Hormone Control of Nodding Thistle

New Zealand Journal of Agriculture, Volume 90, Issue 5, 16 May 1955, Page 515

Hormone Control of Nodding Thistle

NODDING or musk thistle (Carduus nutans) has become a serious weed in many parts of Canterbury. Because of its rapid spread the Ashburton County Council declared it to be a noxious weed many years ago. In this article W. A. McKellar, Fields Instructor, Department of Agriculture, Ashburton, discusses the weed's habits , and suggests methods of control. The plant is a biennial, similar to and often confused with the Scotch or spear thistle (Cirsium lanceolatum). It is often wrongly called star thistle. It strikes readily from seed and during its first year forms a rosette of large, spiny, dark green leaves up to I Sin. in diameter. Plant food is stored in a large, deep taproot, from which one or more seed heads up to 6ft. high appear during the following spring. THE flowers are bright crimson to purple and. are surrounded by strong spines. The flower head is usually larger than that of the Scotch thistle and grows upright while it is developing, but it droops later, and it is this habit that gives this plant its name. Occurrence Nodding thistle grows on all types of land from heavy arable to the very light and even appears in shingly riverbeds. The plant naturally grows more strongly on high-fertility land, and under these conditions the rosette of leaves formed in the first year smothers out pasture growth in a circle up to 18in. in diameter. In addition, stock avoid grazing close to this circle of spiny leaves. The loss of grazing in a paddock may appear small to the farmer concerned, but the reduced grazing caused by these thistles over the whole of the Ashburton County, for example, is responsible for a considerable loss in production.

Control Until recently the only methods of control were fallowing to kill the young seedlings as they germinated or topping pastures as the seed heads appeared from the rosette in the second year of the thistle’s life. The latter method did not prevent the smothering of the pasture while the thistle was in the rosette or first-year stage. The bare ground left in the

pasture after mowing allowed further thistle seeds to strike with a resultant heavier infestation than the original one. With the advent of hormone weedicides it is now possible to control thistles by spraying. Of the two most suitable hormones for use when spraying thistles in pasture, the waterbased salts of M.C.P. are preferable to those of 2,4-D, as they damage the. white clover least, except in young

pastures. In a young pasture waterbased 2,4-D preparations should be employed at Jib. acid equivalent per acre. The thistles are most susceptible to hormones in the seedling stage, when Jib. acid equivalent per acre of M.C.P. or 2,4-D should give excellent control. Where rosettes have formed and seed heads are appearing from the crown, the pasture should be grazed short to bare the leaves to obtain maximum spray coverage. The hormone should be applied at up to lib. acid equivalent per acre in this instance. For best results spraying should be done in spring (early October) when fine weather is likely to follow for 1 to 2 days after the application. .

Method of Application

The accompanying photographs illustrate the excellent results obtained on : a thistle-infested pasture near Ashburton. Approximately one-third of its area was covered in thistles from the seedling stage up to 2ft. high. On the advice of an officer of the Department of Agriculture the farmer fitted a volume sprayer to his tractor, with a 45-gallon drum to hold the spray. The paddock was sprayed on. a hot, calm day in October, with lib-

of M.C.P. in 11 gallons of water per acre. Some difficulty was experienced in obtaining a complete coverage of the paddock, as there was no marker fitted to the tractor. In addition, an ■. , . allowance was made for a certain amount of spray drift which did not take place through absence of wind.

Little change in the thistles was apparent in the first week, but shortly afterward necrosis in the veins of the fleshy leaves . and twisting of the flower stalks showed that the hormone was killing the plants slowly and sure ly. The photographs show the almost complete kill of thistles in the 14ft. widths covered by the spray

boom. The Ift. to 2ft. strips unsprayed show how thick the thistles were in the paddock before spraying. Conclusion Nodding thistles produce large quantities of seed, and prevention of seeding should be the. aim of every farmer. Where seed is present in the soil it should be fallowed through the summer to germinate these seeds for later destruction. Seedlings appearing in crops or pastures tolerant to hormones, such as cereals or potatoes, can be killed with water-based salts of M.C.P. or 2,4-D, preferably M.C.P., as it causes least damage to most crops. Care should be exercised when pastures containing white clover are being sprayed. White clover seedlings are particularly sensitive to hormones, and the use of M.C.P. should be confined to mature clover plants at a rate not exceeding lib. acid equivalent per acre. A good dense turf will prevent thistle seedlings from appearing. The following recommendations are therefore offered as a guide:— 1. The use of adequate lime and superphosphate. 2. The efficient use of D.D.T. for grassgrub and porina (Oxy conus') control. 3. The removal of headed grass-seed straw. 4. The avoidance of pugging of pastures in wet weather. 5. An application of superphosphate after spraying to strengthen the pasture and assist regrowth of pastures on the bare patches left by the dying thistles.