Ivy is a plant that divides opinions. Attractive foliage, good for wildlife, cools buildings… Invasive, ruins walls, harbours pests…. Research and opinion on all these issues has been in the news increasingly in recent years and now the most comprehensive book on ivies ever produced has been published by The Royal Horticultural Society.
Hedera: The Complete Guide by Hugh McAllister and Rosalyn Marshall is exactly that – the complete guide. With more than four hundred pages, and impressively illustrated in colour throughout, this is the second in the RHS Horticultural Monograph series which began with last year’s excellent book on kniphofias.
This is an attractively designed book – far more appealing than most gardeners expect from a serious plant monograph even though the predominant colour of ivies is, well, green. All the species are described in detail, but not in torrents of baffling technical language; two hundred cultivars are illustrated and described, and the fat descriptive checklist covers all the other names that have ever been used, over two thousand of them.
One very useful feature of the book is its use of the Pierot system of classification. This was created by Suzanne Pierot, the first President of the American Ivy Society, in the 1970s and divides ivies into nine convenient groups based on easily-seen features of the foliage. Brits are largely unfamiliar with this approach so its use in this book should help it stick.
But what does the book have to say about those pros and cons?
Attractive foliage? Obviously.
Good for wildlife? Yes. “Ivy berries are eaten by at least 17 species of bird in Britain indicating the importance of this plant group for the support of vertebrate wildlife…. They have a high energy, though low protein, content and form a large part of the diet of several species at this time of year (winter).” “The autumn flowering of ivy provides nectar for a wide range of invertebrates at a lean time of year.” Not to mention ivy as a valuable nest site.
Cools buildings in summer, warms buildings in winter? Yes. “Measurements on ivy-covered stone walls across several historic sites in England… showed that an ivy covering resulted in cooler walls in summer and warmer walls in winter.” “Recent research on the climate of southeast UK… suggests that a 21-37% reduction in winter heating could be achieved.”
Invasive? Sometimes. While most ivies can develop vigorous growth in their native habitats, it is almost always H. hibernica that causes problems as a non-native plant. “All ivies need not be banned in climates where invasive ivies are a problem. There are enough dwarf, miniature and slow-growing cultivars of H. helix to provide a good range of safe and attractive plants for indoor and outdoor use.” In my Pennsylvania garden, as soon as an ivy shoot penetrates the deer fence the deer eat it.
Damages walls? Sometimes. “Any increase in relative humidity due to wall plant cover is offset by lack of rain reaching the building.” But: “ivy can root into weakened historic walls or buildings, and can lift blocks of stone off walls” and also damage walls built and pointed using soft mortar.
Harbours pests? Sometimes. Ivy is attacked by viburnum scale which also attacks Viburnum tinus. “In pots, by far the most serious pest of ivy is vine weevil” – which attacks a wider range of other plants. I’d be interested to know if outdoor ivy has a role as a reservoir of vine weevil infestation.
This is an impressive book by one of our most seasoned horticultural taxonomists and a relatively young recruit to the RHS botany team. The RHS has more in the series on the way, I look forward to the series building into an invaluable resource.
Hedera: The Complete Guide by Hugh McAllister and Rosalyn Marshall is published by the Royal Horticultural Society at £40.00/$53.45.