Earthworms, and their casts, are a problem in some lawns particularly those mowed short or on turf surfaces used for sport. They can cause an unsightly mess on ​the surface as well as bringing up weed seeds from below the soil to cause decline in the quality of the lawn. 

Many products historically used to control worms in lawns are no longer available, but most lawn care professionals should be able to offer sound advice or fairly effective treatments on how best to deal with these unwanted guests.  To a large extent worm activity in the soil is a good thing as it acts as natural aeration, but the casts can cause problems for mowing, unevenness and weed germination.

Worm casts in lawn

A few Worm facts

There are 28 species of earthworm native to the UK. Each species exhibits unique behavioral and physiological characteristics, their distribution being influenced by soil pH, soil texture, organic matter, mode of feeding, lifestyle and their location within the soil profile.

Maybe surprisingly, only a small number of species produce the casts which are such a nuisance. The most abundant of the casting species is Lumbricus terrestris, which is also the UK’s largest species at 90-170mm in length. This earthworm is nocturnal and forms a permanent vertical burrow opening onto the soil from which it forages for food.

Although all earthworms are hermaphrodites (they have both male and female reproductive organs), not all can reproduce asexually and most, including L. terrestris, must find a mate with which to exchange sperm cells. Mating usually takes place on the surface at night when temperatures are lower, humidity levels are higher, and birds are not about. Frequent mating takes place throughout the year, but inactive periods are brought about by unsuitable environmental conditions – for example the soil is too dry during the summer or too cold during the winter.

The advantages and disadvantages of earthworms

Advantages

Their most important role is in the decomposition process within the soil which increases nutrient availability and soil fertility, but other benefits include increased infiltration rates, the modification (improvement) of soil structure and soil aeration, thereby leading to improved rooting of plants.

A healthy soil houses 500 kg of worms per acre (20 kg per ave 150 m2 lawn) Which can process 40 tonnes of enriched soil per year, 5 times as rich as available N, 7 times as rich as available P, 11 times as rich as exchangeable Potassium, Twice as rich as exchangeable Potassium.

Disadvantages

Earthworms may encourage other pests, as they are a rich food source for creatures such as moles.

Worm cast can be messy and will create potential weed pockets where casts are formed.​

Cultural control

The presence of casting worms should be minimised by cultural control methods, such as brushing with a besom/broom on a dry day to disperse the cast.

Worms apparently don’t like acid conditions and much prefer neutral or alkaline soils. So, if your lawn is on an alkaline (high pH soil), you could acidify it by applying Sulphur to the lawn. Modifying soil pH to create more acid conditions has become one of the more effective lines of control. Sulphur and Sulphur-based products oxidized in moist soil to form sulphuric acid, which lowers the pH. Additional benefits of Sulphur can include flocculation of the fine clay particles into larger ones, increasing pore spaces and improving soil structure and drainage.

To help acidify turf the regular use of ammonium sulphate based fertilisers is recommended, especially as research has shown acidifying fertilisers have the added benefit of favouring the development of the finer grasses. Similarly, applications of iron sulphate will have a comparable acidifying effect and, in this instance, will also help control moss and harden the sward against disease.

Similarly, irrigation water should be tested for pH, alkalinity and salinity on a routine basis to ensure that it will not influence the alkalinity of the soil.

Many pesticides historically used to control worms in turf, such as Carbendazim, are now banned but there are various organic products that are proving to be effective. Contact your local lawn care professional for up to date information and advice.

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