What are coarse grasses in lawns?

In a lawn free of weed grasses, it should not be possible to easily make out the different grass species. Where different grasses are clearly distinguishable, coarse grasses are probably present. Early removal is important as there are no chemical controls. Coarser leaf blades may have poor drought tolerance or a tendency to flower at a low height.

Appearance

Coarse grasses are easier to spot in a lawn than you might expect;

• A lawn invaded by weed grasses will appear patchy, with some of the grass being of a different colour and texture to the rest of the lawn
• In the case of Poa annua, commonly called annual meadow grass, flowering can occur below the height of cut so that small, pale seed heads give the lawn a peppered look
• Some types of creeping grasses tend to become more apparent in periods of drought, particularly on light, quick-drying soils. The leaf-sheaths rapidly turn brown, giving the turf a burnt appearance

Cause

How do coarse grasses take hold?

• Coarse grasses may be introduced by birds as seed, or may arrive in un-sterilised loam used as a lawn top-dressing
• On occasion, coarse grasses may be present in the original seed mixture and become apparent only if the condition of the lawn deteriorates. In a new or fairly new lawn, the seeds could have been dormant in the soil, springing up after the lawn was laid
• Coarse grasses can become invasive, especially where the lawn suffers from compaction, drought, or excessive mowing
• In winter they become more prevalent as the fine grasses stop growing, while the coarse grasses continue to develop
• Some fine grasses, such as fescues and bents, are prone to becoming scorched in freezing weather, checking their growth and allowing the coarse grasses to take over
• Poa annua is a small native grass known as annual meadow grass that seeds freely, even when turf is mown very close. Although it is usually an annual (living for one year only), there are strains which tend to be biennial (living for two years), or even perennial (living indefinitely)

Control

Non-chemical controls

Try cultural methods first;

• Apply a spring lawn fertiliser, high in nitrogen, in March. This will boost desirable lawn grasses as they come into growth
• Mow regularly, as fine grasses are better adapted to being cut, while coarse grasses are weakened by this treatment
• Rake the lawn lightly with a spring-tine rake to lift up the patches of coarse grass before mowing. This will allow a better cut and aid coarse grass removal. Take care not to mow too low
• Remove individual patches of weed grasses by hand
• For large problem areas, fork out the worst patches in September or October (adding soil to re-establish the level) and then re-seed with a fine lawn seed mixture, or lay patches of new turf

Poa annua (annual meadow grass):

• For specific problem areas, fork out the worst patches in September or October (adding soil to re-establish the level) and then re-seed with a fine lawn seed mixture, or lay patches of new turf
• If infestation is extensive, control is much more difficult. Avoid frequent watering in dry summers, as meadow grass is shallow-rooting, so should be discouraged by drought
• Poa annua will not survive if the soil is very acid, so avoid applying lime to the turf or using lime-based fertilisers such as nitro-chalk. Use sulphate of ammonia, which is acidic, to feed the turf, but only in moderation – if conditions become too acid this might adversely affect other desirable grasses

Coarse creeping grasses:

• Rake the lawn regularly before mowing in summer and autumn, using a spring-tine rake. This lifts up the stems so they are more easily cut by the mower. Do not rake vigorously in spring, as this is the time of year when new shoots of fine grasses are rooting into the soil. Disturbance of this rooting process could result in sparse turf that is more open to weed infestation
• Apply a proprietary lawn dressing (or a mixture of equal parts good loam soil, sieved leafmould and sharp sand) in September
• Feed the lawn annually in spring to encourage the growth of desirable turf grasses

Chemical controls

There are no selective weedkillers that can be used against coarse grasses in lawns without also killing the turf grasses.