature and water specialist gardens. us to grow species that are poorly adapted Domestic is subject to the Wells and is on its way Maintaining plants is particularly difficult Heritage gardens to the climate in the United Kingdom, such whims of horticultural fashion, promoted by to Teesside.” during periods of higher summer temperatures as alpine plants and mediterranean species, magazines and television programmes. Most plants that are currently cultivated in and drought conditions, especially in parts Bisgrove and Hadley (2002) and also to manage the garden environment Historically important gardens are maintained heritage gardens are likely to be maintained of southern England where to allow cultivation of highly bred cultivars as cultural assets – to perpetuate significant over this century by the use of suitable soil are already limited. Even short periods of which could not thrive unassisted. Current layouts, period styles, plant collections and moisture conservation techniques and extreme stress can kill many annuals and horticultural skills will need to be refined to the accumulation of past owners’ influences. in summer, but at increasing cost. perennials, and may weaken perennials and ensure the survival of existing garden plants In such gardens, maintaining their character Difficult decisions over suitable replacement trees, making them vulnerable to stress in and allow the cultivation of new species. and current combinations of species will planting will arise when extreme weather subsequent years. require special attention and will become events, such as storms, lead to loss of Hardiness Historic collection of plants, such as this at increasingly difficult and expensive, if Gardeners in areas with summer water Biddulph Grange, will become increasingly difficult mature trees. When individual plants are shortages can plant drought tolerant species, to maintain (Nick Meers, NTPL) not unsustainable. Some 85 per cent of garden plants originate unique or of particular historical, botanical but these will require skilled cultivation. from areas with warmer climates than our Destruction of gardens because of climate or biological importance, then strenuous and Such species do not tolerate saturated soil own. Higher average winter temperatures change in the medium term is only likely expensive efforts will need to be made to around their roots and will be vulnerable to will reduce frost damage to marginally in those few coastal gardens that could maintain them in the face of climate change, disease in the warmer, wetter winters that be directly affected by sea level rise, or as will be the case for archaeology hardycalifornica species, such as Abutilon vitifolium, can be expected in the 21st century. perhaps in some that are subject to contained within historic parks or gardens. Bottlebrushfruticosa) (Callistemon spp.), Carpentaria and Jerusalem sage (Phlomis prolonged and persistent flooding inland. Balancing consideration of future climatic , and will extend their cultivation In such cases, documentary recording is conditions with issues of historical northwards. In general, higher average probably the only option. authenticity will become an increasingly important factor in the maintenance of winter temperatures should enhance the Domestic and contemporary gardens winter survival prospects for the majority heritage gardens. Garden conservation of species grown in our gardens. Climate change could bring about rapid management plans offer a tool to changes in garden style. Adventurous plan ahead. Frost will remain a risk – albeit a gardeners have always tried to explore the diminishing one – throughout much of the limits for cultivating unusual and demanding UK, even in a warmer climate, at least until species, so climate change could provide the end of the 21st century. By judicious new variations on familiar challenges. Their Climate change is likely to extend the range of species soil preparation and recognition of local The horticultural trade is already meeting consumer success will depend on careful soil that can be cultivated (Derek Croucher, NPTL) microclimates within large gardens, demand for exotic species (Pardee Tree Nursery) preparation and plant maintenance during gardeners already extend the limits for periods of drought or excessive rain. cultivation of tender species. This skill will Currently, between 10,000-15,000 plant become even more important with climate species and varieties are cultivated in British change, to minimise the risk of frost damage gardens; climate change is likely to extend to precocious buds in spring. the range of species that can be cultivated outdoors throughout the year, especially by gardeners in northern Britain. Difficult decisions will have to be made when mature trees in heritage gardens are lost through extreme weather (George Wright, NTPL)

10 11 Gardening in the Global Greenhouse: Summary Report Rewriting the garden manual Gardening in the Global Greenhouse: Summary Report Rewriting the garden manual

Rewriting the garden manual: climate change and key garden components

All gardeners will need to plan for new planting opportunities, a longer growing season, Herbaceous perennials preparation and irrigation. Higher temperatures will favour use of a much wider range of These are so diverse in their climate optima improvements in garden drainage and watering facilities, continuous grass mowing and half-hardy bedding plants that will no longer that it is difficult to generalise on their be subject to frosts. Separate spring, summer difficult growing conditions under glass. performance. Traditional and autumn bedding schemes will be species such as aster (e.g. Aster novi-belgii), desirable when the length of the growing delphiniumpaniculata) (Delphinium x cultorum), lupin season increases. (Lupinus x regalis spp.) and phlox (Phlox , requiring deeply cultivated, Some of the challenges gardeners will Fruit trees and bushes The plants grown in traditional cottage gardens will fertile, moisture retentive soil, will not adapt require more care (Neil Chrichton, NTPL) face, and some possible solutions, are well to drier summers and will need more set out below: Many existing cultivars have a minimum winter chilling requirement for bud break intensive care, as will old cultivars of , Trees and flowering, which may not be met in which tolerate dry summers but are killed by It should be possible to select tree future mild winters, so maintaining historically “Whilst the challenge for many winter waterlogging. Selecting species and provenances that will be better adapted to important cultivars will become more difficult. gardeners may be the introduction cultivars that are drought resistant and future climates than present cultivars by, for of new and exciting species into tolerant of waterlogged soils in winter will Breeding new cultivars with a lower chilling example, choosing disease-resistant species. be a challenge for plant breeders and requirement, or complete substitution of these their gardens, as the climate Careful management will help avoid large scale storm Many existing long-lived trees will suffer stress nurseries. More thorough staking of may be needed. , cherries and progressively changes a much damage in woodland (NT) from climate change and will require careful herbaceous plants will be required, to withstand Lawns will need year-round maintenance peaches, for example, could replace apples greater challenge to gardeners.. (Mike Sleigh, RHS) management programmes to deal with: periods of intensive rainfall in summer. in some locations. will be to create the traditional • summer drought, minimised by skilled English .” Lawns soil management; Species propagated from bulbs and Bisgrove and Hadley (2002) Fine lawns – the icon of British gardens – are • waterlogging, avoided by planned Most problems that afflict trees will also underground storage organs typically require high-maintenance features particularly suited drainage measures; apply to shrubs, but to a lesser extent, and low winter temperatures to stimulate root the same management procedures apply. to our current climate. Hot summers and • damage and loss from high winds, development and, in some cases, high Shade-adapted shrubs can be damaged by wetter winters will increase browning during requiring planned long term replacement summer temperatures to initiate flower bud intense sunlight and weed competition after droughts, promote soil compaction during wet programmes, planting in suitably sheltered development. Requirements of different storm damage to the tree canopy, requiring weather and increase risk of diseases. A longer site, and perhaps judicious crown species vary widely, but if warmer winters temporary shading during a period growing season will demand year-round Peaches could replace apples reduction of vulnerable trees; lead to failure of root initiation, lifting and (Neil Campbell-Sharp, NTPL) mowing and lawn care. More extensive moss of acclimatisation. refrigeration of bulbs would be needed to • large scale storm damage in woodland, growth in winter is likely, although summer Sub-shrubs ensure normal development. Higher soil and minimised by avoiding uniform drought will minimise its impact on the lawn. air temperatures in spring may alter the and planting a diversity Higher temperatures and fewer frosts will These changes will raise maintenance costs Trees that are resistant to storm damage balance between flower and foliage Source: White (1994) of tree species in new woodlands; allow wider use of sub-shrubs such as in large gardens, making conversion to more development in species such as hyacinth Fuchsia, Indigofera and Penstemon, which natural meadow areas an attractive proposition Acer pseudoplatanus Sycamore • risk of damage to conifer hedges by fire (Hyacinthus orientalis). are marginally hardy at present in many in less formal areas. The cost of lawn irrigation, x Cupressocyparis leylandii in periods of extreme drought, reduced locations. In northern locations such species Warmer summers will benefit summer and if permitted at all in areas of water shortage, Magnolia (tree species) Magnolia by limiting access and establishing fire will still require careful management, to autumn flowering subjects and will permit will be an important consideration. Ilex aquifolium Holly prevention measures. guard against late damage from frosts, Faster-growing foliage may obscure hyacinth cultivation of a wider range of species in the Metasequoia glyptostroboides Dawn redwood which are unlikely to be totally eliminated (David Sellman, NTPL) open border. and some crocus cultivars Robinia pseudoacacia Black locust during the next century. do not tolerate waterlogging, so will either Sequoiadendron giganteum Wellingtonia require excellent soil drainage regimes, or will baccata Yew need to be lifted and stored over the winter. Annuals Higher temperatures in spring will accelerate germination, development and flowering of hardy annuals, and lead to their earlier death. This trend might be minimised by better soil 12 13 Gardening in the Global Greenhouse: Summary Report Rewriting the garden manual Gardening in the Global Greenhouse: Summary Report The bottom line

The bottom line: the commercial impact of climate change on gardens

In gardens open to the public, managing Ponds, lakes and water supplies Many major gardens depend on revenue from visitors. Garden managers will need to plan for visitor access to lawns to minimise soil High summer temperatures will lead to compaction will be important. Unpredictable the financial impacts of climate change. They will need to meet the cost of storm and flood repairs increased frequency of algal blooms in summer rain and prolonged grass growth ponds, promoting stagnant conditions. Water and rising insurance premiums, and to adapt to the effects of climate change by installing water through winter will force greater flexibility in features in gardens will need more intensive storage facilities and flood protection. They will also need to prepare for a longer visitor season mowing regimes to avoid lawn damage, maintenance, with frequent topping up of raising costs when contracted labour small ponds in summer. Seasonal fluctuation and greater visitor impact on the garden infrastructure. There is scope to develop environmentally is employed. of water levels in large lakes and ponds will techniques further. In the longer term, conversion to lawns require the use of spillways and sluices in constituted from coarser, more drought winter and improved marginal planting in resistant grasses that tend to be less summer, to minimise the visual impact of Visits to heritage gardens Gardeners need to be proactive in managing gardens in ways that make minimum tolerant of close mowing may be needed, falling water levels. Climate, or more precisely prevailing demands on scarce water resources, and in to reduce costs of irrigation and weather, is just one of many factors that will Supply of water for irrigation during summer demonstrating sound environmental sward maintenance. influence future trends in garden visits. The cost of lawn irrigation will be a consideration could be problematic across the UK. practices to their visitors. This is important, (David Sellman, NTPL) Many social, cultural and economic factors Garden paths and wooden structures Rationing for all but essential uses is likely in both in terms of managing immediate critical areas. Water charges may also are involved, including population dynamics, Paths act as conduits for storm water run- problems caused by the current phase of increase, encouraging installation of more disposable income, competing attractions off, and so are prone to rapid erosion. Path climate change, and in modifying behaviour efficient irrigation systems and the use of and access to gardens by public transport. design and choice of surfaces can minimise and attitudes to prevent further undesirable ‘grey’ water from domestic activities. Water this hazard. In winter and early spring Improved weather, especially early in the climate changes in the future. Gardens open conservation measures, either via soil slippery algal growth on paths is likely year, may attract more visitors to gardens, to the public could help demonstrate mulching or collecting of rainwater in water to increase. while prolonged autumns with 10-20 per sustainable horticulture techniques. butts, will become a summer priority and cent less rainfall and enhanced autumn Garden industries In addition to unpredictable storm damage, large gardens may need to construct foliage colour are likely to extend the cycles of summer drying and intensive and reservoirs for irrigation purposes. Enhanced autumn colour will extend the visitor season (Mike Sleigh, RHS) visitor season. Exceptionally high summer Nurseries and garden centres can benefit prolonged rain will lead to more rapid Hot work temperatures would be a deterrent, unless commercially from climate change, but will deterioration of wooden structures. More gardens incorporate design features such experience increased operational difficulties. robust construction and use of more durable Extreme summer temperatures represent a as shady woodland and lakeside walks to Most plants sold at horticultural retail timbers from sustainable sources will be health hazard. Those working on energetic increase visitor comfort. required. Shading and better ventilation in tasks will need to guard against dehydration outlets are grown in containers, with roots Warmer weather will give rise to more algae in ponds will be needed in summer, for and sunburn and will require better protective Gardens will benefit from investment in confined in small volumes of growing (NT) the benefit of plants and gardeners who clothing when working with chemicals in visitor facilities, such as glasshouses, shelters medium. Increased labour costs for watering work under glass and, as winters become hot conditions. Coping with effects of climate and information centres, if summer weather will favour the installation of automatic warmer and wetter, good ventilation will be change will entail more work and stress, becomes even more unpredictable. watering systems. Contingency planning will be required to ensure reliable supplies essential to deter fungal diseases. resulting from increased routine maintenance “The gardening community Garden managers need to be aware that tasks and the need for greater flexibility in during drought periods. has the potential to set an climate change will favour increases in the working practices in large gardens. number and variety of biting insects that Increased incidence of stormy weather will example of good practice breed in water, which may deter visitors. require better staking and support of large which will further increase container-grown plants, which are The major impact of more garden visitors public appreciation and inherently unstable. will be increased wear and tear, especially support for gardens..” after heavy rain, which could be minimised Working conditions under glass during hot Bisgrove and Hadley (2002) by contingency planning for managing summer weather will deteriorate, although visitor movements. Gardens can draw on a milder climate in general will make year- experience from the sports turf industry in round outdoor working more amenable. Brown lawns will become a more common sight coping with wear and compaction of lawns. (English Heritage)

Working in extreme heat represents a health hazard (RHS)

14 15 Gardening in the Global Greenhouse: Summary Report The bottom line Gardening in the Global Greenhouse: Summary Report Next Steps

Next steps: research and action Key challenges It is difficult to plan for devastating extreme As undesirable effects of climate change There are serious limitations in our knowledge of the likely impact of climate change and opportunities weather events, such as storms and hail, but become obvious to gardeners, there will be on gardens. Domestic gardens businesses in areas with increasing risk of greater demand for solutions to gardening This study has highlighted challenges and opportunities for the gardening industry Challenges include: flooding may need to improve water run-off problems. There will be new marketing to consider, as well as a number of areas for future research and recommendations • managing drier soils in summer and wetter measures, face rising insurance premiums opportunities for environmentally benign for action (see below). soils in winter and consider relocation. For garden centres solutions such as composting and organic The partners will explore what research and practical action they can take together, • maintaining soil fertility and nurseries, contingency planning for techniques, water storage equipment and as well as using this study to inform their own policy and decision-making. • intensification of pest, disease and extreme weather damage will become biological control methods for dealing with weed problems increasingly important. Extreme weather pests. Greater interest in irrigation systems Research: Recommendations: • maintaining a smooth, green lawn conditions, such as drought and flooding, is also likely. • meeting the needs of drought-adapted Extreme weather conditions are damaging to garden The study has identified a need for • encourage networking between major sales businesses (Notcutts) are also indirectly damaging to garden sales perennials and bulbous species that do Climate change will lengthen the growing research to: gardening organisations and institutions businesses, since public interest in gardening not tolerate waterlogging in winter season and extend peak sales periods for to exchange and coordinate observations, rapidly declines during such periods. • establish the degree of exposure • year-round plant growth, requiring garden plants and sundries such as garden ideas and innovations, disseminate of Britain’s gardening heritage to continuous maintenance leisure equipment. Demand for replacement information and inform policy decisions climate change plants for those lost through drought or Opportunities include: Plants likely to perform better • initiate a Hortus Europaeus, to map the in a warmer climate waterlogged soils in winter will increase. • identify resources and level of investment • increased range of plants suitable for Source: Emmett (pers. comm.) distribution of garden species in Europe, required to maintain the integrity of those cultivation in the open garden Genus Species There could be opportunities for marketing a allowing large scale monitoring of effects gardens of greatest historical significance • northwards spread of optimum growing Agavaceae Agave wider range of perennial exotic species and Cordyline (Torbay palm) of climate change on garden plant species conditions for some plants Yucca plants that are currently half-hardy annuals, • establish the criteria for the • produce a list of indicator plant species • warmer and drier summers and autumns Aizoaceae Carpobrotus (Hottentot fig) which will survive in the open garden in management of significant plant for enjoying the garden Delosperma likely to be sensitive to climate change, Drosanthemum milder winters. Faster growth and flowering collections on a national basis allowing nationwide monitoring of its Heritage gardens and large gardens Aloaceae Aloe will encourage gardeners to replant bedding • develop a greater understanding of how progress and impacts open to the public Araceae Alocasia and containers more frequently. garden plants respond to climate change, Challenges include: Chamaerops • implement practical actions to enable Trachycarpus Palms Exploiting opportunities for marketing a specifically in relation to: • maintaining historic gardens, their associated Many plants, such as cannas, will Phoenix gardens to adapt to climate change. wider range of plants requires advanced – frost sensitivity and hardiness archaeology, wildlife interests, and their specimen perform better in a warmer climate Bromeliaceae Fascicularia Forward plan for flooding, drought, (Mark Bolton, NTPL) Puya planning, since basic research is required to – winter dormancy plants as they adapt to a changing climate soil changes and structural impacts Cannaceae Canna (Indian shot) identify optimum cultivation techniques, – flowering and autumn colour • managing the impact of visitors during less on buildings, features and facilities Crassulaceae Aeonium especially for perennials, shrubs and trees – susceptibility to pests and diseases predictable summer weather Crassula Echeveria which will need to tolerate summer drought – interactions with symbiotic soil fungi • dealing with storms and flood Sedum and waterlogging in winter. Research will and beneficial micro-organisms • increased maintenance costs, especially for Musaceae Ensete also be required in ‘hardening off’ fine grass swards Musa (Banana) • identification and monitoring of Myrtaceae Callistemon (Bottle brush) procedures for species that are marginally Opportunities include: potentially invasive species Metrosideros hardy, although this will become less • potential for a longer visitor season Oleaceae Olea (Olive) problematic by the 2080s. • minimise soil nutrient loss and • developing an educational role, as centres Proteaceae Banksia Leucadendron reduce the impact of water logging of excellence in environmentally sustainable Grevillea and drought on plants gardening techniques Protea Restionaceae Elegia • develop methods for mowing grass that Retail horticulture outlets Restio minimise compaction of wet soils Challenges include: Zingiberaceae Hedychium • maintaining container-grown stock during • improve methods for water management summer drought in the garden and heat management • minimising the impact of extreme weather on in glasshouses business activities • refine models for regional impacts, • marketing a wider range of plant species allowing more accurate prediction and varieties of future local climates Opportunities include: • extension of peak periods of gardening activity • investigate garden visitor response The season for visiting gardens will lengthen • marketing water conservation equipment to climate change (Chris King, NTPL)

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