Annual meadow-grass

Annual meadow-grass
Other names: 

annual bluegrass, cause-way grass, spear grass

Latin names: 

Poa annua L.

Weed Type: 
Occurrence: 

Annual meadow-grass is an annual or short-term perennial grass found on arable land, grassland, trackways and in gardens. It is a pioneer species in disturbed habitats and is common throughout the UK. It does not thrive on acid soils or those low in phosphate. Annual meadow-grass is favoured by improving the water holding capacity of soil with organic matter. It can withstand periodic waterlogging. It is sensitive to drought and is easily damaged by air pollution. Annual meadow-grass is shade tolerant and can withstand considerable trampling. In fact it is one of the most wear resistant of all grasses.

It is one of the main weeds in grassland. In young swards it may account for up to 20% of the composition where establishment of desirable grasses has been poor. In older swards it invades areas damaged by poaching, intensive grazing or where other grasses have died out. It is considered a grassland weed because of its poor productivity and low digestability. Annual meadow-grass is also frequent weed in spring and autumn-sown arable crops and field vegetables. Although the plant is relatively small it often emerges in sufficient quantities to smother crop seedlings.

Annual meadow-grass is a very adaptable species and many ecotypes have been recognised, some of these are perennial. Annual forms have erect growth while the perennial ones form a mass of tillers many of which grow horizontally and root at the nodes. Many named varieties and subspecies are recognised. A perennial form has been bred for use in sports turf. Some populations of annual meadow-grass have developed resistance to certain widely used herbicides.

Annual meadow grass can become infected with ergot and in cereals may act as a source of crop infection throughout much of the year. The fungal fruiting bodies are smaller and less obvious on the grass flower panicle than on the cereal ear. It can be the host of a number of nematode species that also attack important crops.

It is common in the headlands of arable fields where it is an important constituent in the diet of many farmland birds including gamebirds.

Biology: 

Flowering is independent of daylength and can occur at any time of year. It usually begins in early spring and continues throughout the growing season. Over-wintered plants come into flower from May while plants from seedlings that emerge in spring flower from July to September. As a general rule the flowers are self-pollinated but up to 15% of outcrossing can occur. Seed is produced abundantly from April to September and beyond, although, high temperatures can hinder anther development and this limits pollination. Seed number per inflorescence is estimated at 80 and the number per plant is said to average 2,050. The perennial forms are said to produce 13,000 seeds per plant. Annual meadow-grass will flower and set seed even when cut regularly to a height of 0.65 cm in short turf. Seed will continue to develop and ripen on inflorescences severed from the plant at any time after pollination. Annual meadow-grass can be found in fruit throughout the year.

Differences have been found in the germinability and mean germination time of seeds from different populations of annual meadow-grass. In some studies, both fresh and dry-stored seed had a light requirement for germination but not in others. Chilling and nitrate have been shown to promote seed germination.

In the field, annual meadow-grass germinates from February to November with the main peaks of emergence in early spring and in autumn. A high proportion of seed will germinate soon after shedding. However, seedling numbers and rate of emergence are affected by prevailing weather conditions. Seedlings may take around 20 days to achieve 50% emergence when moisture is available but dry conditions will delay emergence, particularly in the summer months. Plants that develop earlier in the year often grow larger and produced more tillers than those emerging later. Seedlings that emerge from August to December overwinter and begin to tiller when growth resumes in spring.

Persistence and Spread: 

Annual meadow-grass seed can form a major proportion of the weed seedbank in both arable and grassland soils. The seed can remain viable in soil for at least 4 years but losses are greater in cultivated soil. Seed mixed with soil and left undisturbed had declined by 76% after 6 years but in cultivated soil the loss was 92%. In soils sown with autumn crops and ploughed annually, the time to 99% decline of annual meadow-grass seed was calculated at 4.3 years with an annual decline rate of 55%. In undisturbed soil the annual loss was 46%. Dry-stored seed was still 98% viable after 3 years.

Annual meadow-grass is a prolific seed producer but there is no obvious dispersal mechanism. Most seeds fall around the parent and being small soon become incorporated into the soil. The seeds are readily transported in mud by vehicular and foot traffic. Viable seeds are often brought to the soil surface in wormcasts where conditions are more favourable for germination. Annual meadow-grass will also colonise molehills before being replace by perennial grasses. Mowing when the weed is in flower will spread the seeds. Viable seeds have been found in horse and cattle dung but viability is lost in manure after a period of storage.

Annual meadow-grass seed has been a contaminant in cereal grain and cultivated grass seed. It was a frequent impurity in grass seeds of Danish, Irish and Swedish origin.

Perennial forms can spread vegetatively by creeping stems that root at the nodes but this is very limited.

Management: 

Control is by surface tillage to encourage germination followed by later harrowing to kill the emerged seedlings. The seedlings have fibrous roots and are easily dislodged by cultivations. In cereals the growth of the weed is suppressed by a vigorous crop canopy and competition from the broad-leaved weeds. The economic threshold population of annual meadow-grass in wheat has been calculated at 714 plants per m² for chemical control but this will vary with climate, growing conditions and may be different for non-chemical control measures. In winter cereals, annual meadow-grass populations increased annually where direct drilling was the practise but declined slightly where ploughing was carried out. However, ploughing may lead to greater persistence and increase future weed problems.

In row crops, hoeing and other inter-row cultivations should keep the weed in check. In a rotation of mainly vegetable crops, different primary cultivations had a pronounced effect on seed numbers of annual meadow-grass in soil. At the end of 9 years, seed numbers were 7, 11 and 23 million per acre respectively for deep ploughed (35-40 cm), shallow ploughed (15 cm) and rotary cultivations (15 cm).

Repeated mowing and close grazing of annual meadow-grass merely causes increased branching. Within the mat of stems below cutting height flowers are still produced and seed set. Hoeing or cutting plants off at or below soil level is more effective in controlling annual meadow-grass than partial burial. Complete burial of seedlings, whether uprooted or not, is more successful. There is the potential for recovery if seedlings are left on the soil surface. Mature plants are able to survive uprooting and may continue to ripen seeds on developed panicles.

Annual meadow-grass has a basal growing point and is not susceptible to flame weeding. Seedlings are also tolerant of UV radiation but seeds in the surface layers of soil are killed by solarization.

A mulch of compost 30 mm deep prevents seedling emergence but so does a covering of soil. In greenhouse tests in the USA, corn gluten meal (CGM) applied to the soil surface or incorporated reduced the growth and survival of annual meadow-grass. Corn gluten hydrolysate (CGH), a water soluble derivative, was even more active.

Phytoparasitic bacteria have been evaluated as potential biocontrol agents for annual meadow-grass. Xanthomonas campestris pv. poannua infects and kills annual meadow-grass damaged by mowing but has a restricted host range and does not cause injury to the desirable grasses. Ground beetles feed selectively on the seeds of annual meadow-grass.

Updated October 2007.

Fully referenced review: