Trees in winter: Frost, snow, and the lame duck syndrome

Derrick Rooney1

Gardeners have it made in frosty If you can get your mouth around the It is an unusual cross in that one weather. They can get indoors, with Latin (alas, there is no simple English of the parents (V. lantana; Fig. 2) a fire or the central heating, and think name) you might find this to be an comes from Europe, and is more or in comfort about how to remodel the interesting in the summer also, less deciduous, depending on the garden when spring comes. in any garden that can give it a bit of severity of the winter, while the other space. (V. rhytidophyllum; Fig. 3) is evergreen Their , other than the favoured and Chinese. The two could never few growing in containers that can hybridise in the wild, but the cross be taken inside, do not have this sometimes occurs spontaneously in soft option. Plants have to sit and gardens where people have brought take whatever the weather throws at the two together. Gardeners often play them. When we have a winter as cold matchmaker in this way for species as the 2015 edition in New Zealand that could never meet in nature. (my garden is in inland Canterbury), damage is inevitable. The main claim to fame of the evergreen parent, V. rhytidophyllum, On the plus side, many garden plants is that it is a big tough shrub capable are capable of resisting the coldest of surviving and sometimes thriving in weather. Dormant grape vines, for dry shade. The flowers, in late spring, example, can tolerate as much are small and dirty white and the as 40°C of frost without damage, fruits are black, so there is no great whereas a summer frost of only two or ornamental value in either. I like this three degrees can put the vine out of shrub, though. The are dark production for a year. To achieve this green, long, and wrinkly and make a level of winter-cold hardiness, plants good backdrop for brighter plants. use a variety of strategies. The hybrid, which varies in the size One of the most interesting winter Fig. 2 lantana ‘Versicolor’ and shape of its leaves, has the same strategies in my garden is that of a (wayfaring ). wrinkly texture and dirty white hybrid viburnum, V. × rhytidophylloides flowers, and is usually evergreen. (Fig. 1). However, unlike the evergreen parent, which merely looks darker and duller green in winter, the hybrid reacts to cold weather by turning various shades of bronze, purple, and beetroot red, depending on the quality of the soil and the severity of the cold. The colder the weather and the poorer the soil, the better the colour. When warmer weather arrives in spring, the leaves revert to their normal dull green. Offhand, I can think of no other evergreen shrub that reacts to winter cold in quite so dramatic a way. One of the challenges that evergreen and face in winter is the phenomenon known as frost- drought. Leafy plants need to transpire continuously, but even if there is plenty of water about they are Fig. 3 Viburnum rhytidophyllum unable to take it up through the roots Fig. 1 Viburnum × rhytidophylloides. (the leatherleaf viburnum). while the ground is frozen. Thus, in

1 PO Box 43, Hororata 7544, Canterbury, New Zealand; [email protected]

2 New Zealand Garden Journal, 2016, Vol. 19(1) a prolonged frosty spell, plants can classified as heavy and wet and is Other factors, including the amount suffer the sort of desiccation and apt to accumulate and stick even on of summer or autumn ripening the damage that occurs after a hot, dry bare branches. This is particularly growth receives, and the frequency spell in summer. noticeable in species such as silver of damaging early-autumn frosts, or birch which are adapted to deep snow spring frosts after growth resumes, Deciduous shrubs and trees partially in their natural habitats but vulnerable can also affect hardiness. This counter this winter risk by shedding to breakage in New Zealand, possibly applies mainly to plants from stable their foliage, so that there is less need because the snow in their natural continental climates grown in for transpiration. Some plants undergo habitats is more powdery and less changeable maritime climates such as complex chemical changes that fill likely to accumulate in the crown. that of New Zealand. An example is their cells in winter with sugars that the silk tree, Albizia julibrissin, which act as natural anti-freeze. Conifers Some evergreen species, including was promoted for horticultural shelter such as firs and spruces have small, rhododendrons and buddlejas, are in the North Island in the 1960s and needle-like foliage with a thick, waxy also vulnerable to snow damage but 1970s and is a common ornamental covering resistant to both cold and have evolved strategies to deal with tree in Christchurch gardens. Known heat. frosty mornings. One is the “lame duck as silk tree in New Zealand and syndrome”. Typically, the leaves of Deciduous broadleaf trees mimosa in the United States, Albizia is plants that adopt this technique curl (angiosperms or “hardwoods”) native to the east Asian continent and up or wilt when the temperature drops and conifers (gymnosperms or is an attractive small to medium-sized below zero, but return to normal as “softwoods”) have different plumbing. garden tree with very fragrant flowers. soon as the day warms up (Fig. 4A–B). Deciduous trees lose most In the United States it is of their ability to circulate rated as hardy in Zone 5b, water after the first freeze and is widely grown. It seeds and need to regrow the freely in Massachusetts necessary tissues in the (Zone 6, -23°C) and has spring. Conifers better been declared an invasive accommodate the transfer pest in some states, of water in winter. They including Florida and have special “check valves” Tennessee. This hardiness that can allow resumption rating implies it should be of water movement should tolerant of winter frosts down conditions be just right, if to about -25°C. However, unseasonable. The conifer on parts of the Canterbury cell walls are stronger than Plains very much lesser those of hardwoods and frosts are usually fatal to it. better able to cope with ice Numerous other plants of expansion. continental origin suffer a Conifers have higher leaf similar fate here. The wonder densities than hardwoods, tree (Idesia polycarpa) is which means that the extra another example. In Rotorua weight of accumulated this is a familiar sight and is snow would damage stems even used as a street tree. and break branches if they A B Its big bunches of sealing had not evolved alternative wax-red berries hanging growth and branching Fig. 4 Buddleja salviifolia (the sage-leaved butterfly bush). A, photographed at 8 am and wilted following an overnight frost. from bare branches are a patterns to counter the B, same plant photographed at 10 am with revived leaves. charming sight in winter. problem. Where hardwood Sadly, in most of the South trees usually have multiple leaders or If a plant could experience emotion, Island away from a narrow coastal main stems (indeterminant growth), a rhododendron on a frosty winter strip sheltered from salt-laden winds, often with narrow branch angles, morning would be the epitome of wonder tree is a non-starter. A native conifers tend to have single leaders depression. Usually, it will bounce of northern Japan, the Korean (determinant growth) and wider back with no apparent harm as soon Peninsula, and nearby parts of branch angles that enable them to as the frosty spell ends, but if the frost China, wonder tree is rated as hardy shed snow without bending as much. is unusually severe and prolonged the to US Zone 6 and should be hardy Conifers also usually have longer large-leaf types may have their foliage everywhere in the South Island except wood fibres that make their stems browned. Some of the more tender the coldest parts of Central Otago and more flexible. species and hybrids may have their the Mackenzie Country. Again, the stems split, with fatal consequences, Broadleaf hardwoods, on the other problem is not extreme winter cold but if the moisture inside them freezes hand, whether they are deciduous or our unstable oceanic climate with its sufficiently to rupture the cell walls. evergreen, are often more vulnerable frequent unseasonable cold or warm to snow damage because lowland snaps and lack of reliable summer snow in New Zealand is usually and autumn heat to ripen the wood.

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