Gardens thrive on diversity

When it comes to choosing plants to create a garden that’s hospitable for wildlife, you are generally advised to chose native species rather than immigrant ones. I have given this advice myself many times in talks and articles.

The idea is that a plant, particularly a large plant like a tree or a shrub, isn’t just an individual living organism, it’s also an ecosystem. It supports hundreds, sometimes thousands, of other living organisms like insects, spiders, fungi and the birds that eat the insects.

A native Irish plant has had millennia to accumulate its ecosystem. But when a plant is brought in from another country or continent it generally leaves most of its passengers behind and its ecosystem is much reduced. Cultivated varieties of plants also lack the luxury of long periods of time during which other species can adapt to living with or on them.

The theory is sound but it is only a theory. In recent years there has been quite a lot of research done, mostly in other countries, to test how the theory works in practice and some of the results have been quite surprising. In fact most of the research suggests that the best gardens for wildlife are the ones with the widest variety of plants and plant habitats and contain both native and exotic species.

This conclusion is particularly relevant to Ireland. We live on an island lying off an island lying off the continent of Europe. Geography ensured that many species failed to make it here by their own efforts after the last Ice Age, and our list of native species is very short. There are fewer than 30 native Irish tree species. There are about twice as many in Britain, and four times as many in the northern part of continental Europe. If we restrict our planting to our short list of native species we may be depriving our garden wildlife of some important advantages.

For example, most wildlife gardeners want to encourage songbirds. They add beauty and interest to the garden and we’re making a contribution to their conservation. So we put up nest boxes and fill up the bird feeders. But the biggest cause of mortality among small birds is not lack of food or a place to nest, it’s hypothermia. What they really need is dense evergreen cover for roosting on long, cold winter nights. But unfortunately the list of native Irish trees and shrubs is very short of evergreen species, particularly the type of evergreen that provides good shelter from cold winds. So the answer is to plant something non-native, even something as unfashionable as a hedge of Leyland cypress.

We have a similar shortage of native plants that flower late in the autumn to provide nectar and pollen for insects at this time of year. Luckily however there is one common native plant that helps fill both these gaps. Thank God for ivy.