Bermuda grass (Cynodon dactylon), a perennial of cosmopolitan distribution often used as a forage, is the most widespread grass weed affecting 90 countries.
Two other perennial grasses, Johnson grass
(Sorghum halepense) and blady grass (Imperata cylindrica) are widespread in tropical areas.
Among the annual grasses, winter grass (Poa annua), of European origin, is the most cosmopolitan as a weed of crops, lawns and waste land. Crowsfoot grass
(Eleusine indica) is mainly found in tropical regions. The two Barnyard grasses
(Echinochloa spp.) are annuals and are major weeds of irrigated tropical crops.
Wild oats (Avena fatua), which is of Mediterranean origin, is found particularly wherever the annual temperate cereals; wheat, barley, oats and rye are grown. Summer grass
(Digitaria sanguinalis) is found in both tropical and temperate regions in both crop and non-crop situations.
The trait of seed dormancy is strongly associated with the persistence of six of the top nine grass weeds (those that are
annuals). These plants rely on a period of dormancy in the seed bank to effect population renewal over several seasons. Other traits such as competitive ability at the seedling and vegetative stage and seed number will have a strong influence on
success as a weed.
