Earthworms: The Garden Ally Already Underground, and How to Keep It

Underneath every lawn and border in a Scottish garden, earthworms are already doing structural work most gardeners never see. They burrow, and those burrows become permanent channels that let air, water and plant roots move through soil that would otherwise compact and waterlog. They eat their way through dead leaves, spent compost and decaying root matter, and what comes out the other end, a worm cast, carries several times the nitrogen and phosphorus of the soil the worm started with, along with a dense population of the bacteria that keep nutrients cycling.

Darwin, working from British earthworm counts, calculated that they could turn over enough soil in a year to bury a football pitch under 15 tonnes of castings, the same casting and burrowing still going on, at whatever scale, in an ordinary garden bed right now. None of this requires a gardener to do anything: earthworms are not something to be introduced. They are already there. The question worth asking is not how to get them, but how not to lose them.

What earthworms are actually doing underground

Three things, mainly: burrowing, casting, and breaking down organic matter, and the mix of species already present in a garden determines how much of each is happening.

Burrowing creates physical channels through the soil profile. In compacted or heavy clay ground, those channels are often the only route water has to drain away from root zones, and the only route roots have to reach past the topsoil. Soil without a working earthworm population can be dramatically less able to absorb water, which shows up above ground as pooling, waterlogging and increased runoff after rain, a bigger practical concern on Scotland's generally wetter, slower-draining ground than in drier parts of the UK, where those drainage channels are doing more work.

Casting is the visible byproduct of feeding: worms process organic matter and mineral soil through their gut and excrete it as a cast, either underground or on the surface. Cast material is enriched relative to the surrounding soil, with markedly higher nitrogen, phosphorus and microbial content than the soil the worm ingested, a natural, slow-release fertilising effect that costs the gardener nothing.

Organic matter breakdown is the least visible but arguably the most valuable job. Worms pull fallen leaves, spent mulchA layer of material spread over the soil around plants to suppress weeds, retain moisture, and improve soil structure as it breaks down. Full definition → and decaying plant debris into the soil, physically incorporating it rather than leaving it to slowly rot on the surface. That incorporation is what converts a mulch layer into actual soil structure over a season or two, rather than a layer that just sits on top.

Not all earthworms do the same job

Britain has roughly 30 earthworm species, of which around 16 are likely to turn up in an average garden, they vary considerably in size, colouring and where in the soil profile they operate. Farm advisers group them into three broad ecological types by behaviour, and a garden with a mix of all three gets the fullest range of soil benefits rather than just one.

Anecic worms build a midden, a small pile of leaf fragments, stones and debris, at the burrow entrance, thought to protect the opening from drying out and predation. If a lawn shows small, tidy mounds of debris rather than smooth grass, that's often a midden rather than damage.

Worm casts on a lawn are a separate, narrower question, covered in full in the piece on worm casts in lawns, including why they're a sign of healthy soil rather than a problem to fix.

How to tell if a garden's earthworm population is healthy

The standard field check is a spade test done in spring or autumn, when soil is moist and reasonably warm rather than baked dry or frozen: dig out a block of soil roughly a spade's width and 10cm deep, break it apart on a tray or sheet, and count the worms found in it by hand. A healthy population typically shows 5 to 10 earthworms in that sample. Fewer than that, especially combined with compacted, poorly draining, or visibly lifeless soil, points to a population under stress rather than one that's simply small by nature.

A few things to look for alongside the count:

• Mixed sizes and types — a population made up only of small, pale, fast-moving worms (more typical of compost-type species) without any larger, sluggish, deep-dwelling individuals suggests the anecic group is missing, which usually means drainage and deep-soil structure aren't getting the benefit they should.
• Visible casts on the surface, especially on lawns in autumn and spring, these are a sign of deep-burrowing worms actively working, not a symptom of anything wrong.
• Middens at burrow entrances in beds or lawn edges, again a sign anecic worms are present and active.
• Bird and animal activity — blackbirds and other ground-feeding birds working a lawn or bed are often following earthworm activity near the surface, since worms are a major food source for birds as well as badgers. Molehills are the same signal underground: moles feed heavily on earthworms, and a mole tunnelling through a lawn or border is a reliable (if unwelcome) sign there's a population worth eating down there.

What harms earthworm populations

How to actively encourage them

• Add organic matter regularly, as compost, well-rotted manure or leafmould rather than as synthetic fertiliser. Organic feeding has been shown to increase earthworm numbers where inorganic fertiliser alone does not.
• Adopt a no-dig approach where practical. Rather than digging organic matter into soil, apply it as a mulch layer on top, 15-20cm for a new bed, topped up with roughly 5cm annually thereafter, and let soil organisms, worms included, work it in themselves. This avoids destroying burrow systems and lets soil develop the fine, stable pore structure that supports both drainage and worm activity, which digging repeatedly undoes.
• Avoid pesticides and go easy on synthetic fertilisers, particularly anything applied directly to soil rather than foliage.
• Keep soil covered rather than bare. Mulch retains the moisture earthworms need and gives surface-dwelling species material to feed on. Leaving fallen leaves and grass clippings on beds over winter, rather than clearing them away, does the same job through the coldest months and gives epigeic species a steady feed exactly when they need it most.
• Reduce hard surfacing where possible, paving over soil removes habitat outright and cuts off the organic matter supply that keeps a population fed.
• Diversify planting. Gardens with a mix of habitat, vegetable beds, shrubs, lawn, support a broader range of earthworm species than a single uniform area, since different ecological groups favour different conditions.

A new raised bed or container garden starts with little or no earthworm population of its own, since there's no surrounding soil for worms to colonise from. Filling it with good-quality compost or topsoil and mulching consistently is what draws worms in over a season or two, without needing to buy and add worms directly.

Composting itself is worth a separate mention: the surface-dwelling species used in wormeries, tiger worms and Dendrobaena veneta, are a different, faster-breeding group from the ones doing structural work in open soil, and a working wormery is one of the most direct ways to turn kitchen and garden waste into the organic matter that then goes back onto beds as mulch or compost.

Does Scotland's climate make a difference?

Places further west and north in the UK, which covers most of Scotland, tend to be wetter and cooler than the east and south, and earthworm activity is genuinely constrained by both extremes of moisture (waterlogging) and by freezing temperatures, which push worms into dormancy. Clay-heavy and loam soils retain moisture and organic matter better than sandy ground, so they generally support higher earthworm numbers, relevant wherever a garden sits on heavier ground, which is common but not universal across Scotland.

The New Zealand flatworm is worth calling out on its own rather than as a footnote: unlike most of what affects garden earthworms, it's a threat considerably more established in Scotland and Northern Ireland than in most of England, arriving originally in imported plant pots and now found in gardens across the country. Where a previously healthy bed's earthworm count crashes with no other obvious cause, checking the soil for flatworms specifically, not just assuming poor conditions, is worth doing before reaching for any other fix.

The species present in Scottish gardens are the same three ecological groups, and broadly the same common species, found across Britain. The practical advice, organic matter, no-dig, avoiding chemicals, watching drainage, doesn't change north of the border. What does change locally is timing: soil that's slow to warm and quick to waterlog in a wet spring pushes worm activity later into a shorter active season than a gardener further south might expect, without changing which worms are there once conditions suit them.

Common questions

Curious about other wildlife your garden might be overlooking? See garden spiders, dragonflies and damselflies and lacewings.

Sources