D. aegyptium is a grass, with characteristic 'bird's foot' digitate inflorescence, up to 50 cm tall.
Annual, never stoloniferous. Culms up to 50 cm tall, up to 5 noded, geniculately ascending, usually rooting from the lower nodes, thus giving the plants a pseudo-stoloniferous appearance, not rarely forming radiate mats, branched from the lower nodes;internodes cylindrical, glabrous, smooth, striate, exserted above, variable in length;nodes thickened and glabrous. Young shoots cylindrical or rounded. Leaf-sheaths keeled, up to 5 cm long, rather lax, striate, tuberculately hairy on the keel or quite glabrous;ligule membranous, about 1 mm long, ciliolate along the upper edge;leaf blades flat when mature, rolled when in bud, linear, tapering to a fine point, up to 20 cm long and 12 mm wide, with 3-5 primary nerves on either side of the midrib, glaucous, usually more or less densely tuberculately hairy along the margins and the keel, less conspicuously so on the adaxial surface towards the tip.
Inflorescence digitate, composed of 4-8 spreading spikes. Spikes 1.5-6 cm long, on maturity often somewhat recurved, greenish-yellow or pallid;rachis keeled, smooth near the base, scaberulous towards the apex, tip mucroniform and curved. Spikelets 4 mm long, strongly compressed, ovate, solitary, sessile, patent alternately left and right on the ventral side of the axis;dense, forming a very flat comb, usually 3-flowered;lower florets bisexual, the upper florets rudimentary;axis without terminal stipe. Lower glume 2 mm long and 2 mm wide, ovate in profile, 1-nerved, sharply keeled, keel scabrid;upper glume 2 mm long excluding the 1.5-2 mm-long awn, oblong in profile, 1-nerved, sharply keeled, keel scabrid. Rachilla slender. Lemmas 3-4 mm wide, the upper smaller in dimensions (but similar), folded about the keel which is scabrid, broadly ovate in profile, lateral nerves delicate and indistinct;uppermost lemma epaleate. Paleas about 3 mm long, 2-nerved, keels scabrid, dorsally concave, shortly bifid at the apex. Three anthers, pale-yellow, 0.3-0.5 mm long, anther cells somewhat remote, with a conspicuous connective. Caryopsis sub-triangular or sub-quadrate, laterally compressed, rugose, light-brown, apex truncate, never convex, remains of pericarp at times visible. (Fisher and Schweickerdt, 1941).
D. aegyptium is a grass, with characteristic 'bird's foot' digitate inflorescence, up to 50 cm tall.
D. aegyptium is usually identified initially by the characteristic 'bird's foot' arrangement of the inflorescence with 4-8 spreading spikes. It is sometimes found as seed during inspections of seed samples.
Producing large quantities of seeds, D. aegyptium is a pioneer grass that quickly colonizes disturbed areas with light sandy soils, often near to coasts or where water accumulates. It is a common component of weed floras throughout the tropics but is rarely reported as an aggressive weed on its own. It is not on federal or state noxious weed lists in the USA and is not recorded on the ISSG database but is recorded by PIER (2016) as invasive on a number of Pacific and American islands including French Polynesia Islands, Micronesia, the Northern Mariana Islands and Hawaii. It is also listed as invasive on islands in the Mediterranean, the USA, Mexico, Costa Rica, Puerto Rico, Virgin Islands and the Lesser Antilles (Vibrans, 2009;Florida Exotic Pest Plant Council, 2011;Chacón and Saborío, 2012;Burg et al., 2012;Rojas-Sandoval and Acevedo-Rodríguez, 2015;DAISIE, 2016;USDA-NRCS, 2016).
D. aegyptium is a ubiquitous weed in many cropping systems around the world. Holm et al. (1977) classified the degree of importance of D. aegyptium on crops in different countries, in decreasing level of severity, as follows: a serious weed of cotton in Thailand;a principal weed of cotton in Australia, Kenya, Mozambique, Nigeria, Sudan, Tanzania, Uganda and USA, of sugarcane in India, the Philippines and Taiwan, of groundnuts in the Gambia and USA, of maize in Ghana and India and of rice in Sri Lanka and India;a common weed of rice in Indonesia, Nigeria and the Philippines, of coffee in Kenya and Tanzania and of tea in Taiwan and it occurs in bananas, pawpaws, cassava, citrus, sweet potatoes and millet in countries of Africa, Asia and Central America.
D. aegyptium has also been recorded in the weed flora of the following crops: aubergines in India;black gram (Vigna mungo) in Bangladesh and India;cassava in the Philippines;chickpeas in India;chillies (Capsicum) in India;cotton in Brazil, South China, India, Nepal, Thailand, USA and Zambia;cowpeas in India;finger millet (Eleusine coracana) in India;groundnuts in Bangladesh, Ghana, India, Senegal and USA;maize in India, Nigeria, Pakistan, Philippines and USA;jute in India;mint in India;mung beans (Vigna radiata) in India;okras in Nigeria;pawpaws in the Philippines;pearl millet (Pennisetum glaucum) in Burkina Faso, Mali and India;pigeon peas in India;potatoes in the Philippines;rice (transplanted) in India, Indonesia and Pakistan;rice (upland) in Cameroon, Gambia, India and Nigeria;sesame in India;sorghum in Australia, India;soyabeans in Ghana, India, Côte d'Ivoire, Pakistan, Senegal;sugarcane in India, Taiwan and Peru;sweet potatoes in the Philippines, Taiwan and USA;tobacco in India;wheat in Bangladesh and India;yams in India and the Philippines.
A. mexicana is an annual herb, up to 150 cm tall with a slightly branched tap root. The stem is erect, branched, usually prickly, pale bluish-green and exudes an unpleasant-smelling yellow sap when cut. Leaves are alternate, without petioles, more or less sheathing the stem, up to 15 cm long, deeply lobed with irregularly toothed, spiny margins;greyish-white veins are conspicuous on the bluish-green upper surface of the leaves. Flowers are solitary, 2.5-4.5 cm in diameter, subtended by 1-2 leafy bracts;sepals 3, prickly;petals 4-6, yellow to pale orange, glabrous;stamens numerous. Fruit is a capsule, spiny, 2.5-5 cm long and 2 cm wide, with 4-6 valves opening at the tip to release numerous seeds. Seeds are brownish-black, nearly spherical, about 1 mm in diameter, covered in a fine network of veins, oily.
A. mexicana forma leiocarpa is a form found in West Africa which has few or no prickles on the stem, leaves and capsule (Lucas, 1962).
A. mexicana is a widespread annual weed primarily associated with agricultural crops and wastelands. It is a major weed of a number of crops in the tropics and warm temperate regions and is persistent as it produces a seed bank. In India in particular, the species is a health hazard and because of its prickliness, is a nuisance to subsistence farmers. In South Africa the seeds of A. mexicana have been declared as 'noxious' as its seeds or bits of seeds may represent a hazard to human or animal health when consumed (NDA, 2001). It is reported as invasive in many countries in Asia, Africa, the Caribbean and Americas, and Oceania (Australia and a number of Pacific island states).
A. mexicana is a weed of most cropping systems, including large- and small-grain cereals, legumes, vegetables, fibre crops (cotton, sisal) and perennial crops (coffee, sugarcane). It appears that any crop has the potential to be infested with A. mexicana if grown within the habitat range of this weed.
The following text is adapted from the Flora of China Editorial Committee (2015). P. arenastrum has procumbent or ascending stems, 15-30 cm tall, branched from base. Petiole is short, articulate at base. Leaf blade is elliptic or oblanceolate, 0.5-2 cm × 2-5 mm, both surfaces with conspicuous veins, base narrowly cuneate, margin entire, apex usually obtuse;ocrea white, 2-3 mm, membranous, 5-7 veined, lacerate. Flowers 3-5, grow in axillary fascicles;with narrowly ovate bracts and acute apex. Pedicel articulate at apex. Perianth is green, 5-cleft to 1/2, veined, margin white;tepals oblong. Stamens 8;filaments dilated at base. Styles 3, very short;stigmas capitate. Achenes (one-seeded fruit that does not open to release the seed) are included in persistent perianth, dark brown, opaque, narrowly ovoid, trigonous, rarely biconvex, 2-2.5 mm, densely minutely granular striate.
P. arenastrum is an annual species native to Eurasia. It is found in field and row crops, orchards, yards, gardens and turf. It readily invades areas compacted by trampling with foot traffic and is therefore frequently found along roadsides, sports fields, vacant lots, gravel parking areas and walkways. This species establishes a taproot, which allows it to survive periods of drought. As a result it can compete with agricultural crops for water and nutrients reducing yields. In California it is reported to have a negative impact on the threatened species Arenaria ursina [ Eremogone ursina ]. P. arenastrum is considered as an environmental weed in parts of Australia and an agricultural weed in cropping systems in Australia and Canada.
Smith et al. (2008) note that P. arenastrum is troublesome in agricultural fields, in particular in alfalfa fields (Medicago sativa), where soil is compacted from wheel traffic.
An annual or short-lived perennial grass growing 20-150 cm in height. Culms (stems) root from the lower nodes, but stems are held upright. The leaf blades are flat, 5-30 cm long;2-10 mm wide. The flowers are clustered in a fluffy oblong or ovate panicle, 5-20 cm long (Langeland et al. 2008;Clayton et al., 2012). Spikelets 2-10 mm long, 2-flowered, the lower floret male, the upper hermaphrodite, densely villous with hairs up to 8 mm long, on very fine pedicels with sparse long hairs. Panicles often have a rosy colour from the long silky hairs attached to the triangular fruits. The colour fades to silvery-white with age (Arizona-Sonora Desert Museum, 2012).
Melinis repens, commonly known as Natal grass, is a short-lived perennial grass native to South Africa, the Arabian peninsula, India, the Seychelles island and Canary Islands. It is already widely distributed in tropical and subtropical regions due to its long use as a pasture grass and ornamental plant. Although considered a weed in many countries, it is not currently regulated. Wind disperses the seeds locally and long distance dispersal happens through the plant and seed trade. It mainly occurs in disturbed areas such as along roadsides and railway lines, but it can spread into natural areas interfering with early successional processes. It is mainly considered invasive in natural grasslands and shrublands. Holm et al. (1979) list it as a ‘serious’ weed in Australia, Brazil and Ghana, and ‘principal’ in Malaysia and Zambia. The dry biomass of the plant leads to an increase in fire frequencies and its dense growth crowds out native early successional species. Currently the main uses promoted for the plant are for reclaiming mined sites and planting as an ornamental plant.
M. repens is considered a significant weed in small-scale cropping systems in northern Zambia (Afors, 1994) and in cotton fields in Brazil (Vieira et al., 1998). It is controlled in citrus crops (Wilcox and Taylor, 1992).
V. myuros is an annual grass of variable size. Culms are erect, slender, tufted or solitary, 5–75 cm tall, mostly glabrous. Leaf blades are finely pointed, 1-14 cm long, 0.5-3 mm wide, with 5-7 pubescent veins on the upper surface. The leaf sheath is split, glabrous or with few hairs. Inflorescences are dense, 5-35 cm long, green-purple in colour. The base of an inflorescence is often enclosed in the sheath of the uppermost leaf. Spikelets on stalks of 1 mm length. Spikelets 5-12 mm long, lower glume up to 1.5 mm long, upper glume 2.5-5.5 mm long. Spikelets have 3-6 florets. Lemmas are 4.5-6.5 mm long, and have awns of 5-15 mm length. Fruits are 3.5-4.5 mm long. (Wallace, 1997;Hickman, 1993).
Vulpia myuros is an annual grass, native to much of Europe and parts of Asia, introduced to the USA, Australia and a number of other countries, and reported as invasive in Australia, the western USA and parts of the Pacific. It outcompetes native species in grasslands of the western US and is a significant agricultural weed. It forms dense swards and its shallow roots suppress growth of native grasses and forbs. Establishment of native plants is strongly hindered once it has become dominant;because it is a winter-annual, it grows rapidly in early spring, thus successfully competing with the slower-growing native perennial grasses. It is a problem weed in pastures and in direct-seed cropping systems. Infested hay can cause injury to livestock due to the sharp seeds. Seeds easily attach to animals and cause losses in the wool industry. Residues of degrading Vulpia plants affect growth of other species including crops.
V. myuros is a weed in direct-seeded cropping systems, especially wheat (Wallace, 1997;Ball et al., 2008). Due to allelopathic compounds and toxic residues of decaying plants, it affects germination and growth of other crops including perennial legumes and grasses. Together with other annual grasses of Eurasian origin, it locally outcompetes native perennial grasses in the western USA including Elymus glaucus, Hordeum brachyantherum, Koeleria cristata, Melica californica, and Nassella pulchra (Brown and Rice, 2000).
Tree to 10 or more metres high, the branchlets usually tomentose when young. Leaves moderately large, an average leaf about 20-foliolate;petiole short, pubescent, eglandular;rachis usually about 2 dm. long, eglandular and otherwise like the petiole;leaflets several to many pairs, lanceolate, 3-8 cm. long and usually about 2 cm. wide, acute apically, obtuse basally, pubescent below, especially along the veins, puberulent to subglabrous above and less dull than below, opposite on the rachis, with 10 or more pairs of prominent lateral veins;petiolules 2-3 mm. long, pubescent. Inflorescence of several terminal or subterminal several-flowered racemes;bracts lanceolate, a few mm. long, caducous. Flowers yellow;sepals 5, obovate-orbicular, markedly unequal, up to 1 cm. long and broad, glabrous to lightly puberulent;petals 5, mostly obovate, markedly unequal, up to 2.5 cm. long and 1.5 cm. broad, subglabrous, venose, short-clawed;stamens 10, 3-morphic;the 3 lowermost the largest, their anthers oblong, about 7 mm. long, short-rostrate apically and dehiscent by terminal pores, the loculi somewhat converging terminally;anthers of 4 median stamens 5-6 mm. long, similar to the 3 lowermost except the rostrum reflexed and the loculi divergent terminally;3 uppermost stamens markedly dissimilar, more or less rudimentary, the anthers distinctly bilobed, each lobe reniform and dehiscent the length of its outer margin;ovary linear, glabrous. Legume linear, turgid-quadrangular, up to 2 dm. long and 1 cm. wide, transversely multiseptate, tardily dehiscent along one margin (Missouri Botanical Garden, 2014).
S. spectabilis is a medium to large tree from tropical America, listed in the Global Compendium of Weeds as an ‘environmental weed’, ‘garden thug’, and ‘naturalised weed’ (Randall, 2012). The species is extremely fast-growing, flowers and sets seed profusely, and re-sprouts readily when cut (Mungatana and Ahimbisibwe, 2010). In Australia it is considered naturalized, has been recorded as a weed of the natural environment and an escape from cultivation, and is labelled an invasive species, indicating its high negative impact on the environment due to its ability to spread rapidly and often create monocultures (Randall, 2007). In Uganda, the species is considered an invasive alien species with high risk to the native flora (Mungatana and Ahimbisibwe, 2010). In Singapore S. spectabilis has been identified as a casual, spontaneous exotic species that survives outside cultivation but does not form self-replacing populations, and relies on repeated introductions or limited asexual reproduction for persistence (Chong et al., 2009). The species is a cultivation escape in Trinidad and Tobago (Irwin and Barneby, 1982) and is considered an invasive species in Cuba (Oviedo-Prieto et al., 2012).
Agroforestry experiments in Kenya showed that while S. spectabilis is useful as hedges for cropping systems, if grown in semi-arid conditions S. spectabilis will out-compete crops for water uptake and suppress crop yields;in the cases recorded, grain yields of maize grown with S. spectabilis or Leucaena leucocephala were reduced by between 39% and 95% (Noordwijk et al., 2004).
The term cropping system refers to the crops, crop sequences and management techniques used on a particular agricultural field over a period of years. It includes all spatial and temporal aspects of managing an agricultural system. Historically, cropping systems have been designed to maximise yield, but modern agriculture is increasingly concerned with promoting environmental sustainability in cropping systems.
The pattern of crops grown on a given piece of land, or order in which the crops are cultivated over a fixed period.