Johnson grass

Botanical Name	Sorghum halepense
Family	Poaceae
Origin	S Europe, N Africa, W Asia

Identification

A perennial summer-growing grass that spreads by seeds and extensive, creeping rhizomes (roots). It's ability to compete with other plants and persist in spite of intensive control measures results form the long, vigorous and highly adaptable rhizomes. These rhizomes may be found about 1 metre deep in the soil, but most are found in the top 200mm where they can be readily distributed by cultivation. Stout, erect stems grow to a height of 500mm to 2m from the rhizomes. Seedlings closely resemble those of maize. Leaf blades are flat, alternate, mostly less than 20mm wide and the midrib is whitish on the underside. Leaf sheaths are ribbed, often hairy on the inside at the junction with the blade, and the ligule is 3.5 to 5mm long and papery. Leaves are often splotched with purple, which is said to be due to a bacterial disease. The flower head is large, pyramidal, purplish, hairy and is 150 to 500mm long. The seed fruits are usually in pairs, although towards the tip of the head they may occur in triplets. The seed is more or less ovate in shape, 4.5 to 6mm long and about 2mm thick. The seed ranges from dark mahogany brown to light reddish, shading to light brown. After maturity, they shatter readily from the seed fruits and can remain viable for 30 months, or longer, in the soil. One plant can produce up to 80,000 seeds per season.

Habitats

It grows in a wide range of temperate to tropical conditions but prefers warm, humid areas with summer rainfall. The foliage is killed by frost but rhizomes can tolerate 2 days with soil temperatures of -3 to -5C.

Impact to Biota and Ecosystems

Johnson grass is an invasive grass that forms dense spreading patches which completely smother other grasses. Overseas, it is the major overwintering host for maize dwarf mosaic, an important virus disease affecting both maize and sweetcorn. Like all sorghums, Johnson grass can be toxic to livestock, especially during adverse growing conditions or periods of new growth. This grass is extremely difficult to control and can become a major problem in pasture and cropping areas.

Dispersal Routes, Vectors, Infestation Sources

Johnson grass can be spread by its seeds or by rhizome fragments during cultivation. The seeds are dispersed by water, on the coats of animals, or in contaminated seeds and feed stocks.

Management

Management
Physical Control	Control work is only to be carried out by persons authorised by AgriQuality. Eradication methods include hand or mechanical clearance, as well as the use of some approved herbicides.
Disposal	Root system and seeds should be burnt at extremely high temperatures.
Chemical Control	Spray with Glyphosate (150mls per 10 litre water) during flowering but before the seed set.
Biological Control
Recommended Approach	Johnson grass is a Notifiable Pest Plant and landowners/occupiers have a legal obligation to report the presence of this plant to the AgriQuality and/or your local Pest Plant Officer who will arrange for the necessary eradication work. It is an offence for anyone to grow this plant. Notifiable Pest Plants are deemed to pose an intolerable threat to NZ and must be eradicated. The Government pays for and organises all eradication work needed on this plant.

Further Comment

A native of Southern Europe, North Africa and Western Asia, Johnson grass was first recorded in NZ in 1908, with occasional recordings until the 1940's. It was presumed to have been eradicated from those original sites but since the 1970's it has been recorded at a number of sites in arable land, pasture, wasteplaces and roadsides. Nine Johnson grass sites have been eradicated from the Whakatane area in the early 1980's through to the late 1990's.

This plant is prohibited from propagation, sale and distribution within New Zealand!