The lowdown on parasitic plants

Many gardeners are familiar with hemiparasites, at least in the form of yellow rattle, which weakens meadow grasses. But complete parasites – plants that lack chlorophyll and steal all their needs from a host – are almost unknown to gardeners, and you can see why when you look at some of them. Dodder (Cuscuta) is related to bindweed, but is even less attractive. A dodder seedling starts off in the ground, but once it twines around and attaches itself to a host, the root dies and it sucks everything it needs from its victim’s stem.

Both our species are quite rare, but the commoner of the two mainly attacks heather and gorse, and a thoroughly infested gorse bush can look like it is completely submerged by a carpet of red string.  Another group of complete parasites are the broomrapes. We have quite a few species, but they’re all fairly rare and rather inconspicuous, so you’re unlikely to notice them at all unless you set out to find them. One or two species can be grown in gardens if you try hard, but since they lack both leaves and chlorophyll and are basically brown, I wouldn’t bother unless your main aim in life is to grow something that none of your neighbours has even considered growing. Again, the RHS Plant Finder doesn’t list any suppliers.  In Spain, one broomrape (Orobanche crenata) is quite a bad weed, attaching itself to beans, peas or sunflowers.

Also in Spain is Cytinus hypocistis, a curious plant that lives entirely inside the roots of cistus and halimium – the only part that ever appears above ground is the striking yellow or red flowers that appear at ground level in spring. These are quite attractive, but I’ve no idea whether anyone has tried to grow cytinus in a garden. You can’t buy it, so your only hope is to take a walk in Andalusia in summer and collect some berries, each of which contains thousands of tiny seeds. Sprinkle these around a cistus bush and see what happens – but don’t hold your breath, and don’t blame me if nothing happens. One parasite that definitely is in cultivation is Lathraea clandestina (purple toothwort), bursting out of the ground in spring like an extra from The Evil Dead.

Purple toothwort is one of those plants that seems to have narrowly escaped being a British native, although it grows wild just across the Channel in Belgium and France. But toothwort has also escaped into the wild in a few places in Britain; like cytinus, it lives completely underground, appearing above ground only at flowering time. The reference books say it’s parasitic on the roots of willows and poplars, and the plant in the Botanical Gardens here in Sheffield grows on a large black poplar. Most other records are also on willows and poplars, but there are also records on hawthorn, birch, beech, hazel and various conifers.

At RHS Wisley, with a vast array of victims to choose from, it has so far been found on silver maple, rhododendrons, holly and hazel. Other recorded hosts include box, hornbeam, bamboo (species unknown), walnut, gunnera and cercidiphyllum (katsura). In Ireland, it has been found on Cordyline australis (cabbage tree), Nyssa sylvatica (tupelo), Lawson’s cypress and even kniphofia (red-hot poker). Some of the weirdest records come from its native France: vines, dogwood, spindle, brambles, Ornithogalum (star of Bethlehem) and – bizarrely – Crithmum maritimum (rock samphire).

How a plant that shows a distinct preference for shady spots by streams ever came to be found on a sea cliff (for samphire grows nowhere else) is a complete mystery, at least to me. Apart from Wisley, other places to see it include Kew, Wakehurst Place, Hidcote, Cambridge Botanic Garden, and the Royal Botanic Garden, Edinburgh. Roy Lancaster is apparently a fan, and the colony at the Sir Harold Hillier Gardens in Hampshire was introduced by him from Cambridge. Roy has also noticed it at Nymans Garden in West Sussex.

Purple toothwort is quite attractive, if a little creepy. If you want to grow it, the Plant Finder lists only one current supplier. The quickest option may be to find someone who already has a plant and can dig you up a section of parasitised root, but even then it may take three years for flowers to appear. The large, round seeds are easy to collect as long as you get to them before they are perfectly ripe, at which point the plant fires them a surprisingly long way, like peas out of a peashooter. Starting with seed, however, is only for the very patient, since plants may take up to 10 years to flower. Worth the wait, though, for anyone with a really tough, shady patch where nothing else will grow. After all, it’s not every day you come across a garden plant that will grow in total darkness.