Pest or Pal?

This week’s question is on an issue that I’m in two minds about. Lorne Bowles from Teddington asks:

‘On damp days in Autumn (and Spring) my grass becomes covered with earthworm casts. I’d like to get rid of them somehow. What do you recommend?’

There are many species of earth worms but only 3 of them make casts. Worm casts are a sign that you have a fairly active soil with good aeration and humus content. Earth worms are useful for mixing and aerating soil, but those which cast can create a muddy and uneven surface on grass and can also encourage weeds, as their casts make excellent seed beds!

Charles Darwin spent a lifetime studying worms, and estimated that up to 40 tons of worm casts per acre can be added to the soil (representing between 45 and 170 worms per square metre!). These casts are invariably richer, finer and less acidic than the surrounding soil, and contain around 50 per cent more calcium, nitrogen, phosphorus, potassium and bacteria.

Earthworm activity is encouraged if cuttings are left on the turf. Worms also pull fallen leaves and plant debris into their burrows and, in doing so, they add organic material to the soil, improving its structure as well as its fertility. For borders and beds, if you are short of compost, a mulch of leaves, preferably chopped up, will not only add structure to the soil, but will also dramatically increase the worm population and therefore the health and fertility of your soil. The use of alkaline or organic fertilisers and dressings can also encourage Worms in lawns. Monty Don tells an amusing anecdote about a grass tennis court which demonstrates worms’ love of alkaline soil:

‘A grass tennis court had been laid on acidic soil and marked out with chalk. Over the years, the calcium in the chalk neutralised the acid soil beneath it, making extremely narrow strips that attracted earthworms. Long after the chalk had been washed away, ‘runways’ made by moles attracted by their favourite food – earthworms – followed the line of them, without realising that they were mirroring exactly the original chalk lines of the court!’

This raises the topic of moles in lawns. A real issue for me here at Old School Garden, despite me trying to persuade the ‘little burrowers’ to take themselves next door. If you have moles, as the tennis court story indicates, this is a sure sign that you have worms in your lawn.

Worm casts on a lawn

So what can you do?

Well one approach (but not one I’d recommend myself) is to try to deter the worms from casting using a fungicide. The law does not permit long life residual chemical build up in the soil, so gone are the days when formulations like Chlordane could be used to wipe out the worms (and possibly some gardeners too). However, Carbendazim is a chemical which is primarily used to deal with fungal diseases such as Fusarium but which also appears to interrupt the feeding of worms near the surface, by making the organic matter in which they feed unpalatable. Deeper feeding, non casting worms are apparently unaffected and continue to benefit the soil structure as normal.

Applying the chemical, which has a non hazardous classification, is said to be most beneficial in Spring and Autumn and must be carried out when the soil is already wet as it needs help in dispersing through the soil. However, be aware that whilst Carbendazim is approved for use in the UK and some other countries, some organisations argue that it is a dangerous substance.  Other options exist such as adding sulphur to the lawn, so reducing its alkalinity, and therefore reducing the attractiveness of it to worms. Manufacturer’s of this type of solution claim that this does not harm the worms or the soil.

However, I’d be a little wary of this, and other chemically – based solutions and try a more organic approach.

This does mean, however, that you’ll need to adopt a more relaxed attitude to worm casts (and mole hills, though it pains me to say so…). Worm activity, on the whole, is extremely beneficial to your lawn, so the best way to deal with the casts is to wait for them to dry and then brush them into the surface, spreading evenly with a Besom or similar broom. In doing so, you are adding to your lawn some fine compost and helping to improve its future appearance. There are also a few other things you can try to reduce the problem of casts:

• Avoid leaving leaves on the lawn surface during the autumn and winter because this warm blanket of organic matter is an ideal ‘restaurant’ for the worms.

• Do not allow a build up of thatch as again this decaying matter is digested by the worm which leads to casting deposits – so scarify your grass in the autumn and possibly also the spring.

• Keep the grass at a reasonable height.

• Avoid unnecessary watering as this attracts more worms. In dry weather the worms will move deeper, and by aerating regularly and ensuring good drainage, you will discourage activity.

If, like me you have a mole problem, I think the only safe, direct (but from experience, not necessarily successful) solution is to trap them, which might involve the services of a mole catcher unless you fancy a go yourself! I’ve found that other ‘solutions’ like noise/ vibration emitting devices seem only to have a temporary effect, if that!

Mole hills on the Old School Garden lawn

More generally, lawns are not attacked by pests, though you may at some point see the effects of the ‘Leather Jacket’ (the larva of the Crane Fly or ‘Daddy Long Legs’) and Chafer grub.

These can cause damage to the roots and stems of grass resulting in poor, stunted growth and bare patches. When a pest problem like this is suspected, the turf should be examined thoroughly to find the culprit. Pests are often found first in stressed areas, such as the edges of lawns or in shady or wet areas. They are not usually distributed evenly so it is advisable to look for spots that have discoloured, stunted or distorted turf. Insects tend to proceed outward from a central point; therefore they are generally most active on the outside edge. In both cases a lawn insecticide could be applied to kill the grubs/larvae….

Chafer Grub damage to grass

However, an alternative, organic solution is to wait for heavy rain (or thoroughly watering any yellow patches in the lawn yourself). Then cover the affected areas with black plastic sacks and leave overnight. The Leather Jackets and grubs will come to the surface and can be collected in the morning and disposed of – or left to natural predators such as spiders and garden birds (especially Starlings). You could also use the biological control Steinernema feltiae, which should be applied while the ground is still moist and warm in late Autumn.

Leather Jackets

Chafer Grub

Old School Gardener

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