Turf Grasses: Annual Meadow Grass (Poa annua)

Introduction

This grass is arguably the bête noire of the majority of the UK's turf managers, as they wish to provide the most suitable grasses for the facility concerned. Other turf managers welcome this grass, somewhat reluctantly, as they feel it is better to have this grass than no grass at all.

Poa annua ideally prefers moist and fertile soils with a high percentage of medium-fine soil particles, having a pH range of 5.5 to 7.5. It can become dominant on compacted soils, which also tend to be those soils which contain a high percentage of fine soil particles.

Seed production of Poa annua is high in April/May and October/November, however, this grass can readily produce seed all year round if conditions are suitable. Seed germination is encouraged by high soil moisture levels, especially where there is a thatch layer present within a fine turf situation. Low, acidic soil conditions, especially in the surface layer of the turf sward, may inhibit the germination of Poa annua seeds and prevent the prolific seed heads from gaining a foothold.

During the summer, Poa annua dies off in hot weather at a high rate, which leaves unsightly bare and thin areas which the desirable fescue and bentgrasses cannot rapidly colonise because of their slower growth rate. One way of preventing this die back of Poa annua is to irrigate in sufficient quantities to maintain the grass throughout the hot, dry spells. Unfortunately, this can encourage surface rooting in this grass, to the detriment of the desirable grasses.

Poa annua also has a particularly high growth rate (in relation to other turfgrasses), which can also contribute to the development of a thatch layer.

Disadvantages of Poa annua

1. Unattractive yellowish-green colour, especially when it is under stress;
2. Produces slower putting and bowling surfaces when compared with fescue and bentgrass surfaces;
3. Susceptible to Fusarium patch and Anthracnose diseases;
4. Higher inputs of chemicals required to maintain a suitable playing surface;
5. High seed head production, making fine turf areas look less attractive;
6. Dies back in dry summers (due to poor drought tolerance) and winter;
7. High nutrient requirements;
8. Relatively high production of thatch;
9. High irrigation requirements;
10. Easily kicked out of football pitches;

Some possible causes of high content Poa annua turf

1. Lack of adequate aeration to maintain porosity within the soil profile, preventing the finer grasses to establish a deeper, more prolific fibrous root system;
2. Excessive amount of irrigation applied, either by frequent irrigation which just wets the surface layer, say the top 25mm, keeping it soft and moist, or by heavier applications where the soil profile is maintained in a wet to moist condition, with only the top layer drying out. In each of these cases the Poa annua will produce surface roots and outcompete the finer grasses. This is not so much of a problem where perennial ryegrass dominates the sward;
3. The overuse of the turf area by players, causing wear and tear which is incompatible with the maintenance of a suitable playing surface;
4. The incorrect use of fertilisers. This can be by continually relying on fertilisers which have a neutral reaction on soil pH, which can include some inorganic compounds, organic fertilisers or especially an over reliance on controlled release fertilisers. Fertilisers can also be applied at the wrong time of year, which can result in forced sappy growth due to late seasonal, i.e. early autumn, application producing weak grass which is more prone to wear and disease attack, and is not conducive with the natural habit of plants to 'harden' themselves for the coming winter months, by a natural reduction in growth rate.
5. If morning dew or surface moisture is not removed, for example by switching or drag brushing, this can encourage disease attack, consequently weakening the sward;
6. If worm casts are not controlled, these will provide an ideal seed bed for Poa annua seeds as well as weed seeds to establish into. In addition, the casts smother the grass, weakening it and encourage disease attack. Earthworms can be controlled by gradual acidification of the soil, a coarser soil texture and the application of chemicals;
7. Carrying out maintenance operations in unsuitable soil conditions can produce surface smearing and soil compaction, both of which benefit Poa annua to the detriment of other grasses;
8. Hollow-tining and then top-dressing when the tine holes are not rapidly colonised by the desirable grasses;
9. Using unsterilised top-dressing, which may introduce weed seeds, including Poa annua ones, into the grass sward;
10. Not having the required experience and knowledge to manage turf correctly, although these can be addressed by further training and guidance.

Controlling Poa annua

1. Use sterilised top-dressings;
2. Buy grass seed which is certified free of Poa annua;
3. Provide a rootzone which is no greater than pH 5.5 for fine turf situations and 6.0 - 6.5 for perennial ryegrass situations;
4. Choose a suitable fertiliser for each situation, and avoid excessive applications of phosphate;
5. Apply fertiliser at the correct times and not too late in the growing season;
6. Control thatch by aeration, scarification and verticutting;
7. Carry out sensible irrigation practices and encourage grass roots to penetrate deeply into the soil profile to extract moisture;
8. Apply sulphate of iron as a turf tonic to acidify the soil surface, although this may not be a good idea on a regular basis for perennial ryegrass dominated turf;
9. Box off grass clippings;
10. Overseed at optimum times to ensure subsequent germination and establishment are given their maximum potential chance of success. Do not be encouraged to delay this operation due to playing requirements as there are only a limited number of optimum 'windows of opportunity' for ideal seeding;
11. Manage usage and wear in an effective way; this helps to maintain the quality of the playing surface;
12. Remove surface moisture and worm casts on a regular basis;
13. Ensure applied top-dressings are properly worked into the sward to prevent smothering the grass;
14. Maintain an adequate aeration programme throughout the whole year;