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IT ISN鈥橳 an exaggeration to say that in the world of horticulture 鈥渘ative鈥� is frequently used as a byword for 鈥渂etter鈥�. Native plants are often considered easier to grow and better for wildlife, while also being less invasive and more resistant to pests.

This belief is so institutionalised that many local planning rules in the UK specify that a certain percentage of landscaping schemes must include native species. Indeed, this conviction runs so deep that some see sharing evidence to the contrary as being hugely controversial, even deeply irresponsible. But accuracy is what matters, so let鈥檚 explore how well this entrenched dogma stands up to analysis.

First, it is important to clarify that, in many cases, native plants are great choices for a garden. What I am examining here is whether they are automatically a superior option for both garden performance and ecological value, in the context of Britain.

The problem with considering a group of plants to be inherently superior is that many measures of 鈥渂etter鈥� are contradictory. For example, one of the key features that makes a plant invasive is it being so easy to grow that it overwhelms efforts to manage its spread, resulting in its escape into natural ecosystems where it can cause havoc. So the idea that native plants are both less invasive and easier to grow can only be maintained if you are very selective with your evidence.

You also have to ignore, for instance, that many native plants, such as bracken, are so invasive that they can swamp huge areas of land, with catastrophic effects on local biodiversity. So if we are concerned about biodiversity, we should also be concerned about invasive native species. Unless, in reality, we are worried only about the 鈥渁lien鈥� part of 鈥渁lien invasives鈥�, not the invasive potential or impact on ecosystems of all plants.

Likewise, the popular claim that non-native plants are far worse at supporting local wildlife, while also being less pest-resistant, requires doublethink. This is largely because the difference between pests and wildlife is cultural. Undoubtedly, the most important way that plants support wildlife is as a food source, but if an animal munches on them in an unaesthetic way, we label it a pest.

This touches on a tricky reality. What does native even mean in the context of Britain? The island has been subject to waves of ecological annexation by giant ice sheets in a series of glacial periods, interspersed with successive waves of colonisation by species from further afield. As such, they cannot be realistically compared with highly specialised ecosystems like, say, those of Madagascar or the Galapagos.

鈥淭his definition makes anything introduced by the Romans, such as olives, native to Britain鈥�

To address this issue, many botanists and ecologists consider only species that we know were in Britain at the end of the last glacial period as worthy of the title native. But this in itself is pretty arbitrary. What is the exact date we are choosing for the glacial period to have finished, considering this process took millennia?

As one solution, others have picked an equally arbitrary cutoff for native plants: they must have been in Britain 500 years ago, based on the idea that most trade in plants occurred after then. This makes anything introduced by the Romans, such as olives and pomegranates, native to Britain.

It is understandable if you think these examples are silly because such species require human-managed cultivation to survive in Britain, but then you must also accept that many of Britain鈥檚 most-loved native meadowland and cornfield plants, such as poppies and cornflowers, are technically also exotic species, introduced by ancient humans and the agricultural methods they brought with them post-glaciation.

The arbitrary nature of the definition of native plants isn鈥檛 just temporal, but also geographic. I could take delicate mosses and ferns from the remnant patches of temperate rainforest in the far south-west of England and plant them in south-east England, in areas with a similar rainfall to Rome or Jerusalem, and still claim them to be native and thus inherently better suited to the environment than Mediterranean plants. This once again shows that definitions of nativeness are really just arbitrary lines drawn on maps and dates picked on calendars.

Using this framework, it is frequently claimed that exotic plants from areas like southern Europe are automatically worse at supporting native British wildlife, despite many animals that are native to Britain also being native to vast swathes of the planet, as far east as Siberia and as far south as northern Africa. Just because they aren鈥檛 native to British people, doesn鈥檛 mean they aren鈥檛 native to Britain鈥檚 animal species.

So while these definitions of native can be useful pointers, we should really consider them in a more nuanced context.