St. John’s Wort

St. John's wort bursts into bright yellow flowers around St. John's Day, 24 June, which gave it its name. It came over from Europe and now grows in meadows, on roadsides and in dry fields. It's toxic to grazing animals, and in some places it's a serious weed.

How to spot it

• Leaves are small, oblong and grow in pairs without stalks. Hold one up to the light and you'll see tiny clear dots, as if someone poked it with a pin. That's the perforatum in its name.
• Flowers are bright yellow, about 2 cm across, with five petals and a burst of long stamens. Look closely and you'll see tiny black dots along the edges of the petals.
• Crush a flower bud and your fingers turn red-purple. That's hypericin, one of the plant's active compounds.
• Stems are branched, with two raised lines running along them.

What it's good for

St. John's wort is one of the best-studied herbal medicines. Clinical trials show it can help mild to moderate depression about as well as standard antidepressants.

The flowers soaked in oil make a deep red oil that has long been used on bruises, burns and nerve pain.

A serious warning

This plant interacts with a lot of medicines. It can stop the birth control pill, blood thinners, HIV drugs and transplant drugs from working properly. Taken with antidepressants, it can cause a dangerous reaction. Talk to a doctor before you use it.