Lawns in damp or poorly drained conditions can suffer from unsightly problems such as cyanobacteria (an algae-like growth), dog lichen and liverworts. Cyanobacteria in particular can make the lawn very slippery.
What is the problem?
Algae-like growths, lichens and liverworts are often found growing in damp or shady places in the garden. Although harmless in other areas of the garden, even lending a mature look to the landscape, they can be problematic on lawns. As well as affecting the appearance of the lawn, they block light from reaching the grass and can make the surface slippery.
Algae-like growths: Dark green or blackish jelly-like growths that often appear in damper, cooler weather over the surface of the lawn, making it slippery are caused by a cyanobacteria called Nostoc. These are sometimes referred to as gelatinous algae or blue-green algae but are technically classified under bacteria, not algae.
Lichens: In turf the most common lichen is Peltigera spp. (dog lichen); it is brown or grey and formed of flat structures that grow horizontally in the turf.
Liverworts: Liverworts on lawns usually have a green, flattened body and no leaves. A common example is Marchantia, which is often topped with umbrella-like sexual organs.
Slime moulds: Wet weather in autumn or spring can also lead to the growth of slime moulds. These growths, which may be white, yellowish or orange, produce small grey fruiting bodies that subsequently release masses of purplish-brown spores. The growths are purely superficial and do not harm the grass but they are unsightly.
Cyanobacteria, dog lichens and liverworts are found on lawns where poor drainage and shady conditions cause a damp surface. Compacted soil is especially prone to developing algae, particularly around the drip line of trees or shrubs.
In very wet weather algae may appear on only slightly compacted lawns and is also frequently found on turf beneath trees.
Correcting the underlying conditions should clear up gelatinous algae and moss growth. Try the following:
- If the lawn is waterlogged for much of the year, consider installing artificial drainage
- If the surface layer is poorly drained aerate to a depth of 7.5-15cm (3-6in) by spiking with a garden fork every 10-15cm (4-6in). Where the lawn is only compacted or waterlogged in patches, aerate during wet conditions every four to five weeks. On heavy soils a hollow-tine aerator will give better results
- After aerating, top dress with sharp sand, working it into the holes to improve drainage
- Avoid walking on the lawn when it is waterlogged as this will only aggravate compaction
- Provide good all-season care, especially in spring and summer and most importantly in autumn
There are few chemical controls available to gardeners, but if lawn mosskillers that contain ferrous sulphate (e.g. Vitax Green Up Mossfree or Doff Lawn Moss Killer Spray) are used on the lawn to control moss, incidental control of algae-like growths, lichens and liverwort should also result. Note: Brinton's Patio Magic (containing benzalkonium chloride) will control algae-like and other growths on artificial turf.
Weedkillers for gardeners (Adobe Acrobat pdf document outlining weedkillers available to gardeners)
Common name Cyanobacteria, dog lichen, liverwort
Areas affected Lawns
Main causes Damp conditions, compacted soil and poor drainage
Timing Mostly in winter but after any wet spell