Are native plants better for garden wildlife?
by David Hewitt
Up until recently, the usual answer to this question was an emphatic yes. Gardeners were urged to include as many native plants such as British wildflowers as possible in their plots to help wildlife, particularly insects such as bees, butterflies and moths. Indeed, as the introduction to the Postcode Plants Database, hosted by the Natural History Museum, advised at the end of the last millennium: “Introduced plants usually offer little to our native wildlife.”
Now though, this long-held view is being subjected to more scrutiny and it seems that picking plants for the garden that will help bugs, and by extension birds and mammals, is not quite so straightforward, even for the experts. “We know that even tidy, well-ordered gardens can be good for wildlife,” says Helen Bostock, Senior Horticultural Advisor at the Royal Horticultural Society (RHS). “What we don’t really know is how planting can help attract and support that wildlife.” It’s an important question because many insects such as bees, butterflies and moths are suffering serious declines in numbers in the UK and plants in gardens can play an important role in providing them with the nectar, pollen and leaf material that they need to survive.
Last month, biologists at the University of Sussex revealed that a plant that’s native to Mexico – Agastache mexicana (or Mexican giant hyssop) – was best for bumblebees in a study comparing the attractiveness of summer-flowering garden plants to insects. And while a native plant, wild marjoram, attracted the most solitary bees, borage, originally from the Mediterranean region, was best for drawing in honeybees.
The question of how the geographical origin of garden plants affects their ability to attract wildlife is not helped by disagreements over which plants should be classed as ‘native’ and which as ‘non-native’. It’s widely suggested that plants should be classed as native if they were growing in Britain during the period following the last ice age and before Britain was separated from mainland Europe by the formation of the English Channel (about 8,000 years ago) but differentiating these species from those that were introduced by people in later periods can be far from straightforward. For some naturalists, because Britain was once part of a greater European land mass, it’s more relevant to expand the definition of ‘native’ to encompass plants indigenous to Western Europe as a whole.
Tucked away in a quiet corner of the sprawling grounds of RHS Garden Wisley in Surrey, the ground-breaking Plants for Bugs project is one of the first attempts to tackle the native versus non-native debate in a scientific way. According to Bostock, who manages the project, this data could prove crucial in helping gardeners to choose plants that will provide the best possible value for wildlife. While the research is focusing solely on invertebrates including bees, butterflies, spiders, ladybirds and beetles it is hoped that the findings will shed some light on the bigger picture. The more bugs plants attract, the more food will be available to other wildlife, including birds and mammals.
Since 2010, researchers have been monitoring 18 separate beds at Wisley and a further 18 on a separate site close to the gardens, each designed to replicate a miniature garden border with shrubs, grasses, ferns, bulbs, perennials and a climber. But, while they may all look the same, a third of the beds have been planted with natives; a third with plants originating in the northern hemisphere and the final third with plants that grow naturally in southern hemisphere countries such as South Africa or New Zealand. The number of invertebrates on the beds is counted at roughly six-week intervals between March and October each year. This vast amount of data is being analysed and some answers to the native/non-native conundrum should be published next year.
For now, the team at Wisley is reluctant to jump to any early conclusions. “Quite simply, we don’t know what the outcome will be,” Bostock says of the research. That said, however, the case for the value of non-native plants seems to be growing stronger. A spate of recent studies highlighting just how rich in wildlife urban and suburban gardens can be despite their heavy bias to non-native planting (the ‘average’ urban garden has 70% non-native and 30% native plants according to findings from the Biodiversity in Urban Gardens (BUGS) project at the University of Sheffield), is helping to support the view that some non-native plants can be good for native fauna. According to Dr Ken Thompson, one of the team behind the BUGS project, it has never really been the case that the majority of British ecologists were strongly against non-native plants anyway. Rather, he explains, “the minority who are against them are just very vocal”.
While gardener-turned-naturalist Marc Carlton is not one of these opponents and considers western European natives to be just as useful to our insects, he is more cautious about the value of exotic plants. He points to fuchsias as a good example of how these plants can favour only the most opportunistic insects, often at the expense of other species. “These plants are pollinated by hummingbirds in South America, where they originated. They are nectar-rich but only a few of our bee species can access this nectar: namely those bees which have jaws strong enough to bite through the corolla tube or honeybees which are small enough to crawl in and which are naturally adventurous. So, to a lay person, a fuchsia bush can look like a great plant for bees, when in fact 95% of our native bee species can’t use it.”
Whatever the eventual results of the Plants for Bugs project, the fact that this research is taking place at all, illustrates the more nuanced view that has emerged over recent years. While our wild habitats, such as wetlands, woodlands and grasslands are vital havens for all sorts of different wildlife and must be protected, they are not necessarily a blueprint for our gardens. “The common belief used to be that, if you wanted to ‘do your bit’ for wildlife, you should recreate natural habitats in your own back garden, by replicating for instance, an English wood or a wild pond,” says Bostock. “But this is not what gardening is about. A garden is completely artificial, but then that doesn’t mean to say it’s bad. In fact, we now know that tidy, well-ordered, and yes, exotic and colourful gardens can be teeming with all kinds of wildlife. Native needn’t always be best.”
Such a sentiment is echoed by Dr Thompson, who warns against gardeners getting too hung up on the idea of native plants and thereby missing the big picture. “Gardening for wildlife is about getting as many people enthused about gardens and wildlife as possible,” he says. “In fact, the biggest danger is people thinking that if they grow just a few native plants then the job is done!”