Sunday, 20 March 2016

Rottboellia cochinchinensis

Rottboellia cochinchinensis itchgrass, pricklegrass, guinea fowl grass

Here is a real nasty emerging weed which I have noticed popping up all along the KwaZulu-Natal north coast. Watch out for it eradicate as soon as it is discovered before it gets out of hand.

A quick search on the internet turned up the following information about Rottboellia cochinchinensis

Rottboellia cochinchinensis

A native of Indo-China, which has naturalised throughout the tropics of Asia, in north-eastern Australia and the savannah zones of Africa including South Africa.
Rottboellia cochinchinensis 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, Rottboellia cochinchinensis is most commonly found on sunny, disturbed sites with high rainfall or irrigation in subtropical and tropical climates.

Distinguishing Characteristics
Rottboellia cochinchinensis is an erect, annual grass that can grow to 3m 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 150mm to 500mm inches long and up to 25mm wide, typically have broad white midribs and scabrous (rough to the touch) margins.

The ligule is a membranous, ciliate flap, to 3mm 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.

Environmental  and Economic impact
Rottboellia cochinchinensis is an invasive aggressive weed under various ecological conditions, in at least 18 crops in 44 countries. It is estimated that more than 3.5 million ha of cropping lands are infested with Rottboellia cochinchinensis in Central America and the Caribbean
Rottboellia cochinchinensisis a serious weed of cotton in Zambia and Zimbabwe and a moderate weed of cotton in Ethiopia, Mozambique, Sudan, Uganda and Venezuela; a serious weed which effects on crop yield,
It competes for soil nutrients, water and light, resulting in reduced crop yields, and also hosts insect pests and diseases that affect graminaceous crops.
Under optimal conditions, Rottboellia cochinchinensis 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.
Recent research (Meksawat and Pornprom 2010) has shown that Rottboellia cochinchinensis is allelopathic releasing 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.

Rottboellia cochinchinensisis a problem to labourers, as the needle-like hairs on the leaf sheaths break off in the skin and can cause painful infections.

Michael Hickman
Landscape Design and Rehabilitation Specialist


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Wednesday, 16 March 2016

Vasey Grass Paspalum urvillei

Vasey grass Paspalum urvillei is a highly invasive grass which is a native of Argentina and Uruguay which is mostly found growing in damp and disturbed localities in our area.

 A monospecific stand of Paspalum urvillei

Paspalum urvillei is a large dominant grass which produces vast amounts of viable seed that displaces other grasses to form large monospecific stands largely by means of allelopathy and direct competition for resources such as sunlight, water and nutrients.

This photo gives one an idea of the average size of Paspalum urvillei

Allelopathy refers to the beneficial or harmful effects of one plant on another plant, due to the release of biochemicals, known as allelochemicals, from plant parts by leaching, root exudation, volatilization, residue decomposition, and other processes.

This photo give a clearly shows that Paspalum urvillei 
completely displaces all other species

What alarms me most about this invasive alien grass and why I see it as a huge threat in areas that are being rehabilitated is that it spreads very rapidly and colonizing vast areas and that there are no selective herbicides available that can be used to selectively eradicate it as in the case of broad leafed and other non-grassy weeds. The only options to remove this invasive grass are to dig it out or the use of non-selective herbicides such as Glyphosate which would destroy all other vegetation in the areas being dealt with therefore the sooner stands of this invasive grass are located and are eradicated the better before their seed bank increases and spreads.

Michael Hickman
Landscape Design and Rehabilitation Specialist


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