WEED IDENTIFICATION AND CONTROL

New Zealand Journal of Agriculture, Volume 101, Issue 1, 15 July 1960, Page 19

WEED IDENTIFICATION AND CONTROL

This is one of a series ! of articles illustrated in i colour to appear in the I “Journal” from time to I time in which identifi- | cation and control of | various weeds will be discussed. 1

By

L J. MATTHEWS,

Principal Scientific Officer, Department of Agriculture, Wellington

QT. JOHN'S wort [Hypericum perforatum) is the type name of the family Hypericaceae. Tutsan also belongs to this family as do many ornamental species.

CT. JOHN’S wort spreads by underground rhizomes and seeds. The stems are erect, two-edged, and branching and carry masses of yellow flowers. Plants, which vary in height from 1 to 3 ft, are easily recognised by the opposite entire (not toothed) leaves which if held up to the light show a number of small transparent dots and by the numerous stamens being united into bundles. There are no stipules. The three spreading styles are between the bundles of stamens. The anthers do not touch the stigma, so that cross and self pollination depend on insect visitors.

Numerous dark brown seeds are set.

Why is it a Weed?

St. John’s wort is widespread in New Zealand in waste areas, particularly on hill country in some parts of the South Island. Eating of the weed by sheep, cattle, or horses causes them to become photosensitivethe lightcoloured patches of unpigmented skin of affected animals become sensitive to sunlight. The severe irritation which develops

causes the animal to rub, scratch, or bite the affected parts. Sheep, because they graze more closely than cattle or horses, are more prone to eat the leaves and stems of young shoots when these are just coming through the ground. Both leaves and stems cause photosensitivity. Control St. John’s wort will not compete with a productive sward; it is mainly a weed of waste areas and open ground. It is included in the first schedule of the Noxious Weeds Act,

but as it is a weed of waste areas and open ground, it cannot be regarded as seriously as a weed that competes with productive pastures. It is a weed that is declining, probably owing to in-

creased improvement of areas now infested by it. Two methods of control are available to supplement its control by farm management.

Biological Control

The St. John’s wort beetle, Chrysolina gemellata, has successfully controlled the weed in the United States, and the same or a similar beetle (C. hyperici) has proved of value in New Zealand and Australia. Control by biological methods is not adequate unless followed by pasture improvement to preclude re-establishment of the weed. Though the St. John’s wort beetle declines as its host plant diminishes, eradication is never complete, because the St. John’s wort builds up more quickly than does the beetle. Use of Weedkiller

St. John’s wort in waste areas can be controlled by the emulsifiable esters of 2,4-D at rates of up to 2 lb of acid equivalent per acre applied at the flowering stage. If spraying is not followed by practices aimed at increasing pasture production, more than one application is required to kill the plants completely. In waste areas the regrowths should be treated at maximum leaf development.