What weeds tell you about your soil
Weeds might be seen as difficult to control or aesthetically unappealing – but they are also a clue to something much deeper. As well as shielding your soil from sun and winter rain, stabilising the ground and providing habitats and food for insects and birds – wild plants like these can be the key to an underground treasure map of your soil.
They can delineate an otherwise secretive soil network of nutrients, structure, pH – and potential deficiencies and problems - that lay out of sight. By paying attention to common garden weeds on the surface of your veg and flower beds, you can learn the best ways to nurture your soil and plan which plants and crops might like these conditions too.
So before you despair at your weed-rich soil, remember fertile soil will probably contain the widest range of weeds because you’re offering them all the good things that your crops will also like.
Indicating your soil type
Wild plants can be good indicators of soil texture i.e. the size of soil particles, and weeds will thrive on a particular consistency. While some, such as chickweed, adapt to looser, sandy soils, others with tolerate compacted soils or heavy clay.
Compaction, flooding and over digging can affect your soil structure and its ability to allow air, water and nutrients to move freely. When particles clump together, it’s only weeds with strong, deep tap roots such as dandelion or horsetail that can survive and tolerate these conditions. The roots of these plants can extend much further down to soak up distant moisture reserves.
Clay soils aren’t bad – in fact they can be high in nutrients – and you can nurture them further by applying lots of organic matter. Growing a green manure such as winter tares or mustard can also help break up compacted ground. They do this through their root networks but also improve structure when they’re dug in. Raised beds can help to improve drainage when flooding is a problem.
Pinpointing pH
Your soil pH influences the availability of certain nutrients. Ox-eye daises, plantain and nettles will thrive on acid soils (soils with a pH value of 6.5 or lower). While plants such as thistles and goosefoot prefer alkaline (above pH7.5).
The good news is that most vegetables will grow within a range of 6-7pH - and fruit at 6-6.5pH – but if you need to make some adjustments, you can add mushroom compost to create more alkaline conditions. Chalky soils will need plenty of well-rotted compost not just to restore deficiencies in nutrients, but also to improve soil structure.
Displaying nutrient levels
As with all plants, weeds will thrive in soils with different nutrient requirements. Clover fixes its own nitrogen so will grow better on lower fertility soils than other plants. Chickweed and nettles, on the other hand, thrive in a rich soil that is high in nitrogen.
Learn to read your weeds like a garden clairvoyant and you may find you tolerate them better and discover more about the wonderful nuances of soil in the process.
Common ‘indicator weeds’
Anton Rosenfeld rounds-up familiar garden weeds and what they may be trying to tell you. (It’s worth remembering that weeds don’t always behave by the rules, so soil testing and note-taking is important too!)
Horsetail = waterlogged
Spore-producing plants such as horsetail (Equisetum arvense) thrives in wet soils.
Creeping thistle = heavy clay
Creeping thistle (Cirsium arvense) regenerates both from seeds and small pieces of creeping root. These pieces of creeping root are more likely to survive on a heavier clay soil that maintains a moist layer around the root.
Pineapple weed = compacted soil
This fragrant plant (Matricaria discoidea) likes to grow in rough environments so you may find it on well-trodden paths and gateways. Other weeds you might find here include creeping buttercup (Ranunculus repens).
Perennial sow thistle = alkaline soil
These plants (Sonchus arvensis) thrive on a slightly alkaline soil and are easy to pull out with the tap root intact, as long you don’t let them get too large. Such soils will be around the right pH for vegetable growing.
Corn spurrey = nutrient poor
This wispy weed (Spergula arvensis) thrives on light soils. These soils are likely to be free draining and good for growing root crops. Another weed to look for is clover (Trifolum spp.). If there is a thick covering, when you are ready to clear the area, cut down and dig in the clover, rather than removing it, as this will add a good dose of nitrogen to the soil and improve its fertility. As clover is often sown as a green manure to improve the soil, think yourself lucky that it has arrived without having to sow it.
Nettles = rich in nutrients
If your ground has nettles (Urtica dioica), it’s a good indication you’re lucky enough to have a soil that is rich in nutrients. Brassicas and potatoes will grow well here, although remember that these plants take a lot of nutrients from the soil. These will have to be replaced by adding manure, compost or growing a green manure.
Head to our garden weed pages to learn more about different weeds and how to tackle them without pesticides.