UK Edition
When is a weed not a weed?
Have a look on the internet for definitions of a “weed” and you will quickly discover numerous variations on “a plant in the wrong place”. Ask any gardener the same question and it’s odds on you’ll get the same reply. But does that help you to understand weeds? Does it even make sense? Let’s find out by looking at a couple of examples.
A while ago I wrote about the surprising appearance in my garden of a twayblade, one of our commoner but less conspicuous native orchids (“A surprise visitor at the back door”, Saturday, February 4).
To my surprise, twayblade is listed in the European Garden Flora, but surely only for the sake of completeness. The RHS Plant Finder even lists one place you can actually buy it, but only from a specialist wildflower supplier, along with other plants that no one in their right mind would grow in a garden, such as dog’s mercury and enchanter’s nightshade (nice name, horrible plant).
In fact, if you were to set out to grow British native plants in your garden ranked in order of some measure of garden-worthiness, you would be a long way down the list before you came to twayblade. Nor have I ever seen twayblade in cultivation, so I’m going to stick my neck out and say it’s definitely not a plant you would expect to find in a garden.
Which makes it, in my garden, a plant in the wrong place. But is it a weed? I don’t think so. It lacks the rapid growth, the territorial ambitions, the annoying air of entitlement, the determined persistence in the face of persecution, in short, any of the things I normally associate with a weed.
My second example is Papaver rhoeas, the common red cornfield poppy. A plant that is, without a shadow of a doubt, a weed, in fact almost the textbook example of a weed. The common poppy is one of a group of obligate arable weeds, plants that are so well adapted to cereal fields that they appear not to have a “natural” habitat at all.
Because such plants are confined to arable fields in northern Europe, botanists usually assume their original, “wild” habitat must lie in the eastern Mediterranean area or south-west Asia, which is undoubtedly where they came from. But even there, many are not found outside arable fields. They may simply have been adapting to farmed habitats for so long that they can no longer live anywhere else.
It therefore follows that as far as the common poppy is concerned, if cereal fields are “the wrong place”, we have a plant that grows only in the wrong place, because it doesn’t grow anywhere else. Far better, I think, to go along with reality and accept that for the common poppy, arable fields are where it belongs.
So there you have it. On the one hand, a plant that has turned up in the wrong place, but can’t possibly be described as a weed. And on the other hand, a plant that couldn’t be weedier if it tried, yet is so perfectly adapted to its one and only, weedy, cereal-field habitat that that must logically be the right place.
Which all tends to suggest that “a plant in the wrong place” is as vacuous and content-free as I always suspected. The truth is that weeds are too numerous, too diverse and simply too interesting for anything useful about them to be conveyed by a six-word soundbite.
There’s a lesson for us all here, one that goes well beyond gardening. Whenever you find your views on any topic seem to be perfectly captured by some glib phrase, it’s always worth checking that the phrase really means what you think it does, or even if it means anything at all.
Ken Thompson is a plant biologist with a keen interest in the science of gardening. He writes and lectures extensively and has written five gardening books. His most recent is Where Do Camels Belong? The Story and Science of Invasive Species