If you want to help wildlife in your garden, should you plant native plants? It's one of those long-standing questions that is an important part of wildlife-friendly gardening. Some would say, "Of course you need to grow native plants. It's obvious!". Others would say, "But some non-native plants are great for wildlife, and some native plants are not".
Add into that the fact that some native plants just don't do very well in the rich soil of gardens, that many of them aren't very pretty, or can be rather too vigorous, plus the arguments about what is native and what isn't, and it gets rather confused and confusing.
So where is the science to get us to a better answer?
Well, bits and pieces have been done over recent years, but this week the RHS released the second set of results from the 'Plants for Bugs' study. It compared a limited suite of 'native' plants (naturally found in the British Isles) with 'near-native' plants (from the Northern Hemisphere) and 'exotic' plants from the Southern Hemisphere, all grown under scientific conditions in trial plots at RHS Wisley and a second site nearby.
The study involved a giant vacuum cleaner that sucked up 22,700 creatures from the plots over a four-year period!
The first set of results, released last year and looking at pollinating insects, was fascinating. Of those plants grown, 'native' and 'near-native' plants were pretty much as good as each other, whereas 'exotic' plants were less good...except that they turned out to be good at the tail end of summer when they helped fill a gap.
This week's second set of results looked at what were termed 'plant-inhabiting creatures'. In other words, all sorts of little creatures that live above ground and use plants but not for their pollen or nectar.
Even the proportions of the different creatures caught was a revelation. Almost half were springtails, those tiny invertebrates almost invisible to the naked eye which can indeed catapult into the air to escape. 12% were 'sucking herbivores', which are typically true bugs such as aphids. And 5% were parasitic wasps and 5% web-spinning spiders.
So which plants were better for wildlife? Well, when adjusted to compare equivalent amounts of cover, native plants supported about 10% more invertebrates than near-natives and 20% more than exotics. (When I say 'more invertebrates', I mean 'more headcount' rather than 'more species'; this study looked at what is called 'abundance', not 'diversity'.)
The survey also showed something else: the greater the density of plants, of whatever type, the more invertebrates there were.
So what should we make of these results? Should we, as a national newspaper reported this week, "Ditch the fuchsia to boost British wildlife"?
It is an understandable knee-jerk soundbite, but I think we need to be careful not to jump to instant conclusions.
After all, if the suggestion of this is that native invertebrates prefer native plants because they have evolved to eat them, then it is difficult to see how growing Spiked Speedwell in Surrey will lure in insects that feed on what may be a 'native' plant but is actually very rare plant from East Anglia.
Conversely, when I think of 'near native' plants, I don't think of plants from Mexico or Japan, which were included in the study. My idea of 'near-native' plants are those from western Europe. Almost all of Britain's wildlife also lives in continental Europe, so maybe growing some of the gorgeous plants from the Alps or France would be perfectly acceptable to 'our' wildlife? Maybe the 33% of 'near-native' plants in the study that came from eastern Asia and central and north America is enough to account for the small difference in the results? We don't know.
Also, we need to bear in mind that successful wildlife-friendly gardening isn't just about abundance. Sometimes it is about encouraging a diverse range of species; sometimes it is about focusing your efforts on helping a rare species to thrive. If abundance is all we're after, then we might as well just grow a bed of Broad Beans - mine are host to about a million blackfly this year, by my reckoning!
In fact, some gardening for wildlife is about having fewer plants! For example, there are plenty of insects that need bare soil, and we know that House Sparrows prefer feeding where plants grow thinly.
But make no mistake, this study is a good one, and takes our knowledge a step forward, and all credit to the Royal Horticultural Society, supported by the Wildlife Gardening Forum, for doing it.
So, informed by these results, here are my headlines, in simple terms:
And here's to the next important piece research to help refine our ideas even further!