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Evans, H.C. (1997). Parthenium hysterophorus : a review of its weed status and the possibilities for its control. Biocontrol News and Information 18(3), 89-98.

The best form of invasive species management is prevention. If prevention is no longer possible, it is best to treat the weed infestations when they are small to prevent them from establishing (early detection and rapid response). Controlling the weed before it seeds will reduce future problems. Control is generally best applied to the least infested areas before dense infestations are tackled. Consistent follow-up work is required for sustainable management.

Parthenium hysterophorus is a much-branched, short-lived (annual), upright (erect) herbaceous plant that forms a basal rosette of leaves during the early stage of growth. It usually grows 0.5-1.5 m tall, but can occasionally reach up to 2 m or more in height.

Ethiopia is currently the worst affected country in the region. P. hysterophorus is currently considered to be the most important weed both in croplands and grazing areas by 90% of farmers in the lowlands of Ethiopia (Tamado and Millberg 2000) with sorghum yields being reduced by 97% in experimental fields with high densities of P. hysterophorus (Tamado et al. 2002). The impact of this species has also been well documented in Australia and India (Evans, 1997) where studies have revealed that P. hysterophorus is allelopathic and that infestations reduce crop yields and that the weed displaces palatable species in natural and improved pasture (Channappagoudar et al. 1990). In terms of animal husbandry it has also been reported that this noxious weed can reduce pasture carrying capacities by as much as 90% (Jayachandra 1971).

Parthenium hysterophorus can be confused with Ambrosia artemisiifolia (annual ragweed), Ambrosia psilostachya (perennial ragweed), Ambrosia confertiflora (burr ragweed) and Ambrosia tenuifolia (lacy ragweed) when in the vegetative stage of growth. However, P. hysterophorus can be distinguished from all these species by its ribbed stems, and also by white flower-heads (capitula) when it is in flower.

Locations within which Parthenium hysterophorus is naturalised include the Indian sub-continent, south-eastern Asia, tropical/subtropical Australia, eastern USA, southern and eastern Africa, Madagascar and many oceanic islands with warm climates.

Agnes Lusweti, National Museums of Kenya; Emily Wabuyele, National Museums of Kenya, Paul Ssegawa, Makerere University; John Mauremootoo, BioNET-INTERNATIONAL Secretariat – UK.

The Siberian Elm is a small to medium-sized tree or a tall bushy shrub. The bark is dry gray and irregularly fissured. Leaves are elliptical and change from dark green to yellow in autumn. The green and brownish-red flowers are rather small. The fruit is flat, round, and paper like. Each fruit or samara contains a single seed. A impressive local example of a Siberian Elm can be seen in Santa Fe at the Palace of the Govenors.

The stems of Russian Knapweed grow out from from a basal rosette of leaves. The leaves, themselves, are long and lobed at the bottom of the plant and become smaller and less lobed towards the top. The urn-shaped flowers are pink to purple and become straw-colored when mature. Fruits consist of white capsules with tufts of hair. The plant has deep roots and can live up to 75 years or more.

Habitat: disturbed soil in open, sunny areas, roadsides, creeks, fields, pastures, and gardens

Saltcedar, Five-Stamen Tamarisk

Dalmatian Toadflax was introduced into North America in the lat 1800s as an ornamental flowering plant. Since then, it has escaped the garden and spread to the majority of US states and most of the Canadian provinces. It is responsible for significantly reducing livestock production on affected land by crowding out valuable forage. It has no value as food and may actually be harmful to livestock, though most animals avoid it.

Saltcedar is a shrub or small tree with bark that is reddish when young and turning brown with age. The leaves are gray-green and have an unusual feathery quality. The blooms are quite showy with clusters of pale pink to white five-petaled flowers. The fruit consists of capsules that each contain numerous tiny seeds which are dispersed by the wind. The plant’s common name comes from the fact that it is able to tolerate salt water by excreting salt through special glands in its leaves. It produces salt deposits which both kill other species and cause increased erosion.

Spotted Knapweed is a short-lived plant with ribbed stems and a woody texture. The leaves are lobed and grayish-green with small bristly hairs. The flower heads are thistle-like with ray florets that are pink to purple. Typically there are 25 to 100 flower heads per plant. The seed heads are brown, finely haired, and elliptical in shape. There is a short tuft of white hairs at the tip of each seed.

Family: Asteraceae (Daisies)
Size: up to 36 in (91 cm)
Growth: forb/herb; perennial