Imperata cylindrica

Imperata cylindrica, cogongrass

Family

Gramineae (Poaceae), the Grass Family

Distinguishing Characteristics

This is a fast-growing, perennial grass averaging 3 to 4 feet in height, but occasionally reaching 6 feet. It is rarely seen as a single plant, as it spreads vigorously by means of rhizomes (creeping, horizontal underground stems), forming dense patches over large areas. Infestations are often circular in outline. The tough, scaly, segmented rhizomes have sharply pointed tips. Individual plants have no aboveground stem, with the leaves arising directly from the ground. The overlapping leaf sheaths give the base of the plant a rounded appearance.

The leaves are very erect, eventually becoming matted and developing into a mass of thatch around the base of the plant. The leaves are yellowish green in color (becoming red in cool weather), often with a prominent white, off-center midrib. Leaf blades vary from 1/2 to 1 inch across, with hairy bases, finely serrated margins and sharply pointed tips. The ligule (a projection at the intersection of the leaf sheath and the leaf blade) is papery and brown with a fringed margin.

In northern Florida, cogongrass typically flowers at the beginning of the growing season (from March to May), but in central and southern Florida, it may flower throughout the year. Flowering also occurs in response to frost, fire, mowing, herbicide application and other disturbances. The flowers are borne in dense, fluffy panicles that are silvery white in color and cylindrical in shape. The persistent, purplish-black stigmas are obvious amongst the white hairs.

The fluffy, plumose seedheads are easily recognized. A single seedhead measures 2 to 8 inches in length and can contain up to 3,000 seeds. The tiny seeds bear silky hairs along their margins, an aid in wind dispersal.

Distribution

Cogongrass is native to Australia, Micronesia, East and Southeast Asia, India and eastern and southern Africa, but it is now naturalized pantropically. In the United States, it occurs in southern states from Texas eastward to South Carolina and a few other scattered states.

Occurrence in Florida

Cogongrass has been documented in all regions of Florida, from the western panhandle to the Keys. It occurs on both moist and dry sites that are disturbed, but not frequently tilled, such as roadsides, utility corridors, industrial sites, pastures, pine plantations and orchards.

Similar Species in Florida

Cogongrass is easily confused with Brazilian satintail (Imperata brasiliensis), a morphologically similar Neotropical species. The two can be distinguished by the number of stamens in each flower — cogongrass flowers have two stamens while those of Brazilian satintail have only one stamen. Brazilian satintail also typically has narrower leaves and shorter spikelets than cogongrass. Brazilian satintail hybridizes with cogongrass, and some botanists have argued that they should be regarded as a single, highly variable species.

When not in flower, Vassey grass (Paspalum urvillei), Johnsongrass (Sorghum halepense), silver beardgrass (Bothriochloa laguroides) and broomsedge (Andropogon virginicus) may be confused with cogongrass. None of these have the dense, cylindrical plume-like seedhead of cogongrass, nor bloom between March and May. Vasey grass, silver beardgrass, and broomsedge all produce an aboveground stem (lacking in cogongrass), and Johnson grass has wider leaves that do not have serrate margins.

Means of Dispersal

Dense colonies form through the growth and fragmentation of rhizomes. Dispersal to new sites occurs by windborne seed. Rhizomes are also transported by construction machinery and in fill dirt.

Comments

Cogongrass was introduced into Alabama from Japan in 1912 as a packing material for citrus rootstock. In the 1920s, it was introduced into Mississippi as a potential forage. More than 1,000 acres of cogongrass were planted in Florida for forage and soil stabilization in the 1930s and 1940s. Cogongrass has proven to be an unsuitable livestock forage because of its sharp edges and low levels of crude protein and mineral nutrients.

Cogongrass is now recognized as one of the most aggressive weeds in Florida, capable of rapidly choking out and displacing native plant species. Over time, the rhizomes can form a dense mat in the upper soil layers and may constitute as much as 60-80% of the total plant mass. This rhizome system makes the species difficult to control. Removal of the aboveground portions of the plant is easily accomplished, but rapid regrowth occurs if the rhizomes are not killed or removed. The rhizomes break readily, and each segment is capable of sprouting into a new plant. Cogongrass is a fire-adapted species, which thrives and spreads in areas that are periodically burned. Because it burns hotter, faster and more frequently than native fire-adapted species, it is altering the ecology and species composition in Florida’s fire dependent pineland habitats.

A red-leaved variant (identified as var. koenigii or the cultivars ‘Rubra’ or ‘Red Baron’) is cultivated as an ornamental. Commonly known as Japanese blood grass, this red form is reported to be relatively slow-growing, shy to flower, and tolerant of colder temperatures. Research by the USDA has demonstrated that the red cultivar can revert to the invasive wild form. There are also concerns that the temperate red variety might interbreed with and confer greater cold tolerance to the aggressive tropical form, leading to an expansion in its geographic range.