Annual meadow-grass is the commonest and most widespread grass in the United Kingdom. Found in a wide range of disturbed and man-made habitats, including grassland (especially where heavily grazed, mown or trampled), arable fields, lawns, gardens, amenity grassland, waste ground, paths and waysides. Despite its name, short-lived perennial forms also occur in some habitats. Rough-stalked meadow-grass (Poa trivialis) is also widespread, especially in grassland, hedgerows and field margins, but also occurs as a weed of arable crops and can be a particular problem in herbage seed. Smooth-stalked meadow-grass (Poa pratensis) is also widespread in grassland and hedgerows but less commonly occurs as a weed of arable crops. Rough-stalked and smooth-stalked meadow-grass are often included in amenity seed mixes used in field margins so there is a potential risk of ingress into the main part of the field.
Leaves of meadow-grasses often have ‘tramlines’ down the middle of the upper leaf surface, which can be a useful diagnostic. Annual meadow-grass seedlings are smaller than most other grass-weed species at the same growth stage, and are often bright green with ‘V’ shaped leaves (see photo). Flowering plants, which may be seen in any month of the year, are usually fairly short (up to 30 cm tall) and the branches in the panicle are usually in pairs. In contrast, flowering plants of rough-stalked and smooth-stalked meadow-grass are taller (up to 90 cm tall) with branches in the panicle usually in clusters of 3 – 7. Rough-stalked meadow grass, as its name implies, tends to have a rough leaf sheath with a long (4 – 10 mm) ligule. In contrast, smooth-stalked meadow grass has smoother leaf sheaths with shorter ligules (1 – 3 mm). Rough-stalked meadow-grass produces stolons while smooth-stalked meadow-grass produces rhizomes but these are often not present or poorly developed in plants in arable situations, which behave as annual rather than perennials.
On an individual plant basis, annual meadow-grass is not very competitive in wheat in the UK; it requires a population of 50 plants/m2 to cause an average 5% yield loss. However, very high populations can occur which can cause substantial yield losses, although there is surprising little data to quantify the effect. High densities tend to form low, sprawling masses of vegetation which can interfere with the harvesting of crops and also promote damp conditions which may favour pests and diseases. A serious weed of herbage seed crops. Rough-stalked meadow-grass is more competitive than annual meadow-grass on an individual plant basis – probably comparable with black-grass.
Agroecology – implications for control
- Annual meadow-grass can germinate throughout the year and, under favourable conditions, flower and set seed within 6 weeks of germinating. In contrast, rough-stalked meadow-grass mainly germinates in autumn and sheds seed in the following summer, like black-grass.
- Several generations per year of annual meadow-grass are possible – in marked contrast to all other major grass-weeds (e.g. black-grass and rough-stalked meadow-grass) in which only one generation a year occurs.
- Annual meadow-grass has high fecundity because of its ability to self-pollinate (estimated 85% self-pollination) and produces many seeds with variable dormancy and high germinability.
- Annual meadow-grass is a very variable and adaptable weed, with the ability to produce heads and seeds even under a regular close cutting regime. Individual plants may produce from 10 to 20,000 seeds with variable dormancy. Many seeds germinate soon after shedding whereas others will remain dormant for long periods.
- Annual meadow-grass seeds often constitute the major portion of the seedbank of arable fields – populations may exceed 30,000 seeds/m2.
- Annual and rough-stalked meadow-grass plants can only emerge from seeds close to the soil surface, so ploughing can give good control, although older seeds may be brought up to the surface.
- Both species are encouraged by shallow tillage and direct drilling, which greatly favour these weeds.
- The relatively uncompetitive nature of annual meadow-grass means that low populations can be effectively suppressed by competitive crops.
- Spring cropping can help control rough-stalked meadow-grass as most germination occurs in autumn.
- In cereals, a wide range of herbicides gives control of annual meadow-grass. For example, the pre-emergence herbicides triallate, prosulfocarb, pendimethalin and flufenacet and chlorotoluron based mixtures. Post-emergence herbicides giving control include ethofumesate and iodosulfuron based mixtures.
- In oilseed rape, propyzamide, carbetamide and metazachlor all give control of annual meadow-grass. Although this weed is ‘naturally’ resistant to many ACCase inhibitors (‘fops’, ‘dims’, and ‘dens’), clethodim and propaquizafop do give some control post-emergence.
- Herbicides that can be used in other crops for meadow-grass control include: nicosulfuron and mesotrione in maize; flufenacet+metribuzin and metobromuron in potatoes; metamitron and ethofumesate in sugar beet.
- Rough-stalked meadow-grass can be controlled by many of the above herbicides, although it is generally considered a harder weed to control.
- While it appears there is a good range of herbicides for control of meadow-grasses, some of the above will not be available longer-term due to regulatory and commercial decisions.
- The ability of annual meadow-grass to germinate throughout the year means that, while herbicides may give good control initially, later emerging plants may not be adequately controlled. This is especially the case with non-competitive crops such as some vegetable crops (e.g. onions) and crops which take a long time for full ground cover to be achieved (e.g. row crops such as sugar beet and potatoes). These later emerging plants can interfere with harvesting, even if they have relatively little effect on crop yield.
- Glyphosate can give good control regardless of growth stage, provided low doses are avoided, so should be effective for controlling plants before sowing or in non-crop situations. Be aware that applying glyphosate several times a year to control repeated flushes of emerging annual meadow grass poses a high risk resistance risk, especially if this is done in the absence of any cultivations.
- No cases of resistance to isoproturon were ever recorded in annual meadow-grass – rather surprising given the widespread use of this herbicide in the past.
- Resistance to the triazine herbicides (e.g. atrazine and simazine) developed in seven European countries (including the UK) mainly in the 1980’s. Paraquat-resistant annual meadow-grass evolved in the UK in 1981. As these herbicides are no longer used in Europe these finding appear to be largely of academic interest now.
- However, metribuzin and metamitron, which are still widely used, are triazinione herbicides with similar modes of action to the triazines. Resistance to these should not be ruled out, although no cases have been recorded in annual meadow-grass so far (but have been in other weeds, including groundsel in the UK).
- In the USA, resistance in annual meadow-grass has been detected to ethofumesate, pendimethalin, ALS inhibiting herbicides (e.g. sulfonylureas) and to glyphosate, although very largely in non-arable situations (e.g. golf courses).
- While annual meadow-grass has evolved resistance to several different herbicides worldwide, the numbers of cases is quite limited compared with many other weed species. Consequently annual meadow-grass can be consider a ‘low resistance risk’ weed species.
- No cases of resistance in rough-stalked or other meadow-grass species have been reported anywhere in the world.
- If resistance is suspected, collect a seed sample and have it tested. Collecting seeds from meadow-grasses is difficult, so take advice on collection techniques.