Ivy
People can readily identify ivy, the evergreen, woody climber that hugs tree trunks, and covers forest floors and hedgerow bases; adding colour to the winter landscape. Ivy attracts horticultural enthusiasm because of its leaf-shape variation and range of variegated types, although this interest is less now than during the nineteenth century. Ivy's symbolic use extends at least as far back as celebrations of Dionysian and Bacchanalian rites in Ancient Greece and Rome. In fourteenth-century England, ivy's associations with alcohol persisted as it became used to identify public houses. Since the eighteenth century ivy has been part of British Christmas decorations, along with mistletoe and holly.
Hedera is a small genus distributed through Europe to East Asia; Hedera helix occurs through Europe into south-west Asia. Despite the ease of identifying Hedera helix, morphological variation within the species is high and there are numerous subspecies, varieties and forms (even within Britain) that are difficult to differentiate from each other. At least one author has gone as far as to state most British specimens of the genus Hedera, housed as reference material in natural history collections, cannot be adequately identified. This has implications for conservation and understanding the detailed distributions of within-species variation.
Hedera helix produces two different types of shoots that are so different to each other they have sometimes been mistaken as coming from different species. Juvenile shoots produce small, distinctly ivy-like leaves, which are alternately arranged opposite each other, climb by adventitious roots and do not produce flowers. In contrast, mature shoots have large, lobed, spirally-arranged leaves which lack roots but produce flowers. In addition to these morphological differences, there are also physiological differences between juvenile and mature shoots, for example, adult leaves have greater freezing resistance but lower levels of photosynthesis than juvenile leaves during late winter and early spring.
Despite popular belief and superficial appearances, Hedera helix is not a parasite. It produces its own food resources via photosynthesis but attaches itself to the stems of trees by adventitious roots. Only when ivy gets very large, making the tree to which it is attached top heavy, is it likely to become a problem.
Ivy flowers during autumn and early winter, and produces fruits into late winter. Consequently, ivy is a very important source of nectar and pollen for late-season insects. It is also an important source of winter food for birds, and a nesting site during the spring.
Further reading
Rehm EM et al. 2014. Spring patterns of freezing resistance and photosynthesis of two leaf phenotypes of Hedera helix. Basic and Applied Ecology 15: 543-550.
Rose PQ 1980. Ivies. Blandford Press.
Sell P and Murrell G 2009. Flora of Great Britain and Ireland. Vol. 3. Mimosaceae-Lentibulariaceae. Cambridge University Press, pp. 245-249.
Stephen Harris