Sundews

What they are

A sundew is any plant of the genus Drosera. With about 250 to 260 species it is the largest carnivorous genus, and the number keeps rising. The name comes from the Greek for dewy, and it describes the trap. Every leaf carries stalked glands, each tipped with a glistening drop of sticky mucilage; an insect that lands is held, and the stalks around it, usually called tentacles, bend slowly towards it. Darwin, who began experimenting on the round-leaved sundew in 1860 and published Insectivorous Plants in 1875, showed that a single tentacle responds to the touch of a tiny fragment of insect and that the leaf secretes a digestive fluid he compared to gastric juice. In many species the blade itself folds around the prey over several hours. Smaller, stalkless glands break down the insect’s body and absorb what is released. One Australian annual, Drosera glanduligera, has dry marginal tentacles that flip a walking insect onto the sticky centre in about 75 milliseconds.

The plants range from pygmies with rosettes 15 to 20 mm (0.6 to 0.8 in) across to fork-leaved plants with leaves 10 to 20 cm (4 to 8 in) long and tuberous climbers that scramble through other vegetation. The flowers are more uniform: five petals, usually white or pink, on a slender stalk, opening for a few hours on sunny mornings, and many species pollinate themselves as the flower closes.

Where they grow

Sundews grow on every continent except Antarctica, but unevenly. South-western Australia holds more species than anywhere else; South America, southern Africa and Madagascar are the other centres. A few hardy species are circumboreal: Drosera rotundifolia grows in sphagnum bogs from Alaska through Canada and northern Europe to Japan, and the south-eastern United States has small rosetted species in bogs, fens and marshes.

All these habitats are wet, acid, nutrient-poor and open; what they do not share is a climate. A northern bog is frozen for months, the sand plains of Western Australia are bone dry all summer, and northern Australia has months of warm rain and then drought. So the soil and water rules are the same for every sundew, and the seasonal rules depend on the group.

The range in cultivation

Botanists now place the genus in named sections; growers still use informal groups based on care, and these are the ones a beginner meets.

The Cape sundew and the rosetted subtropicals. Drosera capensis and a hundred or so low rosetted species from southern Africa, Australasia, Asia and the Americas form the easy group: D. aliciae, D. spatulata, D. venusta, D. slackii and more. They grow all year, need no dormancy and tolerate about 10 to 35 °C (50 to 95 °F). The taller Brazilian highland species such as D. villosa are not beginner plants; they want cool nights of 10 to 16 °C (50 to 60 °F).

Temperate sundews with winter buds. Drosera rotundifolia, D. anglica, D. intermedia, D. filiformis and D. linearis stop in autumn and form a tight bud of scales, a hibernaculum, which must spend the winter cold and just damp. Expect about six months of rest from the first two and up to nine from D. linearis, the hardest of the genus. These are plants for an outdoor bog in a cold climate.

The fork-leaved sundew. Drosera binata from eastern Australia and New Zealand has long stalks ending in a blade that forks once or many times. It is a big plant for a large pot, and every form dies back to ground level at some point.

Pygmy sundews. More than fifty tiny species from south-western Australia grow through the mild wet winter and sit out the summer as a bud of silvery hairs. Each autumn they make gemmae, pinhead-sized buds that rain splashes off the plant and that root into clones. They want intense light, a very sandy mix, and to be left alone: the single long root does not survive transplanting.

Tuberous sundews. Some seventy species, nearly all Western Australian, spend the dry summer as an underground tuber, sprout in early autumn, grow and flower through the cool wet winter and die back by late spring. The pot is dried off for summer and re-wetted in autumn, the plants are kept below about 25 °C (77 °F), and nothing is moved except as a dormant tuber. The D. peltata complex, the D. stolonifera complex and D. menziesii are the ones to start with.

Woolly sundews, the Petiolaris complex. About thirteen species from monsoonal northern Australia with hairy, often white-felted leaf stalks. They resent anything below 18 °C (65 °F), enjoy the low 30s °C (high 80s °F) in summer, and are given less water in winter rather than a rest. D. paradoxa and D. petiolaris are the easiest.

The Queensland rainforest trio. Drosera adelae, D. prolifera and D. schizandra suit a terrarium or a cool, humid greenhouse. D. adelae is tolerant; D. schizandra declines above 25 °C (77 °F) and burns in direct sun; D. prolifera is scorched by fertiliser.

Growing them

Light

Nearly every sundew wants strong light, and a lack of it is the first thing to suspect when leaves are green and dry. The subtropical species want more than a windowsill gives; pygmies and tuberous species want full sun or a bright lamp on a natural day length. The Queensland species are the one group grown out of direct sun.

Water

Rain, distilled or reverse-osmosis water, never tap water. Most groups stand in a tray through the growing season; the temperate species sit in water all season and can be flooded in hot weather. Tuberous sundews are lifted from the tray and allowed to dry for the summer rest.

Growing medium

Sphagnum peat with washed sand or perlite, or long-fibre sphagnum, with nothing added. A general mix is three parts peat to one of sand or perlite; use one part peat to two of sand for pygmies and tuberous species, because sand controls moss. Peat-free mixes of farmed sphagnum and pine bark also give good results.

Temperature

The subtropical group tolerates about 10 to 35 °C (50 to 95 °F). The temperate species need cool summers and a cold winter; D. rotundifolia is difficult in warm climates. Tuberous sundews are kept as cool as possible in growth and woolly sundews warm all year.

Dormancy

None for the subtropical rosettes, the woolly sundews or the Queensland species, though all slow in winter. Temperate species rest about six months as a hibernaculum kept near freezing and barely damp; in a mild climate, lift the bud after die-back and keep it in the refrigerator in a sealed bag with a little damp sphagnum. Tuberous species rest dry through summer. Pygmies rest in summer heat as a stipule bud. The fork-leaved sundew rests in winter in every form.

Feeding

Sundews feed themselves wherever there are gnats and fruit flies. Indoors with nothing to catch, soak dried bloodworms and lay them on dewy leaves, and never fertilise the medium of any sundew. A monthly foliar spray at a quarter of label strength or weaker can be used with care.

Common mistakes

The commonest mistake is treating every sundew like a Cape sundew: a temperate species on a warm windowsill never gets its winter, a tuberous species kept wet in summer rots, a woolly sundew in an unheated room dies of cold. Tap water and fertiliser in the medium kill sundews slowly. Dim light produces green, dewless leaves that growers mistake for a humidity problem. Pygmies and tuberous species die when transplanted; they are moved only as gemmae or dormant tubers. Avoid soap-based insecticides on sundews.

Which to start with

Begin with the Cape sundew. It tolerates a windowsill, a terrarium or a greenhouse, catches its own food and needs no rest. Drosera spatulata and D. aliciae belong beside it: small year-round rosettes that take the same water, mix and light.

The round-leaved sundew is the right second step for someone with an outdoor bog container in a climate with real winters. It is harder than the Cape sundew because it must have a cold six-month rest and dislikes hot summers; it is not a houseplant. Leave the tuberous and woolly sundews, D. schizandra and D. linearis until the easy species are second nature.

Going further

From the easy rosettes the usual routes are the fork-leaved sundew, the pygmies and then the tuberous species. The methods of propagation differ by group; see Propagating Drosera. The guides on water and growing media, light and dormancy cover the three things that decide success.