The question of what to grow to benefit wildlife is being answered. But whether that includes non-native plants remains open to debate, with parts of the international plant trade under threat from EU legislation.
Growers and garden retailers have been under pressure from environmentalists and the EU to produce and sell plants that are friendlier to invertebrates. The importance of bugs is that "the world's ecosystems would collapse" without them, according to naturalist Sir David Attenborough. Latest evidence shows gardeners can help to save bugs, but there are mixed messages as to how they can do it.
The European Parliament environment committee is considering a draft bill on banning certain invasives. Species "of Union concern" would be barred from being introduced, transported, placed on the market, offered, kept, grown or released in the environment.
Biosecurity is the issue but trade bodies including the HTA say a banned list is "potentially horrific" for the industry and could lead to UK horticulture companies going out of business.
The RHS says an "unlimited" list is a concern and wants an evidence-based, taxonomic approach. Last week, the RHS/Wildlife Gardening Forum reported after the four-year Plants for Bugs research project. The headline conclusion is that non-natives are just as valuable for invertebrates and pollination as natives.
RHS entomologist Andrew Salisbury agrees with the HTA that only a very small number of imported plant species are an issue. "The number of invasive plants that have become a problem seems to be quite small, such as Japanese knotweed. Most plants in the garden have not escaped," he says.
"Only a handful have been a problem. Buddleia only thrives on disturbed habitats and cotoneaster is probably similar, though we're not sure. It's a situational thing. Either plant on nature reserves is not good but in a garden can be good. Cotoneaster is more clear-cut. Buddleia is more of a problem. It's great in gardens but will establish where you don't want it."
Nevertheless, buddleia is "the obvious one" when it comes to a non-native that is good for invertebrates. Salisbury cites Jennifer Owen's 2010 Wildlife of a Garden: A Thirty-Year Study treatise on her own back garden, which found, for instance, that non-native plants are surprisingly good at attracting wildlife, spotting 18 species of moth on her buddleia bush.
Owen's 30-year study shows wildlife numbers in gardens have plummeted since 1972. The ecologist found hoverflies numbered 43,749 from 1972-86 and 16,987 from 1987-2001. Butterflies recorded were at 172 in 1973 but fell to 19 by 2001. Moth numbers dived from 622 to 132 over the same period and common wasps fell from 123 in 1972 to seven in 2001.
Gardeners are clearly keen to do their bit, as evidenced by the wildlife market, which is worth around £365m for wild bird food alone.
No simple picture
Plants for Bugs "isn't a simple picture initially", Salisbury adds. "Initial analysis shows native is not always the best when it comes to pollinators and that's the most important thing. Sometimes other plants in gardens can be just as good but I emphasise that's just in gardens and not outside them."
His big recommendation for growers is not to produce doubles, which are bad for pollinators. Salisbury's research project tested plant "communities", not individual plants, and he will publish peer-reviewed findings later this year, possibly in The Journal of Plant Ecology.
Of more immediate use is a study linked to Plants for Bugs published late last year by the University of Sussex, in which scientists counted the number of insects visiting the plants in their garden. Their results were published in Functional Ecology.
PhD student Mihail Garbuzov used 32 different varieties of popular garden plants. He found "great scope for making gardens and parks more insect-friendly" by selecting the right plants.
The study found that borage, lavender, marjoram and open-flower dahlia varieties were best for insects. Erysimum 'Bowles's Mauve' was good, geraniums less so.
Marjoram attracted honey bees, bumblebees, other bees, hover flies and butterflies. Borage was best for honey bees and lavender, particularly hybrids, and open-flowered dahlias were most attractive to bumblebees.
Pests - International plant trade blamed
Buglife chief executive Matt Shardlow asks: "What good has the international plant trade ever done us?"
He says pot plants, for instance, should not be imported because they have brought us harlequin ladybirds, oak processionary moth, New Zealand flatworm, Australian flatworm, lily beetle, rosemary leaf beetle, Spanish slug, girdled snail, Australian landhopper, light-brown apple moth, Asian hornet and Argentine ant.
A new risk is New Guinea flatworm, now in France and a threat to native snails, he warns.
Plants for Bugs
Good for bugs:
- Open-flower dahlias
- Double blooms
- Imported pot plants