It is very likely that you may see small toadstools appear in your recently laid turf. They often concern parents of small children, worrying that they may be harmful if eaten.
Why do they grow in my lawn?
When turf is being harvested, damage is caused to the roots and other underground parts of the turf. Microscopic bacteria and fungi, present in all soils, decompose this dead and dying tissue once the turf has been laid. This process of biological decomposition, which breaks down organic matter in the turf, occurs naturally throughout the garden.
For most of its life cycle the fungi that feed on the dead material in the turf are too small to easily see. However, under certain weather conditions the fungi move into a reproductive phase and produce the small brown toadstools found in the turf. The spores (serving the same purpose as seeds) released from the toadstools fall to the ground or are carried long distances on the wind. The air is full of the microscopic spores of various fungi and they will only grow if they find suitable conditions. In the case of newly laid turf, the spores only grow if a suitable food source remains in the turf. Once the dead material in the turf has gone, the toadstools will go as well. It is very unlikely that they will return in quantity.
When do toadstools appear?
Toadstools are commonly seen whenever the weather is warm enough. When you water your newly laid turf, you provide an ideal warm, moist microclimate for the fungus that produces the toadstools to grow. In suitable conditions toadstools are produced a few days after laying the turf. They emerge in a flush of growth and may persist for a few weeks at most, then disappear. Occasionally, a second brief outbreak may occur, possibly during the next period of warm moist weather. If turf is laid in cold weather the fungi will not appear until the temperature rises and conditions are moist. If decomposition is not complete when winter comes and cold weather stops growth of the fungi, more toadstools may form the following year. It is most unusual for toadstools of this type to be produced in the year after that, however.
What are they?
The most common of these toadstools is the brown hay cap, but other species may be involved.
Is my lawn diseased?
In some gardening books, the only reference to toadstools in turf is in connection with fairy rings, which can be a very serious problem on mature lawns. The small brown toadstools found in newly laid turf are completely different from those found in fairy rings, and have no long term consequences. If toadstools of colours other than brown are found, it is likely that they are living on buried dead wood, for example tree roots. In this case, the toadstools will be found immediately above the decaying wood. Removing the wood will prevent fungi of this type from growing.
Are they poisonous?
Without accurate identification, it is impossible to say whether toadstools are poisonous. As a precaution, it would be wise to keep small children away from them since they may be harmful if eaten in quantity, as is the case with many things found in gardens.
Can they be treated?
There are no fungicides recommended in the UK for use against these toadstools. However, since they are not damaging to the turf, and are part of a natural and temporary process, there is no need to treat them. If you are worried about children eating them in the garden, the best approach is to remove the toadstools from the lawn by mowing the affected area on a daily basis until no more are produced. Provided the lawn is not mown at less than 25 mm height of cut this will not damage the turf. Alternatively, breaking their stems by brushing the toadstools will allow them to dry out and disappear they are composed mainly of water.
To summarise: it is very common for small brown toadstools to grow in cultivated turf in the weeks after laying from whichever grower it is obtained. They are not damaging to the turf and will be a temporary problem.
Advice from the TGA (Turf Growers Association).