I hAve been doing a bit of work in the garden recently, but how would you react if I told you I'd planted Japanese knotweed and some rhododendron and would be happy to see them spread into the wild; indeed, I'd encourage it?

Would you think me a criminal, or an enemy of Scotland perhaps, or of the natural world, or of the entire planet?

That might sound a bit over the top, but that's what the discussion of native and non-native species is like. Even though dividing plants into native and non-native is as fatuous as assuming a fir tree knows where the border between two countries is, millions of pounds is spent supporting futile attempts to control certain plants and animals. And in case any of us should doubt the point of the whole process, those who take part justify it all with the worst B-movie vocabulary: words like "invasion", "swarm" and "monster".

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Rhododendron ponticum is a good example. The plant has had all the usual pejorative terms thrown at it ("dirty", "invader" and even "evil") and James Fenton, an ecologist who has worked for the National Trust for Scotland, said recently that if we took no action to control it and came back in a thousand years, the landscape of Scotland would be just one dark rhododendron forest.

This kind of hyperbole is rarely questioned in the discussion of non-native species, even though it defies common sense and the history of the plant itself. Rhododendron ponticum can certainly do well in certain kinds of soil but it is no more or less likely to spread than so-called native plants like bracken, for example, or ivy.

And plants do not spread ad infinitum anyway - that's not how nature works. As the plant biologist Ken Thompson points out in his book Where Do Camels Belong?, Americans used to get hysterical about the purple loosestrife plant. It was accused of reducing diversity but the long-term evidence was different: there was a first wave, it declined, and diversity reasserted itself. With the exception of very small islands, that's what new species do: they increase diversity rather than reduce it.

The only reason we defy this logic and organisations such as the Forestry Commission and Sepa go on trying to control certain plants is because they have been labelled non-native, but what does that actually mean? Rhododendron ponticum was in this part of the world before the last Ice Age so how old does a plant have to be, exactly, before it is considered native?

The idea of native and non-native is also pretty much a modern idea. It was invented by the Victorian botanist Hewett Watson and has spread ever since, like a swarm in fact, even though no-one can agree on a definition of native. In the search for one, some of those working in invasion biology try to go back to a year zero but how far back do you go? And if you go too far back, the problem is you reach the point where the world was one great continent anyway and every plant and animal was free to roam where they pleased.

Other attempts at defining native and non-native rely on human intervention (in other words, if the plant or animal has been introduced by humans, it is non-native) but again this breaks down quickly. Many apparently native British plants, for example, were introduced by humans but there were introduced a long time ago, so that is deemed to be OK.

And even if we could agree on what is non-native, we cannot agree which non-natives should be controlled. There is a consensus that grey squirrels (introduced 120 years ago) should be culled, but there are no attempts to control the collared dove, which has only been in Scotland since the 1950s. Should I shoot the collared doves that come to my garden, or at least send them back to Asia, their "home"?

Despite all this confusion, the desire to keep things in their place goes on even though none of it will matter in thousands of years. By that point, plants and animals will have moved on; perhaps no-one will even remember that there ever was a Scotland, and that's the point. Instead of talking in terms of native and non-native, humans should be looking beyond their own lifetimes and their own borders. Species evolving in one spot, spreading and then dying back, is perfectly normal. Let's leave nature to get on with it.