Transplanting plant species between continents can have disastrous effects, writes Jane Powers
We gardeners are benign creatures, bumbling around our patches with trug and trowel, carefully tending our individual sections of the planet. Or so it may seem. In fact, gardeners have wreaked some dreadful havoc on this earth, albeit (for the most part) innocently. This was brought home to me a few years ago while travelling through rural Massachusetts in late summer. Our minor highway threaded through a typical eastern United States landscape of woods, wetlands and small lakes (or ponds, as they are called). And wherever there was watery ground - at pond-edge or in marshy soil - there were stands of purple loosestrife, its brilliantly-coloured spires of flower shooting skywards.
It was a stirring sight indeed - or it would have been, if it weren't that this European interloper, Lythrum salicaria, is one of the most invasive plants in north America. It arrived early in the 19th century, introduced by gardeners, herbalists and bee-keepers, among others. In a congenial habitat lacking the pests that keep it checked in Europe, it has steadily spread, choking waterways, crowding out native species (including wetland orchids), starving wildlife and disrupting the ecology. Each plant can spawn up to two million seeds, and it can reproduce vegetatively as well, from scraps of stem or root.
Ironically, the salvation of America's wetlands may lie in the importation of more European aliens: beetles and weevils that feed exclusively on purple loosestrife. Lythrum salicaria will never be eradicated from north America, but trials involving the introduction of its natural enemies show that some balance may be restored to the habitats that it has colonised.
Ireland, of course, has its own alien invaders. Giant hogweed, Japanese knotweed and Indian balsam were introduced as stately garden plants during the Victorian era, but have become ineradicable nuisances. And Rhododendron ponticum, which was planted on demesnes from the end of the 18th century, is a serious problem in many counties, smothering native species with its light-excluding and "allelopathic" growth (it exudes chemicals that prevent other plants from germinating). The pinky-purple-flowered evergreen can produce 7,000 seeds per flowerhead, while a single plant can spread 100 square metres by layering (rooting where the branches touch the soil).
I garnered these last bits of information from a book that I've been dug into over the past couple of days. Plant, edited by Janet Marinelli (and published with the imprimatur of the Royal Botanic Gardens at Kew), is a new kind of guide for gardeners. It aims to teach us (in the most alluring way, with plenty of lovely photos and lively snippets of knowledge) to think differently about working with plants. Instead of plants serving us, maintain the authors, we really should be serving plants, and the planet that we're all growing on.
Human activity, I read with a chill, has accelerated the natural process of extinction a thousandfold. Up to a quarter of this earth's plant species could die off in a matter of decades - as a result of the spread of civilisation. Habitat loss, pollution and global warming are our gift to our fellow denizens. But what can we, as gardeners, do to help? Of course, we all know that we should garden as sustainably as possible, cutting down on water, chemicals and the unnecessary use of power tools; we know we should recycle our waste and be kind to our wildlife. But how many of us have a clear idea of how to plant for the planet?
Not giving a home to invasive species is an obvious strategy. Welcoming in plants that are endangered will also help. We may not all have room for a dawn redwood or a ginkgo, but surely we can find space for an heirloom vegetable or flower variety that is no longer available in commerce? And if we do acquire plants that are rare or threatened in the wild (the book includes 350 pages of them), we should be mindful of their provenance, buying only from reputable sources.
In the past, plant hunters were immensely rapacious, particularly in the acquisitive Victorian era. Millions of orchids, for instance, were stripped from forests in Asia, and in central and south America; most died on the voyage back to Europe, leading to the extinction of countless species. Bulbs, likewise, were collected willy-nilly, seriously depleting populations in their native habitats. Now, collection from the wild is regulated, but many plants are still illegally traded.
Would any responsible gardener really want such plants, especially if it contributed to their dying out in their native habitat? Yet, there is still a market for the cacti, succulents, carnivorous plants, orchids, bromeliads, bulbs, trilliums, tree ferns and cycads that are illicitly harvested. Some gardeners, no doubt, buy these plants in blissful ignorance, but perhaps it's time we educated ourselves and asked questions about where they came from?
Over-collection is just one toll in the death-knell of some species. Goats, introduced all over the world to supply food, are deeply destructive. Hardy and agile, they are able to scramble across poor terrain with ease, eating all that goes before them. In Guadalupe, they have eaten every single seedling of the native palm, Brahea edulis. When the existing adult palms die from old age or disease, there will be no infants to replace them, and the species will be extinct in the wild. Fortunately the Guadalupe palm is alive and well in horticulture, and if the plague of goats is ever dealt with, it could be reintroduced.
Things are more complicated for the deliciously scented chocolate cosmos (Cosmos atrosanguineus) from Mexico. It was first collected around 1860, and was found on only one occasion since, in 1902. All the plants in existence are sterile, and are clones of the same plant, propagated from cuttings for over a century. With no genetic variation, and an inability to set seed, its chances of survival in the wild are minimal.
And then there is the rare cycad, Encephalartos woodii. Like the chocolate cosmos, it is extinct in the wild. And as all the 500 specimens in cultivation are male, there is no chance of this African native ever reproducing sexually. Or is there? Well actually, in rare cases cycads may change sex. Doesn't nature have a wonderful bundle of tricks?
Get out of that Garden
Six Irish natives not welcome in certain other countries:
Hawthorn (Crataegus monogyna): in the US, birds prefer the berries of this to those of the American native, leading to increased numbers of the European species at the expense of the indigenous one.
Ivy (Hedera helix): invasive in Australia, New Zealand, Brazil and the US, where it makes trees vulnerable to storm damage and reduces diversity of native forest-floor species.
Holly (Ilex aquifolium): competes with native species in old-growth forests of the Pacific northwest in US.
Yellow flag (Iris pseudacorus): a noxious weed in the USA where it crowds out shoreline vegetation, and makes it difficult for waterfowl to move between water and land.
Purple loosestrife (Lythrum salicaria): now dominates wetlands in northern US.
Dog rose (Rosa canina): invasive in southern Australia and the US, where it may form dense thickets.