Controlling the cursed Californian thistle

Press, Volume CXIII, Issue 33419, 28 December 1973, Page 4

Controlling the cursed Californian thistle

“Cali” is the abbreviated name unaffectionately given to a species of thistle which has invaded the countryside. One can find them almost anywhere. Californian thistle is a nuisance wherever it is found. It has been declared a noxious weed in some places, is of no value to anyone and is cursed by all. The only things which may delight in its presence are the great variety of insects which are attracted to its strongly honey-scented flowers. The plant, officially known as Circium arvense, is also known as creeping thistle and Canadian thistle. It is a perennial by nature and its aerial parts usually die down in the late autumn. In spring the round white almost fleshly creeping horizontal roots send up a rosette of leaves. From this develops a stem which may grow to three feet. The stem supports a number of v a r i a b 1 y-leafed branches from the tips and axils of which flowers emerge, but not all of the plants’ numerous adventitious roots form a flowering shoot. SEXUAL DIFFERENCES Examination of a number of these plants in flower will reveal differences in their characteristics, for male and female flowers are borne on separate plants. A

reasonably reliable and; quick indication of the sex of a plant is by means of the shape of the ovary —, rounder in the male, more; cylindrical in the female.! Seed is rarely produced ini male plants and only viable! from the female if there is a! male plant in its proximity, when the characteristic plumed seed of the thistle is produced in great profusion. The difficulty in control and eradication of Californian I thistle lies not only in the fact that seed is dispersed far and wide by wind but also in the persistence and activity of its root system. The roots not only remain alive from year to year but they spread and thicken in the process. A lateral spread of 40ft in one season and a vertical penetration of 20ft has been recorded. A further cause of land infestation occurs when isolated plants or patches of the thistles are cultivated and or distributed without a planned follow-up. The re--suiting fragmentation of the! roots stimulates further de-i velopment. ; Control in the garden can! i be achieved several ways! (and it must be positively! I pursued if it is to be successful. Individual plants and even! newly-formed clumps canj sometimes be dug out. Digging must be deep and thor-i lough to ensure that roots or! portions of them are not left 'behind. Every little bit! (missed will develop into an-! iother plant. REPEATED HOEING Another method is re-i peated hoeing at regular intervals, preventing any [ shoot from emerging above; ground level. In time this: will cause the death of the root system, but it is a slow: and laborious process and even then not always sue-: cessful. Californian thistle is sus-i ceptible to a wide range of (herbicides, and here thorjoughness and timing of ap-i (plication and possible fol-i jlow-up is the key to success.! (Established stands present; (the major problem and con-; Itrol is usually not of a per-: : manent nature unless repeti-j

tive spraying is maintained over a number of years. Amitrole T is a widely, used weedkiller which has given good control of Cali(fornian thistle. Although not (a hormone spray it must i still be used with great care, (as with any herbicide, for it is non-s’elective. It can be used around ornamentals and fruit trees and as a spot treatment on lawns and in vegetable plots in safety, provided care is exercised in its application. The combination of picloram and 2-4 D is a hormone mixture which has proved good for use in waste areas, lawns and where spot treatment can be applied in safety to plants between the full-leaf and early-flowering stages; PPCPA, MCPB, 2-4 DB and mecocrop formulations have [also given control. Whatever you use read the proprietary brand label and instructions first.