Guest post by Sarah Gerhardt

Weeds have a bad reputation. As gardeners, we are often taught to see them as plants that need to be removed as quickly as possible. We hoe them, dig them out, poison them, cover them up, and generally try to prevent them from appearing in the first place. But the closer I look at weeds, the more I realise that the plants we call weeds are often doing something useful.

They can tell us about the condition of our soil, protect disturbed ground, provide food and shelter for wildlife and sometimes even improve the conditions for other plants. Not every plant we call a weed is beneficial in every context. Some can cause real problems. The relationships between plants, humans and the wider environment are more complex than simply for the gardener to decide whether a plant is “good” or “bad”.

Before deciding what to do with a weed, it is worth asking a more basic question: what actually makes a plant a weed?

In the first part of Rethinking Weeds, I look at where the idea of a weed comes from, what weeds can tell us about the conditions in our gardens and the ecological role they play. In the second part, I will look at how we can live with weeds in a practical and thoughtful way.

What Makes a Plant a Weed?

“Weed” is not a biological category based on shared characteristics. It is simply a plant growing where humans do not want it. The same plant can be a weed in one place and desirable in another; it can be unwanted to one person and valued by another. 

The idea of a “weed” is therefore based on human judgement and reflects how we tend to measure the world by its usefulness to ourselves. We could also say that weeds tend to be plants that are particularly successful in the environments we create. We usually notice them because they grow quickly and reproduce effectively, competing with plants we have deliberately chosen to grow.

The language we use around plants reflects this judgement. We talk about weeds “invading” gardens, “taking over” borders or “colonising” disturbed ground, and about “controlling” or “eradicating” them. These words create a story in which plants become enemies and humans become the people responsible for restoring order. They make it easy to forget that these plants are simply responding to the conditions around them.

This does not mean that we need to pretend that plants cannot cause serious ecological and practical problems. However, it is worth being aware of the language we use. Terms such as “invasive”, “alien”, “colonising” and “enemy” can carry strong associations that shape how we think about plants and their place in the landscape. “Invasive species” is widely used in science and policy, but even its definition and use remain debated. Language does not simply describe the plant world; it also influences how we see it.

Many plants considered weeds share certain characteristics that make them particularly good at taking advantage of disturbed ground and changing conditions.

Many “weeds” are ruderal, meaning they are the first to grow in an area that has undergone change or disruption, for example, after a forest fire or flood. They use the conditions created by this change to grow and reproduce quickly, before larger plants establish and eventually crowd them out. The term ruderal comes from the Latin rudus, meaning rubble. Berlin ecologists coined the term after the Second World War, referring to plants that spontaneously grew on the city’s ruined grounds.

Often these ruderal plants have a lifespan of only one or two years. As a result, they put a lot of their energy into seed production. For example, a single wild plantain (Plantago sp.) can produce up to 20,000 seeds, and a single dandelion flowerhead contains around 180 seeds, sometimes up to 400. Their seeds have evolved to travel easily and establish in new areas. 

Plants we consider weeds are often adapted to low-fertility soils, grow quickly, and reproduce prolifically. Their ability to spread and persist is one reason they can be frustrating in gardens, but it is also why they are so effective at responding to disturbed environments.

What Weeds Can Tell Us About Our Soil

No single plant can give us a complete diagnosis of our soil. Weeds are influenced by many factors, including climate, disturbance, light, and the availability of seeds. However, certain plants are associated with particular soil conditions, and looking at which weeds repeatedly appear in an area can give us useful clues. 

For example, weeds with deep taproots, such as dandelion or burdock, can thrive in compacted soil where more delicate, less specialised root systems struggle. Their thick vertical roots can exert enough pressure to pierce through dense layers of earth where more delicate, less specialised root systems fail.

These plants can not only grow in such conditions but may also improve them. By pushing through the soil with their taproots, they break the dense clay that stops other plants from growing, and they move moisture and minerals below the compacted zone. The growing taproots create physical channels in the hard soil and thereby aerate it naturally. When the dandelion and burdock die, their decaying roots leave a pathway for air and water to enter the subsoil. In this way, their presence can improve the conditions for plants that come after them and would otherwise struggle to grow in the compacted soil.

Compaction can also contribute to waterlogging. When soil is dense and poorly structured, rainwater may struggle to drain away, creating a wet surface layer. This is an environment that suits plants such as ferns, mosses, redshank (Persicaria maculosa) and creeping buttercup (Ranunculus repens). Creeping buttercup has a shallow root system and spreads sideways using creeping runners. Instead of fighting compaction, it moves over the top of the hard, wet surface like a blanket. 

While creeping buttercup and redshank can be signs of a wet surface layer, the presence of horsetail (Equisetum sp.) can indicate waterlogging deeper in the soil profile. Horsetail is an ancient fern-like weed that loves wet, swampy, or waterlogged soil. Its deep roots pull up silicon from deep down in the soil to build its tough stems. When the plant dies, it returns the silicon to the topsoil, which helps to improve the overall strength and structure of the soil.

A lawn that struggles to grow while clover thrives may be low in nitrogen and other nutrients. Clover can grow in soils where lawn grasses struggle and can actually improve the conditions for the surrounding grass. It converts nitrogen from the air and fixes it in the soil, where it acts like a slow-release fertiliser for the surrounding grass plants. In addition, clover is much better equipped to survive dry spells because it has deeper roots than lawn grass and can remain green while the grass around it turns yellow. It is low-growing, can outcompete other lawn weeds, and can tolerate dog urine. There is a lot of value in allowing clover in a lawn, especially in difficult spots. The RHS has more info on clover in lawns.

The presence of stinging nettle, fat hen (Chenopodium sp.), chickweed (Stellaria media) or common groundsel (Senecio vulgaris) can suggest a soil high in nitrogen and organic matter. While groups of stinging nettles prefer undisturbed areas like compost heaps and rich woodland edges, fat hen, chickweed and groundsel pop up quickly in freshly dug garden beds. A thick carpet of chickweed also suggests that the soil is moist and has a balanced pH, though its shallow roots mean it can happily carpet compacted surfaces, too. All of these plants pull up large amounts of nutrients such as nitrogen, iron, calcium and magnesium. When they die and decompose, they act like a natural fertiliser, returning rich minerals to the topsoil for surrounding plants.

Ultimately, the presence of plants such as stinging nettle or groundsel can be a useful sign for the gardener. It may indicate fertile ground that suits hungry vegetable crops, roses, and many lush ornamentals. However, nutrient-rich soil is not ideal for everything. Plants that prefer poorer soil, such as native wildflowers or Mediterranean shrubs like lavender, may struggle.

The Ecological Value of Weeds

The plants we call weeds are crucial to the health of the ecosystem. Weeds do a great deal to protect soil disturbed by events such as floods, fire, landslides, or direct human intervention like clean tillage. 

Bare soil is vulnerable. It has been stripped of its normal supply of organic compounds from living plant roots, causing soil life to go hungry. In addition, the exposed soil surface is at risk of erosion by rain or wind, especially if root systems inside the soil have been removed as well.

They are adapted to grow in these harsh conditions and establish quickly in the exposed soil, cover it, and restore it.

We can think of exposed soil as an open wound that nature tries to heal. The plants that rush in to cover it are often ruderals that we call weeds.

Their roots help hold the soil in place against wind and rain, while their foliage casts shade that lowers soil temperature and helps conserve water. 

They can absorb and recycle soluble nutrients that might otherwise leach away and capture carbon from the atmosphere. When they die and decompose, they build up organic matter and provide food for microbes and other soil organisms.

And they provide a habitat for insects and other animals. Many weeds, such as dandelion and deadnettle (Lamium sp.), provide early-season nectar and pollen for bees and other beneficial insects when little else is in flower.

When a Plant Becomes a Genuine Problem

Some plants that we call weeds can overwhelm smaller or slower-growing species, damage structures, interfere with food production, or create risk for humans and other animals. Recently introduced species can sometimes spread particularly successfully in new environments where the ecological relationships that limited them in their native range are absent. Japanese knotweed and giant hogweed are well-known examples.

Even when a plant causes genuine problems, it is worth remembering that the plant itself is not acting with intent. It is simply responding to the conditions available to it. 

Conclusion

As we have seen, the term “weed” is largely a human value judgement. Human activities can turn plants into weeds by disturbing the soil and creating ecological niches for ruderals to grow. Environmental events such as floods or fires, many of which result from human activity, can have the same effect.

Weeds are often a symptom of certain soil conditions or issues, and the presence of certain weeds can give us clues to what is happening in the soil. Several of them can improve the conditions around them.

By shifting our perspective, we can see these resilient plants not simply as enemies to be eradicated, but as ecological indicators, soil healers, environment improvers, and wildlife lifelines.

Perhaps the question should not be “How do I get rid of weeds?” but “Which plants I see as weeds are genuinely causing problems here and which could be understood, managed, or valued in other ways?” 

In part 2, I will turn to the practical side of the question: which plants to leave, which to manage, and when intervention is genuinely necessary.

About the author: Sarah Gerhardt is a gardener, linguist and punk musician based in Edinburgh. She was head gardener at the Dean Gardens, Edinburgh for 9 years and runs her own gardening business Gerhardt’s Garden Service. Find out more via her Linktree: https://linktr.ee/gerhardtsgardenservice