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Holcus is a genus consisting of eight species of grass. These grasses, often called soft-grass, are originally from Asia, Africa, and Europe, although they also grow on some Atlantic Ocean islands. The plants are sources of food for some species of moths and butterflies in the Lepidoptera order. Though most of the species are not invasive, at least one poses a problem when it escapes cultivated land.
One species, Holcus lanatus, or Yorkshire fog in the United Kingdom, has velvety green leaves; hence its common American name of velvet grass. It makes a good pasture grass, as it can thrive under a variety of conditions, though severe frost and trampling can kill it. The plant is considered an invasive species or weed in several countries, including the United States. It is capable of disrupting ecosystems by killing off native plant species. Yorkshire fog prefers soil that isn’t rich in nutrients and has poor drainage. Therefore some countries reduce the grass’s population by increasing the phosphorus and potassium or drying the soil.
In 2006, California classified the impact of Holcus lanatus as moderate. Alternative methods for controlling this invasive grass species are being explored as of 2010. The species reproduces rapidly due to numerous factors. Yorkshire fog produces many seeds that fall off at the slightest movement. Disturbed ground, such as ground rooted by animals like feral pigs, aids the plant in reproducing. Humans are guilty of spreading the invasive species by mowing the grasses down and then using the same equipment to mow elsewhere without first cleaning it.
Another species, Holcus mollis, also called creeping soft grass or creeping velvet grass, is native to west Asia and Europe and flowers in early summer. The species has a soft, feathery appearance much like Holcus lanatus, and it is of average height for a grass. It prefers very shady areas, but it is known to thrive even after all shade is removed. This species is considered a weed in England, though variants have been introduced that are sterile and are occasionally cultivated in gardens.
A third species, Holcus setiger, or the annual fog, is native to South Africa and flowers during the spring. It is an annual grass that is usually a shade of white or green, though it may have a greyish tint. One can identify this species from the Yorkshire fog plant by its distinct awned glumes. As of 2010, this species is believed to have naturalized in several countries due to reported sightings away from its native habitat.