While most flora have both male and female organs on the same plant, dioecious plants have either male or female individuals.
“We have just 20 or so species like this in Britain, plus the willows and docks,” says botanist Trevor Dines of Plantlife. “Coincidentally, they include the three we associate with Christmas – holly, ivy and mistletoe.”
Other dioecious species are red and white campion, dog’s mercury, stinging nettle, yew and juniper. Ash trees are often polygamo-dioecious – that is, there are a few male flowers on otherwise female trees, and vice versa; sometimes their branches are either all-male or all-female.
Dioecy is important to the conservation of juniper. “Many relict juniper populations in northern England and on islands are single plants, so as lone males or females they are doomed,” says Trevor. “Plantlife is introducing specimens from the other sex to help them start setting seed again.”
Dioecious plant examples
Dog’s mercury
Forms carpets in shady ancient woodland; flowers February–April. Flowers on male plants resemble yellow catkins, and on female plants develop into paired fleshy fruits. But spreads mostly by rhizomes, creating single-sex patches on woodland floors.
Red campion
Abundant plant of woodland, hedgerows and verges, with a long April–August flowering period. Female flower has five tall white styles (the structures that bear the stigmas) at the centre; the male flower has a cluster of 10 yellowish stamens.