The St John's wort family
Hypericaceae carries its chemistry where you can see it. Every leaf in the family is dotted with glands, and what is in them explains the folklore, the medicine, the warning that goes with it and the red stain on your fingers.
around nine genera
roughly 700 species
Worldwide in temperate regions and on tropical mountains
Everything in this guide that belongs to Hypericaceae
Take a leaf from anything in this family and hold it against the light. It is peppered with small translucent spots that look exactly like pinholes punched through the blade. They are not holes. They are cavities full of clear essential oil sitting inside the leaf, and the best known species is named perforatum precisely because of how they look.
There is a second kind of gland and it is the one that matters. Along the leaf margins, on the sepals and thickly over the flower buds sit small black or dark red dots, and these hold something else entirely: a deep red pigment.
Crush a few unopened flower buds between finger and thumb and your skin turns blood red, instantly and unmistakably, from what looked like a yellow flower. It is the single best party trick in the plant world and it is also the key to everything else on this page.
The common name comes from timing. The plant flowers around the shortest nights of the year, close enough to the feast of St John the Baptist on the twenty fourth of June for the association to stick across most of Europe. Wort is simply the old word for a useful plant.
The red juice did the rest. Squeezing the buds on that particular date and getting what looks like blood was read as a sign, and the plant was understood to be marked with the blood of a beheaded saint. It was hung over doorways and windows at midsummer, burned in bonfires and carried against storms, illness and the evil eye, and it acquired the Latin nickname fuga daemonum, the flight of demons.
Folklore usually attaches to plants that do something, and this one does. The pigment that made it holy is a genuinely active compound, which is a rare case of the superstition and the pharmacology pointing at the same molecule.
St John's wort is one of the best selling herbal products in the world and it is also the one with the longest list of documented interactions with prescribed medicine. This is not a caution about it being weak. It is the opposite.
It switches on the liver enzymes and transport proteins the body uses to clear foreign compounds, which means it clears out other drugs faster than intended and drops their levels in the blood. The consequences are documented across a long list including hormonal contraceptives, blood thinners, transplant anti rejection drugs, some HIV and cancer treatments and several antidepressants. Anybody taking it alongside a prescription should be running that past a pharmacist rather than assuming a plant cannot interfere.
The same pigment causes a separate problem in animals. Livestock grazing large amounts of it become severely sensitive to sunlight, and pale skinned sheep and cattle burn badly on the muzzle and ears. It is a well known agricultural nuisance in several countries where the plant has escaped.
The flower is a plain yellow saucer with a boss of stamens in the middle, and if you look closely those stamens are not evenly spread. They come in three, four or five distinct bundles, joined at the base into groups, which is a family character and one of the easier ones to see without a lens.
Most of the family is a scruffy yellow flowered plant of roadsides and hillsides, and a good part of it is tropical trees and shrubs nobody outside the tropics meets. The florist crop is a modern invention: a set of hybrids selected not for their flowers at all but for the size, gloss and color range of their fruit, bred over the last few decades into a genuine commercial line.
It is an unusual piece of plant breeding. Taking a medicinal weed and turning it into a berry stem is a long way from where the family started.