The control of worms in the lawn is a controversial subject, as they are very beneficial to the soil. A high population of worms is an indication that the soil is healthy and ideal for growing healthy turf.
However, worms produce casts which create numerous problems, including sticky, muddy conditions as well as damage to lawn mowers. Of all the species of worms found in the UK, very few actually produce worm casts.
The worm population is heavily influenced by environmental factors such as soil texture, moisture content, pH levels, soil temperature, inorganic salts and organic matter levels.
Worm casts are more troublesome on heavier clay type soils rather than on light sandy soils. Clay is water retentive and doesn’t have the free draining properties of sand and worms favor moist conditions. This is the reason why worm-casts are troublesome from October to April, because the soil is generally wetter during the autumn and winter months.
The soil pH also has an influence on earthworm numbers. Most species prefer a soil that is neutral or alkaline (high pH), they will not tolerate acidic soils (low pH).
A soil that contains a high percentage of organic matter is likely to be high in worm numbers, as organic matter is their food source. Organic matter is dead or decaying matter like leaves, grass shoots, roots, stems etc. Clay soils are typically higher in organic matter than sand based soils.
There are many advantages of having a healthy population of worms in the lawn, some of the key benefits are:
- Break down organic matter to humus, this increases soil fertility (reducing fertiliser requirements) and helps maintain acceptable thatch levels.
- Helps increase the micro-organisms or soil bacteria population. They require air to survive and are important for the breakdown of thatch and reducing soil pathogens.
- Aerating the soil and in turn improving the soil structure.
- Help relieve soil compaction and reduce the need for aeration.
- Improve the surface drainage of the lawn.
- Helps reduce toxic gases in the soil.
The only disadvantage of having worms in the lawn is the fact they produce casts, which create a host of problems for the gardener:
- Worm-casts are unsightly and spoil the look of the lawn.
- They can damage machinery, especially bottom blades on cylinder mowers. They are also prone to sticking to rollers and can alter the height of cut of the mower.
- In wet conditions worm-casts stick onto the soles of shoes and create muddy conditions.
- Can smother finer leafed grasses, such as fescues and bents.
- Worm-casts create an ideal seed bed for weed seeds.
- They can expensive to control and control is not always 100% effective.
- Encourage moles (worms are the primary food source of moles).
Should worms be controlled in the lawn
The million dollar question to which there is no easy answer. If you are a turf care specialist who is preparing golf or bowling greens, even a hockey pitch or croquet lawn, then worms have to be controlled. Golfers, bowlers etc. demand high quality playing surfaces and worm-casts are totally unacceptable as they interfere with the play-ability.
However gardeners are not preparing a surface for sports, they just want their lawn to be healthy and look it best in the spring and summer. Remember the most active times for worms is during mild periods of the autumn and winter months, when most lawn receive little or no traffic.
In most cases gardeners could get by without having to resort to treating worms if the situation is managed correctly. When worm-casts appear in the winter they can be left until they dry out. During these months, dry spells or strong winds are not uncommon and they are quick dry the casts out.
When this happens they can easily be scattered over the lawn with a stiff brush or spring tine rake. A besom (witches broomstick) is very useful for this task (old technology but still very effective).
Removing casts (prior to mowing) is also beneficial, if the lawn is to be cut during the winter months. Many gardeners still give their lawn a ‘light topping’ to tidy it up during this period, especially when the weather is relatively mild and the surface is firm and dry.
Although the above won’t prevent all of the damage caused by casting worms, it will certainly make a big difference in that it prevents casts from smothering the turf. In fact if the soil is loamy or sandy, then scattering the casts when they are dry may also benefit the lawn, by giving it a light top dressing.
A case could be made for using a worm killer, if the autumn or winter was very wet and mild and growth was still strong. A combination of worm-casts and regular mowing can turn a fine lawn into an unsightly mess, especially on heavy, water retentive clay soils.
In this situation treating the worms could be an option. When considering whether to spray for worms it is always worth remembering the benefits of earthworms, before you make a decision.
Prevention and control of worms
Although it is impossible to totally eradicate worm-casts using cultural methods, there are numerous things that can be undertaken to minimize the problems. These include:
- Removing leaves from the lawn during the autumn will help as they are a primary food source of worms. A blanket of leaves will also increase the temperature of the lawn surface and encourage earthworm activity.
- Regular scarification and aeration will help reduce the thatch (organic matter) layer, another food source of worms.
- Box of grass clippings, as this is also a food source as it adds to the thatch layer.
- Avoid unnecessary watering of the lawn, especially late in the season as this may encourage worms to cast earlier than normal.
- Light treatments of sulphate of iron may help discourage surface activity. Most species of worms do not like acidic conditions.
The above may help discourage worm activity and reduce casting to a degree on certain soil types. However in some situations, especially on clay or heavy loam soils it may have little effect. In this situation, the best way to reduce casting is to enlist the help of a lawn care specialist and treat with carbendazim.
Carbendazim is not available for amateur gardeners, its can only be applied by lawn and turf care professionals with the relevant spraying qualifications. Although it is sold for worm control these days, it was originally used as a turf fungicide. However, it was also found to have worm control properties too.
Currently, it is the only approved chemical available for the control of worms in turf. Although it is listed as a worm killer, it doesn’t kill them, it only irritates them. The worms simply move away or go deeper into the soil. This product should only be applied when worm activity and casting occurs.
Applying it in dry conditions when there is no activity is a waste of time and money as it will not be effective. Carbendazim is most effective during mild periods when the soil is moist and worms are active. It is also advisable to spray it when rain is forecast, to wash the chemical into the soil when the worms are.