WEED IDENTIFICATION AND CONTROL

New Zealand Journal of Agriculture, Volume 99, Issue 4, 15 October 1959, Page 313

WEED IDENTIFICATION AND CONTROL

By

L. J. MATTHEWS,

Senior Scientific Officer, Department of Agriculture, Wellington This is one of a series of articles illustrated in colour to appear in the “Journal” from time to time in which identification and control of various weeds will be discussed.

CALIFORNIAN thistle (Cirsium arvense), one of New Zealand's worst weeds, is widely distributed, except in North Auckland, in pastures, waste areas, and crops. It competes with good pastures, spreading by seed and underground rhizomes and is difficult to eradicate in established pastures.

CALIFORNIAN thistle, also known as Canadian thistle, field thistle, etc., belongs to the daisy family (Compositae). It is dioecious, that is, the male and female organs are produced on separate plants, often in separate colonies. The above-ground parts, which can reach a height of 5 ft, die back each autumn. Control of Established Plants Established plants in old pasture are difficult to control selectively. Ploughing up and cultivation of the old pasture and treatment of regrowth would give better control. However, if control without taking the pasture out of production is to be attempted, repeat applications of MCPA, 2,4-D or MCPB, and 2,4-DB will be required annually. The first application should be made after the plant has reached the maximum leaf stage, that is at flower stalk initiation, with the watersoluble phenoxyacetics and phenoxybutyrics. If the application is delayed

until the early seed-head stage, best results are obtained with emulsifiable esters of 2,4-D. Alternatively the thistles should be mown and treated in autumn with salts and amines of 2,4-D, MCPA, MCPB, or 2.4- Normal rates of application are 1 lb per acre for the phenoxyacetics and 1 to 2 lb for the phenoxybutyrics. Control in Young Pastures Californian thistle is best controlled in newly established pastures or in hormone-resistant crops after cultivation. Cultivation should aim at cutting the rhizomes into short lengths to activate growth so that each plant arises from a short root length. After cultivation the thistle is controlled or killed in new pastures by MCPB or 2.4- at 1 to 2 lb, in peas by MCPB at 1 lb, in lucerne by 2,4-DB at 1 lb, in cereals by MCPA, 2,4-D, MCPB, or 2,4-DB at 1 to 2 lb, MCPA/TCB

(CP 18: 15) at 4 pints, and 2-(MCPP) or mecoprop at 2 lb, and in maize with MCPB, 2,4-DB or MCPA, and 2,4-D at up to 1 lb. It is not controlled by simazine in maize crops. In field brassicas Californian thistle is checked by monochloroacetate at 15 lb per acre. It is controlled adequately by 2 lb per acre of amitrol applied with dalapon for turf destruction before pasture and crop establishment. Amitrol at up to 8 lb per acre has given good control in waste areas; 2,3,6-TBA is also promising, at up to 20 lb per acre. In waste areas thistle has been killed by the substituted ureas (monuron and fenuron) at over 40 lb per acre. It is more effectively controlled by the more soluble soil sterilants, the chlorates and chlorate/borates, than by the largely insoluble preparations such as simazine and the substituted ureas. Control by these chemicals is more costly than with amitrol 'or mixtures of amitrol and 2,4-D.