Have you been seeing small coiled mounds of hardened earth on the surface of your lawn or garden? They are worm casts or worm castings: essentially, earthworm poo. In flower beds and vegetable gardens, they pretty much go unnoticed, but in lawns they can leave the soil uneven and can even make it slippery. On sports fields, they can sometimes cause injury to players.
Usually A Good Thing
Under normal circumstances, worm castings are useful to gardeners. Earthworms dig deep galleries (up to 12 feet/3.5 m deep!) and aerate the soil as they go. Their efforts allow water to penetrate more readily and excess water to drain away, bring air to the roots and loosen the soil so roots can grow better, giving nearby plants a boost. In addition, earthworms bring up minerals from the soil layers below back up to the surface: nitrogen, phosphorus, calcium, potassium magnesium, etc. Worm castings contain 2 to 11 times more minerals than the surrounding soil and, as such, are a kind of natural fertilizer. In addition, they are rich in beneficial bacteria that help soil particles clump together, further improving drainage.
Moreover, vermicompost or earthworm compost, made from earthworm dejections, is generally considered the best compost available. You can produce your own or buy it in many garden centers… but in your lawn and gardens, earthworms are producing it for free and with no effort on your part.
Logically, then, any gardener should therefore be happy to have worm castings on their property.
Not So Popular in Lawns
It’s usually only lawn owners who get upset about worm castings. Not only do they make the lawn a little bumpy, but when you cut the lawn very short, their presence is quite visible: ugly little brown spots in an otherwise perfect green carpet. So much so that many lawn owners seek ways of getting rid of castings, even if this means killing off all the earthworms. But that is never a good idea.
First, any product you apply to the soil to kill earthworms, such as chemical insecticides (earthworms are not insects, but are sensitive to most insecticides), will also poison the beneficial microbes and insects that live there. So by eliminating one problem, you’d be causing another, because the quality of your soil will take a major dive and soon the grass will become weaker and thinner.
Some Partial Solutions
If you can’t completely eliminate worm castings, there are ways of reducing their impact… without in any way harming the earthworms that produce them.
For example, earthworms produce fewer and smaller surface castings that break up more rapidly in light, airy soils and much more numerous and long-lasting ones when in heavy clay or compacted soils. That’s because in light soils where tunneling is easier, earthworms don’t have to move to the surface to deposit their dejections; they can do so as they dig. And lawns grow far better in light, airy soils. By improving the quality of the soil of your lawn through topdressing, you’ll have a more beautiful lawn with fewer visible worm castings: a win-win situation!
Also, earthworms tend to deposit more castings above ground when the soil is very moist, because then they have to come up to the surface to breathe … and while there, empty their intestines. So castings are much more numerous during rainy periods than dry ones. The use of irrigation systems, which often keep the soil very moist, greatly increases the number of castings. If you simply stop watering your lawn, even it that means it will go into summer dormancy, you can greatly reduce their number. And since lawns are fully capable of recuperating from drought, no harm is done to them.
Also, if you increase the height to which you mow your lawn (3 inches/7.5 cm is the generally recommended height), worm castings will be hidden from view… and out of sight, out of mind. However, many lawn lovers are still given to scalping their lawns, cutting them very short, to 2 inches (5 cm) or even less, then become upset when casting seem to appear everywhere. They are, in a sense, creating the problem they complain about. Mowing high and ignoring the castings is a perfectly legitimate practice… and gives you a healthier, greener lawn.
Just Spread Them Around
If worm casting bother you, there is a simple solution: get out and break them up with a lawn rake. This is what sports field superintendents do just before a game … except they use mechanical lawn rakes to do the job. By breaking up the castings and spreading them around, the lawn will benefit from the minerals the castings contain. Alternatively, you can also rake them up and add them to the soil of the nearby flower beds and vegetable gardens if that’s what you prefer.
Personally, worming casts in the lawn have never bothered me. In fact, they are welcome in the ecological lawn that I try to maintain. But for gardeners who see things differently, the tips above may be helpful.