Gramineae (Poaceae), the Grass Family
This is an erect, annual grass that can grow to 10 feet high. Stilt roots often develop at the base of the stem. Plants produce abundant side shoots and grow into large clumps. The leaf sheath and the lower portion of the leaf blade are usually covered with stiff hairs that have a small swelling (tubercle) at the base. These hairs break off on contact, penetrating and irritating the skin. The leaf blades, which measure 6 to 20 inches long and up to 1 inch wide, typically have broad white midribs and scabrous (rough to the touch) margins. The ligule is a membranous, ciliate flap, to 1/8 inch long. The slender, cylindrical, unbranched inflorescence is segmented. Each heteromorphic segment consists of two types of spikelets: a sessile (stalkless), fertile spikelet that is embedded in the axis of the inflorescence and a pedicellate (stalked), often sterile spikelet with the pedicel fused to the internode of the inflorescence axis. The spikelets are awnless, narrowly ovate to triangular in shape, and somewhat flattened. At maturity, the seedhead breaks into segments, with the paired spikelets attached. A small, cylindrical gland-like projection, known as an elaiosome, is located at the base of each segment. Rich in lipids and proteins, the elaiosome attracts ants that help to disperse the seed.
Itchgrass is native to tropical Africa, tropical Asia and northern Australia. It has been introduced throughout the Caribbean, Tropical America and the southeastern and central United States. Although it may occur under a variety of moisture, light and soil conditions, at a wide range of altitudes, itchgrass is most commonly found on sunny, disturbed sites with high rainfall or irrigation in subtropical and tropical climates.
This species has been reported from scattered locations throughout Florida, where it occurs in hammocks, clearings, pastures, roadsides and other disturbed sites. It has been vouchered with herbarium specimens from Escambia County in the panhandle to Monroe County (Florida Keys) in the southern peninsula.
Itchgrass may be confused with other grass genera in Florida that have cylindrical segmented inflorescences, such as Hackelochloa, Hemarthria, Coelorachis, Manisuris and Tripsacum. Itchgrass can be distinguished by the combination of larger stature, wider leaves, irritating hairs on the leaf sheaths, hairless inflorescence axis, heteromorphic and awnless spikelets, and pedicillate spikelets fused to the inflorescence internode.
Itchgrass seed is dispersed by ants, birds, livestock, water, and farm machinery. It is also distributed as a crop seed contaminant.
Itchgrass has become a major weed of upland rice, sugarcane, corn, sorghum, soybean and various vegetable crops throughout the tropics. It competes for soil nutrients, water and light, resulting in reduced crop yields, and also hosts insect pests and diseases that affect graminaceous crops. Recent research (Meksawat and Pornprom 2010) has shown that itchgrass is allelopathic (contains chemicals that inhibit the germination and growth of nearby plants). The irritating hairs on the leaf sheaths discourage foraging by livestock and wild herbivores and can cause infections among agricultural laborers. Under optimal conditions, itchgrass plants may begin producing seed six to seven weeks from emergence and continue to produce seed throughout the growing season. A single plant can yield between 2,000 and 16,000 seeds.