Variegated thistle and its control

New Zealand Journal of Agriculture, Volume 81, Issue 2, 15 August 1950, Page 141

Variegated thistle and its control

By

A. R. DINGWALL,

~ Assistant

Fields Superintendent, Department of Agriculture, Christchurch.

VARIEGATED or milk ▼ thistle (Silybum marianum) is an annual or biennial weed occurring extensively in several North Island areas, particularly in Hawkes Bay and Wanganui (coastal) districts, and less extensively in isolated parts of the South Island. The plant forms a broad rosette of large, shining, variegated green and white leaves with thorn-tipped segments. The tall, robust, branching flower stalks, which attain a height of 7 to Bft. on fertile soils, bear large, spiny, purple flower heads up to 3 in. in diameter. The thistle grows usually in isolated but very dense communities that can completely suppress pasture species and form an almost impenetrable barrier to man and beast. FAVOURED by free-textured, warm soils. variegated thistle gains ingress to thin, open swards on dry. sunny hill slopes; to ground bared by stock camped around trees and along ridge crests: to spoil from newly

excavated drains; to roadsides, stream banks, and waste places, or to ploughed or cleared ground subject to infestation from nearby established stands. With some variation due to district, location, and seasonal influences, seedlings develop strongly with February and March rains, and plants grow to moderately sized rosettes to 18in. in diameterat which they remain throughout the winter. Rapid growth restarts with the advent of warmer spring weather (early September) and flower stalks begin to appear by mid-October. Flowering begins early in November and continues through December and seed is set and ripens prolifically in the new year. Spring-germinating seedlings flower and set seed later in the autumn. Control Measures Control measures . for variegated thistle, in addition to the elimination of existing plants, must include the suppression of the crop of seedlings arising from dormant seed in the normally bare ground that remains.

Ploughing or Grubbing Where ploughing followed by cropping or summer fallow or both and sowing down to a vigorous pasture sward are possible, control is not very difficult. Variegated thistle can prove a serious weed and will continue to propagate in summer-sown winter forage crops, especially of non-inter-cultivated roots. However chou moellier, rape, and greenfeed cereals — especially the last when autumn sown have a more favourable smothering effect. On non-ploughable hill country control is much more difficult and costly. In the past it has been largely confined to hand grubbing, usually in October, when most of the thistles are in the large rosette stage, and before the development of flower stalks. As many small rosettes are missed in grubbing and as weather conditions

about this time are not always favourable for successful oversowing, reinfestation quickly occurs. . Consequently "systematic grubbing over a period of years, often with twice-yearly grubbmg (early summer and autumn), is needed to gain a reasonable measure

of control. This method has become increasingly more difficult and costly with increasing shortages and costs of both permanent and casual farm labour. The 5 per cent, sodium chlorate-lime mixture, as for ragwort, has been used to some extent as a

chemical control. It has not proved very satisfactory mainly because sodium chlorate is not fully effective on variegated thistle and it can also cause severe injury to any pasture species present. Hormone Weedkillers Trials conducted by the Department of Agriculture using hormone-type weedicides as a basis for chemical control of variegated thistle have shown promising results in comparison with hand-grubbing and sodium chlorate methods. Both 2, 4-D and M.C.P. compounds have been successfully used in both spray and powder forms, though dusting offers the more practical means of control on hill country where access and the use of large volumes of water, as in spraying, are difficult problems. Hormones can give complete kills when carefully applied and are less injurious to pasture swards than sodium chlorate, though they retard clover species for a time. Applications as low as 2|cwt. of 1 per cent, dust or 561 b. per acre of 5 per cent, dusts give satisfactory results if evenly distributed, preferably during fine weather but when foliage is damp after heavy dews or recent rain. The smaller quantity of 5 per cent, dust used can be evenly applied if bulked up to about 2cwt. per acre with lime, superphosphate, or serpentine superphosphate. Using the phosphates as the spreading agent has the added advantage of encouraging competition from pasture species without detriment to the weedicide properties of the hormones. Time of Application Hormone treatments should be applied in August-September and again in the following February together with surface sowing of pasture species and topdressing (if topdressing is not applied as a spreader) 4 to 6 weeks after final weed-control treatment. Where long-established, dense stands of thistles exist it may prove more advisable to give two main treatments in consecutive springs, followed by a lighter application or “spot” treatment of individual plants in the second autumn before surface sowing. Whichever method is adopted, isolated plants will continue to appear and will have to be dealt with systematically in the rosette stage either by grubbing or “spot” treatment with hormones. Seeds Mixture and Grazing The surface-sown seeds mixture should contain at least 21b. of white clover and 21b. of subterranean clover in addition to 12 to 151 b. of perennial ryegrass, 51b. of Italian or shortrotation ryegrass, 21b. of crested dogstail, and 1 to 21b. of browntop. If a mob of sheep can be used to trample in the seed after sowing, so much the better. Instances have been recorded of stock poisoning where cattle have eaten variegated thistles during dry periods of feed shortage. Cattle will readily consume the wilted foliage of treated thistles and should be kept off any extensively treated area for at least 4 weeks after treatment. Treated areas should be leniently grazed throughout the period of treatment and until a reasonably dense sward has been established on affected areas.