Weed of the Month: St John’s wort

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St John’s wort (Hypericum perforatum) is a Declared Weed under the Weed Management Act 1999 and is a high priority weed for the Southern NRM Region in Tasmania. St John’s wort aggressively invades roadsides, pastures, grasslands, woodlands, railway lines, forestry areas and riverbanks. Now is the best time to keep your eye out along roadsides and in paddocks for this serious weed.

At this stage its known distribution in southern Tasmania is limited to south of Dover, Franklin area, Cygnet area, Huonville area, Lucaston area, Ferntree/ Mt Wellington area, Glenorchy area, Granton area, Risdon Vale, Sandford area, Lindisfarne and Seven Mile Beach. Infestations are generally small and discrete and most commonly found along roadsides in the above areas. However, in NSW and Victoria where the plant is more widely distributed, farmers have been forced to abandon vast areas of formerly productive pasture due to large, dense infestations of St John’s wort.

St John’s wort is an upright, hardy, perennial herb that may grow to 120cm tall. The plant grows from a tap root, and produces other roots which grow out from the plant, from which new plants may sprout. Branches of this weed are positioned opposite one another and have two slight ridges running along them. Leaves also sit opposite one another in pairs on the branches.

Leaves 5-30mm long and 1.5 – 5mm wide. They have distinctive small translucent glands that look like pin pricks when held up to the light. Flowers are bright yellow with 5 petals and appear from October till March peaking over the summer months. A mature plant can produce up to 33,000 seeds each year and can remain dormant in soil for at least 20 years.

A range of control options exists for St John’s wort, including registered herbicides. Integrated management strategies are required to effectively control infestations.

For the best control method(s) for St John’s wort contact DPIW Weed Management Branch on 1300 368 550. Please report all plants to the STCA Project Manager Weeds Sandy Leighton or phone 0437 450 143.

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