Do weeds really love poor soil? Not if you look at the science

One of the most common claims in traditional gardening lore is that weeds only grow in poor soil. Improve the fertility of your plot, and the weeds will disappear. If true, this simple trick could save gardeners hours of work, just by adding plant food. That’s an attractive offer – so let’s take a look at the evidence.

First, what do we actually mean by the word “weed”? Rather than a closely related group of plants, “weed” is really a catch-all term for any species growing where humans don’t want it. If this sounds arbitrary or culturally constructed to you, that’s because that’s exactly what it is.

Many notorious weeds in some contexts live secret double lives as prized plants in others. Take the dandelion. It is easily the most common species on weedkiller packaging in the UK. Yet in Singapore, where dandelions are considered exotic, I’ve seen seeds selling for nearly $100 on online auctions.

In fact, all five of the most invasive plant species identified in a recent global study were originally introduced as garden plants. This blurs the neat distinction between “weed” and ornamental plant, arguably to the point where the word itself begins to lose much of its meaning.

What many commonly cited weeds do have in common is a vigorous growth habit. Their ability to establish quickly, reproduce readily and tolerate a wide range of conditions is precisely what allows them to appear where they aren’t wanted. These traits often make them the first plants to colonise disturbed ground or neglected soil – places where other species struggle to gain a foothold. But tolerating poor conditions isn’t the same as preferring them.

In fact, many classic weeds actively favour rich soils. Stinging nettles, for instance, are strongly associated with nutrient-dense ground. Dandelions also thrive where nitrogen is abundant, not where fertility is low.

So where did the idea that weeds signal infertility come from? Like many gardening myths, this contains a kernel of truth. Improving soil fertility can allow a wider range of plants to grow, reducing the competitive edge of the most resilient pioneer species. This effect was demonstrated in the farmlands of 20th-century Europe, when synthetic fertiliser use increased. Vigorous grasses were finally able to outcompete troublesome weeds such as cornflowers and poppies – to the point where some of these species are now on the brink of extinction in the UK. The great irony is that these exact same plants are now highly fashionable wildflowers.

So where does this leave us? With our views about plants constantly shifting, it seems fair to say that weeds aren’t so much reliable indicators of soil quality as they are reflections of human whims and preferences.

These articles are posted each week at
newscientist.com/maker

James Wong is a botanist and science writer, with a particular interest in food crops, conservation and the environment. Trained at the Royal Botanic Gardens, Kew, in London, he shares his tiny flat with more than 500 houseplants. You can follow him on X and Instagram @botanygeek