WEED IDENTIFICATION

New Zealand Journal of Agriculture, Volume 95, Issue 6, 16 December 1957, Page 547

WEED IDENTIFICATION

VARIEGATED THISTLE

By

D. I. GLUE,

Instructor in Agri-

culture, and L. J. MATTHEWS, Weeds Research Officer, both of the Department of Agriculture, Wellington.

V/ARIEGATED thistle {Silybum marianum) is a serious weed of ’ waste places and open pastures, particularly in coastal areas. It is often prevalent on soils that dry out badly in summer, such as unconsolidated sands or light stony soils. Though variegated thistle can cause severe nitrite poisoning in cattle, and to a less degree in sheep, its seriousness as a pasture weed is due mainly to the fact that it is the largest and quickest growing of the common thistles. TN favourable situations variegated thistle reaches a height of 7ft. with a rosette spread of 3ft. The spikes are so large and strong that stock give the plant a wide berth. Variegated thistle is annual or biennial in habit. Seedlings appear in autumn and grow rapidly through winter and spring. As the plant develops, its large rosette leaves lie close to the ground and completely smother out all other nearby vegetation. Seeds are set in summer, and, though equipped with a hair “parachute” (a pappus), are so large that distribution by wind is relatively ineffective, and most of them fall on to ground that has been bared at the bases of parent plants. Conditions are thus created which are ideal for the perpetuation of the infestation. To break the cycle it is necessary to do two things:— 1. Kill existing thistles before they can smother out the surrounding pasture sward or have had a chance to set seed. The control of seedlings and small rosettes is readily carried out by a blanket application of water-soluble

MCP and 2,4-D preparations at rates as low as gib. of acid equivalent per acre. Spraying should be done as soon as most of the seedlings have struck and are competing with the pasture. Spraying should not be delayed until the plants are fully established, as they build up resistance as they approach maturity. A heavier rate of weedkiller per acre must then be used and greater clover damage occurs, both from smothering by the large plants and the action of the weedkiller. In most districts control' measures may be taken as early as July or August. In some areas autumn spraying is also practised. Variegated thistle is also readily controlled by aerial spraying, the usual weedkiller being a solution of 2,4-D ester in diesel fuel oil. . Water-soluble preparations are less satisfactory for aerial. application. 2. Encourage the development of a dense sward containing vigorous, high-producing pasture species. Variegated thistle seedlings are unable to grow through a vigorous, dense sward, and in practice chemical control has proved of little use unless followed by sward improvement. This may take the form of complete pasture renewal, or the thickening up of the existing sward by applying phosphate, lime, potash, or molybdenum as required, the incorporation of better grass and clover species by oversowing or the use of the sod seeder, and the avoidance of . pasture-management practices such as pugging or overgrazing which lead to a weak and open sward.