WEED OF THE MONTH: St John's Wort (Hypericum perforatum)

St John’s Wort is an upright perennial herbaceous plant with bright yellow flowers with five petals and occasionally small black dots. The leaves are opposite, oblong, about 1 inch long, with tiny clear dots that can be seen when held up to light. It reproduces by rhizomes, creeping stems and seeds. One plant can produce 100,000 seeds in a year.

St John's Wort competes with useful plants in pastures and large infestations reduce property values. The plant contains the toxin hypericin, which causes photosensitisation in sheep, cattle, horses and goats. The skin damage associated with this problem leads to weight loss, reduced productivity and, in extreme cases, death. St John’s Wort also adds vegetable fault to wool.

The most cost-effective and practical control techniques to use will depend on the scale of the St John’s Wort infestation and the topography of the infested land. All techniques should aim to remove the weed and replace it with introduced or native pastures.