Earthworms and Worm Casts in Turf
What are Worm Casts?
Do all earthworms produce casts?
Quite simply, the answer is no! The majority of our native earthworm species do not produce casts and are actually beneficial to soil recycling, drainage and aeration. The three main species of earthworm responsible for causing unsightly casts in the UK and Ireland are Lumbricus terrestris (lob or common worm), Aporrectodea calignosa (grey worm) and Aporrectodea longa (black-headed worm). The lob worm is the most commonly encountered species, and is usually most active at night, or during periods of prolonged surface moisture. On a positive note the presence of Earthworms is generally indicative of a healthy and balanced soil ecosystem!
When do worm casts occur?
Worm casts can occur at any time of the year but are most prevalent during prolonged cool wet periods of weather. The Earthworms digest a mixture of soil and organic matter and will generally excrete the arisings underground. However, when conditions are conducive, they are drawn upwards and deposit their excretia at the surface forming the cast.
There are limited methods for turfgrass growers to control the incidence of worm casts in their turf due to the loss of chemical control methods. It has therefore never been more difficult for growers and turf managers to control unsightly earthworm casting in production, and on existing and newly laid turf installations.
What problems do “casting” earthworms cause?
Earthworms themselves are generally not a problem, however their castings are. As well as looking unsightly the casts can bring undesirable weed seeds, from the seed bank below, to the surface and create the optimum conditions for them to germinate in the fresh soil generated with the cast.
Mowers, and in particular cylinder mowers can be badly damaged by the soil particles within the cast, and particularly on sports surfaces, the levels and performance can become badly affected.
What can I do to discourage earthworm activity and limit damage caused by casting?
Allow the casts to dry and then brush back into the surface before mowing to reduce any "smearing" effect, and avoid mowing your lawn in wet conditions whenever possible.
Collect and dispose of all grass clippings responsibly, by either composting or taking to your local recycling centre Green Waste point. The collection of the clippings will reduce one of the worms key food sources within your lawn.
The application of a specialised, abrasive, high sand content top dressing will help to create a drier surface and allow the casts to be broken down more easily.
Earthworms generally prefer a non-acidic soil, therefore applications of Ferrous Sulphate and other acidifying fertilisers can often help to discourage their activity. Always seek advice and follow manufacturers recommendations before any application.