St John’s Wort is rather pretty, having a ferny ground cover and a tall stem of small yellow flowers. In medieval times it was favoured as a herb for treating dysentery, rheumatism, gout and hypochondria, and as a charm against witchcraft. It was introduced to Australia as a garden plant in 1875.
However, because of its toxicity to livestock, it has now been declared noxious over all parts of NSW and the ACT. This means all property owners and occupiers must control the weed on their property.
The toxicity of St John’s Wort is caused by the chemical hypericin, a fluorescent red pigment which is contained in oil glands in the leaves. When eaten, this chemical causes the lighter parts of a grazing animal’s skin to become sensitive to sunlight, leading to sunburn and irritation (photosensitivity). Secondary infections may then set in.
While symptoms in cattle and sheep can be extremely acute and lead to death, horses rarely eat enough of the weed to be seriously affected. Horses having no white markings may not show any effect and animals which are accustomed to the weed are less susceptible to its effect than horses which have never been exposed to it. Horses which do react readily to St John’s Wort are those which have white legs and/or white noses. The affected skin becomes dry and scaly and on the lower legs a secondary infection will often take over, creating a far greater problem than the original reaction.
If you find your horse with a skin condition which you otherwise can’t account for, check the pasture to see if you can find any St John’s Wort. In summer and autumn there will be a profusion of yellow flowers, standing on long leafy stems, 30-60cm high. In winter and spring these stems will persist as dead stalks. If your horse has eaten enough to cause a reaction, the plants will usually be quite abundant.
If possible, remove the horse from the paddock and put it in a covered yard or stable for a few days to reduce exposure to sunlight. If you cannot move your horse out of the sun, provide some cover such as a summer rug, a mesh fly veil and a canvas flap hanging down from the nose band of the halter. If the skin condition is severe, call a vet.
Origin: Native of Europe, western Asia, North Africa.
Flowers/Seedhead: Flowers: About 2 cm wide with 5 bright yellow petals. Flowers late spring and summer.
Description: St John’s wort is a serious weed that spreads by seeds and lateral roots. Seed is also spread over short distances by wind, but over long distances by water, machinery, humans, livestock or feral animals. The sticky seed capsules adhere to animals and are also carried in the digestive tracts of animals. Because roots of St John’s wort sucker and grow from fragments, cultivation can spread the weed unless the roots are brought to the surface and dried out.
St John’s wort 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, excludes useful plants from pasture and reduces property values.
Distinguishing features: Distinguished by numerous translucent glands on the leaves (obvious when leaves are held to the light), and yellow flowers often with black glands on the petal margins (look like dots—see photo).
Dispersal: Spread by seed, growth of rhizomes and movement of cut sections of rhizomes.
Flowers with black gland dots on petal margins
Notes: There are two varieties, var. perforatum with broad leaves and var. angustifolium with narrow leaves but intermediates are also common. One plant will produce thousands of seeds and these may remain viable in the soil for many years. Introduced to Australia in 1800s, and still spreading, especially on roadsides and cleared land. Major weed in USA and Canada. In Australia biocontrol has been partly successful but work is still continuing. Hypericin is concentrated in oil glands and causes photosensitisation in light skinned stock especially sheep.
Some more pics of it: