P. hysterophorus is an erect, much-branched with vigorous growth habit, aromatic, annual (or a short-lived perennial), herbaceous plant with a deep taproot. The species reproduces by seed. In its neotropical range it grows to 30-90 cm in height (Lorenzi, 1982, Kissmann and Groth, 1992), but up to 1.5 m, or even 2.5 m, in exotic situations (Haseler, 1976, Navie et al., 1996). Shortly after germination the young plant forms a basal rosette of pale green, pubescent, strongly dissected, deeply lobed leaves, 8-20 cm in length and 4-8 cm in width. The rosette stage may persist for considerable periods during unfavourable conditions (such as water or cold stress). As the stem elongates, smaller, narrower and less dissected leaves are produced alternately on the pubescent, rigid, angular, longitudinally-grooved stem, which becomes woody with age. Both leaves and stems are covered with short, soft trichomes, of which four types have been recognized and are considered to be of taxonomic importance within the genus (Kohli and Rani, 1994).;Flower heads are both terminal and axillary, pedunculate and slightly hairy, being composed of many florets formed into small white capitula, 3-5 mm in diameter. Each head consists of five fertile ray florets (sometimes six, seven or eight) and about 40 male disc florets. The first capitulum forms in the terminal leaf axil, with subsequent capitula occurring progressively down the stem on lateral branches arising from the axils of the lower leaves. Thousands of inflorescences, forming in branched clusters, may be produced at the apex of the plant during the season. Seeds (achenes) are black, flattened, about 2 mm long, each with two thin, straw-coloured, spathulate appendages (sterile florets) at the apex which act as air sacs and aid dispersal.
P. hysterophorus is known to reduce the yield of various crops and to compete with pasture species in various countries. However, the yield loss and specific effects on the crops have not been quantified in all countries (Rubaba et al., 2017).;In Australia, the main impact of P. hysterophorus has been in the pastoral region of Queensland, where it replaces forage plants, thereby reducing the carrying capacity for grazing animals (Haseler, 1976, Chippendale and Panetta, 1994). Serious encroachment and replacement of pasture grasses has also been reported in India (Jayachandra, 1971) and in Ethiopia (Tamado, 2001, Taye, 2002). The weed is also able to invade natural ecosystems, and has caused total habitat changes in native Australian grasslands and open woodlands (McFadyen, 1992).;In India, the yield losses are reported as up to 40% in several crops and a 90% reduction of forage production (Gnanavel, 2013). P. hysterophorus is now being reported from India as a serious problem in cotton, groundnuts, potatoes and sorghum, as well as in more traditional crops such as okra (Abelmoschus esculentus), brinjal (Solanum melongena), chickpea and sesame (Kohli and Rani, 1994), and is also proving to be problematic in a range of orchard crops, including vineyards, olives, cashew, coconut, guava, mango and papaya (Tripathi et al., 1991, Mahadevappa, 1997, Gnanavel, 2013).;Similar infestations of sugarcane and sunflower plantations have recently been noted in Australia (Parsons and Cuthbertson, 1992, Navie et al., 1996), whilst in Brazil and Kenya, the principal crop affected is coffee (Njoroge, 1989, Kissmann and Groth, 1992). In Ethiopia, parthenium weed was observed to grow in maize, sorghum, cotton, finger millet (Eleusine coracana), haricot bean (Phaseolus vulgaris), tef (Eragrostis tef), vegetables (potato, tomato, onion, carrot) and fruit orchards (citrus, mango, papaya and banana) (Taye, 2002). In Pakistan, the weed has been reported from number of crops, including wheat, rice, sugarcane, sorghum, maize, squash, gourd and water melon (Shabbir 2006, Shabbir et al. 2011, Anwar et al. 2012).;In Mexico, the species is reported as a weed in cotton, rice, sugarcane, Citrus spp, beans, safflower, sunflower, lentils, corn, mango, okra, bananas, tomato, grapes, alfalfa, chili peppers, luffa, marigolds and other vegetables and fruit orchards. It is also a weed in nurseries. In Argentina is reported as a weed of tobacco fields (CONABIO, 2018).;Gnanavel (2013) also reports the following detrimental effects of P. hysterophorus on crops: it inhibits nitrogen fixing bacteria in legumes, the vast quantity of pollen it produces (ca. 624 million/plants) inhibits fruit setting, it is an alternative host for viruses that cause diseases in crop plants, and it is an alternative host for mealy bugs.
Plants typically cespitose, occasionally stoloniferous or rhizomatous, especially when heavily grazed or under frequently mowed. Culms 20-80 cm long, stiffly erect;nodes glabrous or short hirsute. Leaves basal;ligules 0.5-1.5 mm;blades 5-25 cm long, 2.0-4.5 mm wide, flat to folded, glabrous or with long, scattered hairs at the base of the blade. Panicles 5-10 cm, fan-shaped, silvery reddish-purple;rachises 0.5-2 cm, typically with 2-8 branches;branches 3-9 cm, erect to somewhat spreading from the axillary pulvini, usually with only one rame;rame internodes with a central groove narrower than the margins, margins ciliate, with 1-3 mm hairs. Sessile spikelets 3.0-4.5 mm, narrowly ovate;lower glumes hirsute below with about 1 mm hairs, lacking a dorsal pit;awns 9-17 mm, twisted, geniculate;anthers 1-2 mm (Vega, 2000).
B. ischaemum is a warm seasoned perennial grass in the Poaceae family which is native to Europe, Asia and Africa. There are two varities of B. ischaemum, var. ischaemum and B. ischaemum var. songarica, which have different native ranges and have been introduced into different countries. This species was introduced into the North American Great Plains in the 1920s to tackle soil erosion and for forage production. This species has since been planted onto millions of hectares of marginal rangeland, roadsides and Conservation Reserve Program lands (Harmoney et al., 2004). B. ischaemum can readily escape original planting sites where it can invade native rangelands, with negative ecological and economic consequences such as the formation of monocultures and the loss of native biodiversity. This species is a particular problem in Texas where dense monocultures are displacing native grass species.