When Is a Weed a Weed? And When a Weed Is Not
- Jun 7
- 5 min read
My mum and I have been having the same argument for about twenty years.
It goes like this. I say I love the yellow cosmos in our compound because those hot little suns add instant cheer wherever they can be seen. Or our nasturtiums, climbing wild over everything, huge weird-shaped peppery leaves and orange trumpets spilling out everywhere. I think they're glorious. She looks at them, and says, "That's a weed."
And every time I want to ask the same, irritated question: says who? So I finally looked it up properly, expecting to be proved right. I wasn't - well not 100%.
There is no such thing as a weed
This was the part that surprised me most. "Weed" is not a botanical category. It isn't a family, a genus, or any kind of scientific grouping. No botanist has ever discovered a weed, because there is nothing out there to find.
The definition that gets repeated most often, credited to a long line of gardeners and naturalists, is roughly this: a weed is a plant growing where someone doesn't want it. That's the whole thing. The definition lives entirely inside that word "someone." A plant becomes a weed the moment a person decides it is in the wrong place, and stops being one the moment they change their mind.
There is a pattern, though
Here is where I have to be fair to mum, though. The word may be subjective, but it isn't arbitrary. When people call a plant a weed, they are usually reacting to a real set of behaviours, even if they couldn't name them. Botanists who study these plants have a rough profile of what makes something "weedy," and it runs something like this:
They germinate fast and grow faster
They produce an absurd number of seeds, sometimes tens of thousands per plant.
Those seeds stay viable in the soil for years, waiting.
They are resilient; Poor soil, drought, being mown, being trodden on, they cope.
They spread on their own, by runner, by root fragment, by seeds that hitch a ride on fur or socks or wind.
A plant doesn't need the full set to earn the name, but the more boxes it ticks, the faster people reach for it. And that’s where my flowers get caught in the crosshairs; Cosmos self-seeds with real enthusiasm; leave the dead heads on and you'll have a fresh crop next year whether you planned one or not. Nasturtiums sprawl, climb, and reseed cheerfully. By this checklist, the plants I love do half qualify. So when my mum calls them weeds, I will begrudgingly agree that she isn't entirely wrong.
What’s in a name
But the word carries more than a description. Folded into it is contempt, and the contempt is old. People have scorned weeds for about as long as they have grown anything on purpose, and it shows up as early as Genesis, where unwanted plants are written into the curse on the ground:
cursed is the ground for thy sake; in sorrow shalt thou eat of it all the days of thy life; Thorns also and thistles shall it bring forth to thee.
Folklore took it further than scripture did. The poisonous weeds that came up in ditches and graveyards, henbane, nightshade, the thorn apple, were blamed on the devil and named after him, which is how datura came to be called devil's snare. These were the plants witches were said to use, and they turn up in old recipes for the "flying ointment" meant to let a witch fly. Feared as the devil's own, and used anyway.
But this negative connotation is often misplaced.
At a practical level, run down the long list of plants we file under "weed" and a surprising share of them turn out to be worth having. Much of the keerai tradition of Tamil Nadu rests on greens that arrive uninvited: ponnanganni, the creeping weed of wet ground that is also eaten for the eyes; manathakkali, a roadside nightshade cooked into kootu and used to calm the stomach; mullu keerai, the spiny amaranth that is a pest in one plot and, a continent away, the grain the Aztecs once held sacred. Purslane works its way through cracks and vegetable beds the world over and happens to be one of the richest plant sources of omega-3s, eaten from Mexico to the Mediterranean. Even the dandelion, sworn at on lawns, is brewed into medicine and wine.
Some weeds, given long enough, stop being weeds at all. The botanist Nikolai Vavilov, who spent his life tracing where our crops came from, found that rye and oats began their careers as weeds, intruders in the wheat and barley fields of the Near East. As farming climbed north into colder, poorer ground, the coddled wheat struggled where the weed did not, until somewhere along the way people stopped pulling it up and started harvesting it. Rye bread and porridge oats are, in other words, weeds that got promoted. The line between crop and weed doesn't only move with our opinions. Plants have walked across it themselves.
In hard times the line between weed and lifeline disappears altogether. When cotton ran short in the First World War, German uniforms were woven partly from nettle fibre. When coffee was blockaded or too dear, roasted chicory root, a common roadside weed, stood in for it, which is why chicory still flavours the coffee of New Orleans and parts of Europe. And through one famine after another, the plants that kept people alive were usually the uninvited ones, the greens already growing for free while the planted crops failed. I guess you could say that a weed, more often than we admit, is just a useful plant waiting for a harder year.
When the worry is real
Some plants, though, do cause real harm. That is a different thing from simply being unwanted, and it has its own name: invasive. The word is worth keeping well apart from "weed," because people muddle the two constantly.
An invasive plant is one introduced where it isn't native, which then spreads aggressively and does measurable ecological or economic harm, crowding out native species, draining water, choking what was there before. That is a claim you can actually test, and I don't have to look far for it. Wattle (Acacia mearnsii and Acacia dealbata) is one of the most aggressive invasive plants in Kodaikanal. Brought in by the British, these trees now blanket tens of thousands of hectares of the Palani Hills and smother the native shola grasslands. That has records, costs, and consequences. "Invasive" is a judgement about damage to an ecosystem, not about whether a gardener finds a plant untidy.
Most of what gets called a weed in a garden is nothing of the sort. The cosmos seeding itself along our compound wall isn't native either, but it feeds bees and harms nothing. The reverse can also be true, though. The wattle now smothering these hills began as a plant people chose and admired. So I'm not saying plant whatever you fancy. I'm saying watch what a plant does where you live, and judge it on that.
The last word
So who's right, me or my mum? Both of us. Neither of us. We were never really arguing about the plant. We were arguing about what a garden is for.
She wants one that looks deliberate, where everything was chosen and stays where it was put. To her, a flower that flings its seed into the gravel and comes up wherever it likes has broken the agreement. It is behaving like a weed, so it is one. I want a garden that feels a little alive and ungoverned, where things turn up on their own and surprise me.
The word "weed" was never going to settle that, because it was only ever a way of saying "this isn't what I wanted here." It tells you about the gardener, not the plant. So now when she points at the nasturtiums and says "that's a weed," I can just say "in your garden, maybe."
Comments