St John’s wort, common St John’s-wort, Klamath weed, St John’s-wort
Hypericum perforatum L.
Perennial Broad-leaved Weeds
Perforate St John’s-wort is an erect, rhizomatous perennial native in dry grassland, hedgebanks and open woodland (Stace, 1997; Clapham et al., 1987). It is a garden rather than an arable weed but can be a problem in lightly grazed or poorly managed pasture. It is more of a problem weed in countries like Australia where it was introduced for ornamental or medicinal purposes. Perforate St John’s-wort is found throughout the UK and is the commonest Hypericum in England. It thrives on calcareous soils and is most abundant on soils with a pH above 7.0. Perforate St John’s-wort has a preference for well-drained, coarse to medium textured soils, low in organic matter and with a short vegetation cover. It is recorded up to 1,500 ft in Britain. Perforate St John’s-wort has a root system that penetrates over 1 m deep and can survive dry conditions but seedlings are sensitive to drought.
Perforate St John’s-wort flowers from June to September. The flowers are self-pollinated and usually apomictic. some authorities believe the mean number of seeds per capsule is around 450, but others suggest around 50 seeds per seed capsule. There may be 73 capsules per plant. The average seed number per plant is of the order of 15,000 to 30,000, but a single plant can produce up to 150,000 seeds. Seed is shed from August onwards but some seeds overwinter on the plant.
Seeds need an after-ripening period of 4-6 months. Fresh seeds have a greater response to light than older seeds.
Persistence and Spread
Common St John’s-wort forms a persistent seedbank. Seeds persisted for more than five years in cultivated soil. The longevity of seeds in soil and in dry- storage was 8-10 years (Guyot et al., 1962). Seed in dry-storage gave 94% germination after 5 years, while seed stored in freshwater retained up to 47% viability after 4 years and 7% after 5 years.
The small seeds can be blown by the wind. Parts of the seedhead adhere to animals but seeds also survive digestion by stock animals. Seeds have been found in animal droppings. The gelatinous seed coat may aid further dispersal by animals.
Vegetative regeneration is by the production of axillary shoots from the base of the plant. This results in limited lateral spread. In shallow soils, new plants may develop from rhizomes at some distance from the parent. Detached roots and rhizome pieces are able to regenerate if soil conditions remain moist.
Perforate St John’s-wort cannot withstand cultivation and is not usually a problem in cultivated crops. It is readily controlled by tillage and rarely invades properly managed pasture.
The sap of St John’s-wort is acrid and grazing animals will usually avoid the plant. Sheep will graze young plants when forage is scarce. In pasture, cutting and pulling promotes regrowth from the rhizome system. Cutting has little beneficial effect unless repeated at 2-week intervals. Mowing and grazing reduce seed production but promote vegetative spread. Cultivating and resowing pasture will give control if the weed is left to desiccate on the soil surface. Perforate St John’s-wort is tolerant of and even favoured by burning and may occur on burned areas of grassland. Burning results in an increase of plant density. Fire may stimulate seed germination.
Updated: October 2007