Family: Clusiaceae

Status: Declared noxious in category W2 in all south coast Local Government Areas (must be continually suppressed and destroyed).

Description:
An erect branched perennial herb, with small light green to blue-green leaves, arranged in opposite pairs. If the leaves are held up to the light, fine translucent oil dots can be seen in them. Yellow flowers are about 20 mm diameter, in spreading terminal heads. Flowering is in mid-summer. The plant dies back to the rootstock over winter, and does not begin growing again until early summer. At some times of year, the growth habit of non-flowering stems may be prostrate and ground-hugging. The seeds are in papery capsules which dry to dark brown.

Preferred habitat and impacts:
Found in pasture and on road verges, generally in drier parts of the region. St John’s Wort is more common on the tablelands and slopes, where it is a major weed of grazing land, and a serious environmental weed of remnant grassy native vegetation.

The plant is poisonous to stock, dry or fresh, causing photo-sensitisation in pale coloured animals. The faces and mouths become itchy and raw, preventing feeding.

It has been promoted recently for its medicinal uses in treating depression.

Dispersal:
Seed sticks to animals or vehicles, or is spread in contaminated soil. It can be introduced in contaminated hay or chaff. Each plant also spreads via underground runners. These can be chopped up and spread during cultivation. Seed is long-lived in the soil.

Look-alikes:
There are two native St John’s wort species (Hypericum gramineum and H. japonicum), but both are very much smaller than the weed, with flowers only 5-10mm diameter.

A larger weedy species is tutsan (Hypericum andosaemum), but this species is found in cool, moist areas such as the Blue Mountains and parts of the ACT and Snowy Mountains. Several other species of Hypericum are promoted by nurseries, but given the weediness of several members of this group, they would be better avoided.

Control:
Smaller plants can be hand-dug, taking care to remove all of the rhizomes (underground stems), and dispose of them carefully. Spot spraying with a selective herbicide will be preferable to non-selective herbicides, since it reduces the amount of bare ground available for seedlings to re-invade. St John’s Wort does not tolerate strong competition from healthy pasture, so avoid over-grazing, and ensure rabbits are controlled.

Various biological control agents have been released for St John’s wort, but they are unlikely to be very effective in the generally small and scattered infestations found on the coast.