According to a short article in BSBI News a few years ago by James Armitage at RHS Wisley, female and hermaphrodite pampas grasses look rather different, with the flowering plumes of female plants generally reckoned to be more attractive. Certainly most of the available cultivars are female, as are the three that have the RHS Award of Garden Merit.
Most plants are hermaphrodite, even if some of them (hazel, for example) keep their male and female flowers apart. But some plants are dioecious, i.e. they have separate sexes. Some of our most familiar wild plants, such as nettle and red campion, are dioecious. If your holly never has any berries, that’s probably because it’s a male.
An interesting situation arises when only one sex of an alien dioecious plant is imported into the UK. This again involves some very familiar plants; perhaps none more so than Japanese knotweed, where all the plants in Britain belong to the same female clone. Thus our Japanese knotweed is incapable of setting seed, although it does occasionally do so by crossing with other (alien) species of knotweed. This means that apart from bits of rhizome being washed down rivers, it can only get around by being moved around in contaminated soil. The frequency with which it does this is a sign of just what a careless bunch we are.
7:00AM GMT 12 Dec 2014
According to an article in a more-recent BSBI News, another plant we’re going to see a lot more of is winter heliotrope (Petasites fragrans), also a garden escapee that has made itself a bit too much at home. With its large, kidney-shaped leaves and heads of vanilla-scented flowers at this time of year, winter heliotrope is already a familiar sight along many a road verge. But our plants are all male so, like Japanese knotweed, it has so far had to rely entirely on bits of plant being moved around.
Pampas grass (Cortaderia selloana) is more complicated. In its native Argentina, about half the plants are hermaphrodite and about half are female, but the hermaphrodite plants apparently don’t put much effort into producing seeds. In other words, hermaphrodites are effectively male and thus pampas grass is effectively dioecious.
But sharp-eyed botanist Arthur Hoare has now found the female plant growing both in and around Borde Hill Garden in West Sussex. No one knows how it comes to be there, but the head gardener confirms that it’s already a bit of a problem in the garden. The inability to produce seeds has hardly held back the male plant in the wild in Britain, where it is already in more than 2,000 hectads. However, now free for the first time in 200 years to enjoy the delights of some female company, this Mediterranean native may manage to do even better in future.
Pampas grass is highly invasive in some other parts of the world, but Armitage reckons that this preference for females by British gardeners may have slowed down its escape into the wild in Britain; for a long time there weren’t enough males to allow all those suburban females to produce many seeds. But once a few self-sown plants did appear, some of these turned out to be male, which allowed the females to produce more seeds, some of which were male… and so on. Certainly pampas grass made a slow start in the wild in this country; despite being grown here since 1848, it was recorded in only 21 hectads (10 x 10km squares) up to 1986, but is now recorded in 425.
The winter heliotrope is a garden escapee originally from southern Europe
Most plants are hermaphrodite, but not all – and an interesting situation arises when only one sex is imported into the UK