Grasses

Some people have developed allergies to grass pollen which can be very abundant in the atmosphere at times when these plants are flowering. Although many wind-pollinated plants contribute to hay fever, grasses are among the most important causes during the early and mid-summer seasons in temperate climates.

Some species of grasses may be deemed to be weeds for other reasons. Crabgrasses (Digitaria spp.), for example, are unwanted in lawns, and for that reason they are considered to be important, aesthetic weeds. Other grasses interfere with the productivity of agricultural crops, and they may be weeds for that reason. The barnyard grass (Echinochloa crus-galli), for example, can be abundant in fields of cultivated rice, causing losses of economic yield in the form of rice grains. Another example is ilang-ilang (Imperata cylindrica), a weed of various types of cultivated lands in tropical Asia. This grass can be such an aggressive plant that it is sometimes referred to as the world's worst weed.

Other weed grasses are non-native species that have been introduced beyond their original range and have become seriously invasive in their new habitats. Sometimes these species can become dominant in natural communities and thereby seriously degrade the habitat for native plants and animals. In North America, for example, the reed canary-grass (Phalaris arundinacea) and giant manna-grass (Glyceria maxima) have invaded some type of wetlands, causing serious ecological damages in terms of habitat availability for native species. In semi-arid parts of the Great Plains of the western United States, the downy brome-grass (Bromus tectorum), a Eurasian species, has become abundant. The highly flammable, late-season biomass of this grass has encouraged frequent fires in this habitat. This too-frequent disturbance regime has converted the naturally shrub-dominated ecosystem into a degraded system dominated by the brome-grass, which supports few of the original, native species of plants and animals.

Clearly, the grass family contains species that are extraordinarily important to the welfare of humans and other creatures. Some of these grasses are consequential because they are such important sources of food. Others species are important because they have been able to take advantage of ecological opportunities provided for them by human activities and disturbances. Especially important in this respect has been the dispersal of some species of grasses far beyond their native ranges. In their new, colonized habitats the productivity and fecundity of these invasive grasses are not limited by the natural constraints that they experience in their original range such as diseases and herbivory. This is how these plants become invasive weeds.