Russian Knapweed, a Potentially-serious Weed of Cultivated Land

New Zealand Journal of Agriculture, Volume 78, Issue 1, 15 January 1949, Page 63

Russian Knapweed, a Potentially-serious Weed of Cultivated Land

'T’HE recognition of a small patch of Russian knapweed ( Centaurea repens) in the vicinity of Blenheim early this year shows that another potentially-dangerous weed has Become established in New Zealand. The patch, which is now about -J- acre in area, has .been on this property for longer than 12 years and fortunately has not spread greatly, but that is probably the ' result of the treatment it has had and its environment, for under suitable soil conditions Russian knapweed would be likely to become a very serious weed on cultivated land in New Zealand. J. P. Beggs, Instructor in Agriculture, Department of Agriculture, Blenheim, describes the weed and the methods used in other countries in attempting to eradicate it.

THE patch of the weed at Blenheim is on a clay loam which is often waterlogged in winter and very dry and hard in summer. Fortunately such conditions have not favoured rapid spread of the plant’s vast underground root system. The fact that the paddock has been in pasture for most of the period since its appearance has also had an important effect on its failure to spread to any great extent. The leafage is eaten by stock, so that under grazing little is noticed of the weed and it has no great opportunity of extending and building up reserves in its root system.

How the knapweed was introduced into New Zealand is not certain, but it is significant that it was first noticed in a lucerne stand, and, as it has been most commonly introduced into other countries in lucerne seed, it is possible that it came here in this manner.

The following closely-related plants are already well established in various

parts of < the country, but- nowhere have they become a menace: Knapweed (Centaurea nigra), star thistle (Centaurea calcitrapa), yellow star thistle (Centaurea solstitialis) , and Malta thistle (Centaurea melitensis) .

Description of Plant

Russian knapweed is a perennial which becomes very persistent and difficult to eradicate because of its

extensive underground root system. At different stages of growth the aerial parts of the plant present markedlydifferent appearances. The young shoots take the form of a rosette with leaves about 3in. long and more or less notched. They present a greyishgreen appearance because of a white down or nap which clothes them. The mature plant has narrow leaves 1 to 2in. long, with an even margin. These leaves are smooth, with no pronounced midrib or stalk.

The plant is a type of thistle, but bears no thorns or prickles, and the much-branched seed head resembles that of other thistles. The flowers are lilac coloured and the thistle-like seeds bear a tuft of hairs at one end, very much like the star thistle seed. The stems of the mature plant, which are about 2ft. high, are woody and tough, being inedible when dried out. However, stock will graze fresh growth quite readily.

The stems extend into the ground to a depth of 6 to Bin., where they join a vast horizontal root system which spreads out in net-like fashion. From this horizontal system vertical roots descend at frequent intervals to a depth of 6ft. or more. The roots and lower stems are covered with a brown to black skin.

This extensive root , system provides a capacious storehouse for plant foods, so that the plant is persistent under continued cultivation. It also allows the knapweed to make very rapid growth in spring, with the result that few plants can withstand its smothering effect.

Occurrence in Other Countries

Russian knapweed originated in the Caspian region of southern Russia. It is a notorious impurity in lucerne

seed from Turkestan, and apparently found its way to the United States of America as such an impurity, for it was first recorded there in a lucerne paddock in 1930. Ten years later it was recorded in all the western and central States. It is also present in Canada, and was recorded in Australia in 1930. In the U.S.A, it is a serious weed in cereal crops, and in Australia it is troublesome in Murray River vineyards.

In all these countries Russian knapweed is regarded as a very serious weed and is responsible for heavy losses in production. In Australia vigorous efforts are being made to eradicate it before the infestation becomes very heavy.

Methods of Control

The ' extensive underground root system of the plant causes it to be extremely persistent in the face of prolonged cultivation. Overseas experience has indicated that cultivation at fortnightly intervals for two years or longer is necessary to eradicate the weed. The farmer who undertakes a programme of cultivation to eradicate Russian knapweed is faced with, a formidable task. Should weather x or other conditions upset faithful adherence to the cultivation schedule, the weed will be allowed to return further food supplies to the roots and the cultivation period will have to be correspondingly extended. The use of hormone weedkillers has not been successful in other countries, and the weed is reported to be resistant to control in this . way.

The main line x of attack on small patches of Russian knapweed in America has been the use of weedkillers such as sodium chlorate and acid arsenical sprays. Heavy applications of these (for example, 60 to 901 b. of sodium chlorate a square chain) are reported to give satisfactory control, but this method of eradication is applicable only to small areas. 5 /.-

The building of a bank round a patch of the weed followed by flooding with water for a period of up to eight weeks has also been mentioned as a method of destroying Russian knapweed, 'but that method has obvious limitations. - Therefore it is apparent that Russian knapweed is a very difficult weed to eradicate, and all known successful methods of control are very expensive to apply. Importance of Early Recognition - Early recognition of all weeds is the main step in their control, and that is particularly true in the case of Russian knapweed. Had nassella tussock been identified within a few years of its introduction into New Zealand, and had its potential danger been recognised, its eradication would have been simple, but now it presents a difficult problem. It is of the utmost importance that farmers become familiar . with all plants growing on their farms and with their potential worth or danger, for should a new and serious weed appear it is extremely important that immediate steps be taken to eradicate it before it has become widely established. Fortunately the eradication of Russian knapweed at Blenheim should present no serious - difficulties, but a careful watch must be kept for its reappearance. Control Measures in New Zealand The farmer on whose property the knapweed is present has fenced the area with the intention of keeping it grazed continually. As the plant is comparatively palatable, that will assist in reducing its chances of spreading. In addition, a trial of various kinds of chemical weedkillers has been laid down. As the plant is strong growing and twitch-like, conclusive results are unlikely to be secured with one application. In the meantime the farmer has treated the area with sodium chlorate in the hope that this may check the knapweed. It appears that this knapweed infestation came to New Zealand in lucerne seed, . but the sale of lucerne and other seeds containing Russian knapweed is prohibited in Victoria and possibly other States of Australia. That- reduces to a minimum the chance of seed of this weed being brought into this country from Australia, and the amount of lucerne seed coming in from other countries is now negligible. ' Illustrations taken from Bulletin 292, Agricultural . Experiment > Station, North Dakota Agricultural College, U.S.A.