One St John’s wort plant can produce 33,000 seeds per year. Seeds can remain viable in the ground for 20 years or more, so it takes only one plant to start a major infestation. Early identification and treatment before plants set seed is crucial, but the time to treat window is very small. St John’s wort plants are hard to identify unless in flower, then it is only a matter of two or three weeks before the seeds mature. Plants do not flower all at once so continuous surveillance and treatment is necessary.

Leaves and branches are always opposite on the stem and when leaves are held up to the light have a perforated appearance. Plants have gold-yellow flowers in October to January, with five petals that grow in clusters at the end of branches.

Seeds are small, cylindrical and black (1mm diameter) encased in a reddish-brown capsule.

St John’s wort spreads by seeds and lateral roots. Sticky seed capsules adhere to animals and pass through the digestive tract of animals where seeds remain viable. Wind will spread the seed over short distances (up to 10m), however, it can be carried long distances by water, machinery and in fodder.

St John’s Wort was introduced from Europe as a medicinal plant in the 19th century and is now a widespread weed.

For more information contact local weeds officer.