Hydrangea Paniculata Neiill Lancaster Bottaniisstt,, RHS Garrden Wiisslley

RHHSS PLAANNTT TRIALSS BULLLEETIN Numberr 23 Deccemmberr 2008 Hydrangea paniculata Neiill Lancaster Bottaniisstt,, RHS Garrden Wiisslley Wendy Weslley Trriiallss Offficerr,, RHS Garrden Wiisslley The RHS Trial of Hydrangea paniculata Background to the trial H. paniculata in the wild and in Although Hydrangea paniculata has been cultivated in cultivation European gardens for well over a century, for most of that H. paniculata is native of cool temperate and sub-tropical period there has been little choice beyond a handful of regions of Japan, China and Sakhalin, where it grows as a very early introductions. H. paniculata , the panicled deciduous shrub or small tree to 8m in height, at altitudes hydrangea, has invariably played second fiddle to the more to about 1300m. It is often stated to be a very variable colourful H. macrophylla , or hortensia, even though it is species in the wild, but not all authors agree on this. generally easier to grow and more reliably hardy. The first edition of Michael Haworth-Booth’s The Hydrangeas , H. paniculata was first described and named in 1829 by the published in 1950, included over 300 hortensias, but only German physician and botanist Philipp Franz von Siebold 3 types of H. paniculata , namely the cultivars ‘Floribunda’, while working for the Dutch East India Company based on ‘Grandiflora’ and ‘Praecox’. Deshima Island within Nagasaki harbour, Japan. Granted limited access to the mainland to carry out his medical After the 1950s, the range of cultivars started to increase duties, at a time when other foreigners were excluded, with the appearance of the first de Belder introductions Siebold used the opportunity to collect and describe the and slowly gathered pace in the 1970s and 1980s as new native plants, including H. paniculata . Siebold’s generations of seedlings were raised from existing cultivars. determination to explore this closed country eventually Because of the long lead-time for raising and introducing resulted in him falling foul of the authorities and he was new plants, only eight cultivars could be listed in the first banished from Japan before the plant could be exported to edition of the RHS Plant Finder in 1987. A decade on, there Europe. Fortunately, he was allowed to return 30 years were still only twelve cultivars, four of the early de Belder later, when he succeeded in sending back the cultivated selections having become more widely available, but form of H. paniculata now known as ‘Grandiflora’. This subsequent years saw a more rapid increase. In the past 10 cultivar, also known as the pee gee hydrangea, is probably years, interest in H. paniculata has taken off with many still the most widely grown. promising new introductions from breeders in Belgium, Holland, France and the USA. Other early introductions from Japan include ‘Floribunda’, discovered by Carl Maximowicz in a Japanese nursery in With so many new cultivars coming on to the market, the the 1860s, ‘Praecox’, raised from seeds collected on RHS decided to hold a trial to compare the new cultivars Hokkaido Island by Charles Sargent in 1897, and ‘Kyushu’, with the old and to select the best ones for the Award of introduced as a wild-collected seedling from Kyushu Island Garden Merit. More than 40 cultivars were assembled in a by Collingwood Ingram in 1926. trial that was considered by many of those involved to be among the most successful woody plant trials yet In the 1950s, at the embryonic Kalmthout Arboretum and undertaken by the RHS. At the same time, the first Eurotrials later at Hemelrijk, Robert and Jelena de Belder began to were conducted in the UK, France, Germany and Holland, experiment with raising and selecting seedlings of providing an interesting opportunity to compare trial results H. paniculata , derived initially from an old plant in their between different sites and growing conditions. garden of ‘Floribunda’. The first and most famous of these, Front cover: Hydrangea paniculata ‘Limelight’ Right: Hydrangea paniculata Trial August 2008 2 RHS Plant Trials Bulletin ‘Unique’, was reputedly the sole survivor of a batch of Inflorescence of Hydrangea paniculata young seedlings that had been ravaged by blackbirds. It showing pink late season colour turned out to be an exceptional plant whose offspring yielded many good new cultivars, including ‘Papillon’, ‘White Lace’, PINK DIAMOND , ‘Burgundy Lace’, ‘Brussels Lace’, ‘White Moth’, ‘Green Spire’ and ‘Melody’. Further generations were raised in the 1970s and 1980s by open pollination of these plants, with PINK DIAMOND and ‘White Moth’, in particular, producing some notable cultivars. The arboretum at Hemelrijk still contains many unnamed plants, and seeds from these are still collected and distributed to nurseries in the hope of raising new cultivars. Breeders in Europe and the USA have built upon the de Belder legacy, producing cultivars with inflorescences of subtly different shapes and shades of pink, white and green, as well as cultivars of more compact habit. Some of the most striking new cultivars have been raised by Peter Zwijnenburg at Boskoop, including ‘Big Ben’, ‘Phantom’, ‘Limelight’, ‘Dolly’, ‘Silver Dollar’ and ‘Pink Lady’, while significant contributions have also come from Darthuizer in the Netherlands and Michael Dirr in the USA. As a garden plant, H. paniculata has a number of advantages over H. macrophylla . It is relatively drought- tolerant, does well in both full sun and partial shade and grows in acid or alkaline soil, as long as the ground is well- drained. Importantly, it is very hardy (to about -30˚C in the USA) and as it always flowers on the current season’s wood, it is not susceptible to late spring frosts. Botanical Description Hydrangea paniculata is a deciduous woody plant with serrate leaves arranged in opposite pairs or clusters of three, and flower heads made up of showy, sterile flowers mixed with smaller, fertile flowers. What sets it apart from other species is the structure of the inflorescence, which is typically a cone-shaped panicle. The only other Hydrangea that has flowers arranged in a panicle is H. arborescens and that is distinguished by having lobed leaves. Habit: In the wild may be a lax shrub to about 2m or a small tree to 8m. Shoots: Somewhat brittle. Downy at first, becoming hairless. Leaves: Elliptic or ovate, serrate, acuminate, rounded or tapered at the base, 7.5 – 15cm long and about half as wide. Arranged in opposite pairs, or in 3’s. Sparsely pubescent above and on the veins below. Flowers: A mixture of small, yellowish fertile flowers and long-lasting (up to about 8 weeks) sterile flowers with 4 or 5 white sepals, usually entire, often ageing to pink or purplish-pink, particularly on the underside. Flowers are borne in panicles on the current season’s growth, opening from the base toward the apex of the panicle. The ovary is semi-inferior. Fruit: A many-seeded capsule. The seeds have no dormancy and typically germinate in 2 to 4 weeks. Hydrangea paniculata 3 The Trial Award of Garden Merit (AGM) Objectives ⅷ To compare and assess new and old cultivars Hydrangea paniculata ‘Big Ben’ AGM(H4) 2008 ⅷ To recommend the Award of Garden Merit to those considered to be the best Hydrangea paniculata ‘Floribunda’ AGM H4) 1993* ⅷ To determine correct nomenclature Hydrangea paniculata ‘Grandiflora’ AGM(H4) 1993* ⅷ To obtain specimens, photographs, and descriptions as a Hydrangea paniculata ‘Kyushu’ AGM (H4) 1993 record to be held in the RHS Herbarium at Wisley Hydrangea paniculata ‘Limelight’ PBR AGM(H4) 2008 Hydrangea paniculata ‘Phantom’ AGM(H4) 2008 Records Hydrangea paniculata AGM(H4) 1993 ⅷ Habit PINK DIAMOND ‘Interhydia’ ⅷ Response to pruning Hydrangea paniculata AGM(H4) 2008 ⅷ Stem colour PINKY -W INKY ‘Dvppinky’ PBR ⅷ Foliage colour (summer and autumn) Hydrangea paniculata ‘Silver Dollar’ AGM(H4) 2008 ⅷ Flowering period Hydrangea paniculata ‘Unique’ AGM (H4) 1993* ⅷ Inflorescence shape ⅷ Flower colour (early, mid-season and late season) * Recommended to be rescinded at the AGM Review in 2012 Entries The following will receive an Award of Garden Merit 68 entries, representing 47 cultivars, were submitted by when available to the public gardens, nurserymen and individuals in the UK, Netherlands, Belgium and the USA. Details of each cultivar Hydrangea paniculata ‘Dolly’ are given in the Selection Table below. Hydrangea paniculata ‘Pink Lady’ Cultivation Hardiness rating Three plants of each entry were planted in May 2004. All H4 = hardy throughout the British Isles plants were cut down to the base after planting. The plants were grown in the open, with no shade, and watered only during periods of drought. Soil analyses were conducted Judging Criteria annually and treatments applied as necessary to eliminate nutrient deficiencies and maintain approximately neutral The Woody Plant Trials Subcommittee assessed conditions. entries in the trial for the Award of Garden Merit using the following criteria: ⅷ Habit Pruning ⅷ Bud / Flower / Faded flower All plants were pruned each year in late March. The three plants of each cultivar were used to test three pruning ⅷ Floppiness of head regimes: ⅷ Foliage 1 Hard pruning (to two buds) 2 Moderate pruning (to four buds) 3 Light pruning (removal of previous year’s flowerheads The AGM is only awarded to plants that are: only) ⅷ Excellent for garden decoration ⅷ Available from nurseries ⅷ Reasonably resistant to pests and diseases ⅷ Of good constitution ⅷ Essentially stable in form and colour ⅷ Reasonably easy to grow Hydrangea paniculata Trial September 2007 4 RHS Plant Trials Bulletin Light Moderate Hard 1 Hard pruning (to two buds) Where there was a response to pruning, the lightly pruned plants were the first to flower and carried more, smaller flowers than the hard-pruned plants, which flowered later, and bore larger, fewer panicles. Those plants subjected to moderate pruning were intermediate in panicle size and time of flowering. Although the hard-pruned plants tended to grow with added vigour, their larger flowerheads were not always advantageous, sometimes producing panicles that could not be supported and flopped.

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