Waterwheel plant

What they are

The waterwheel plant is a single species, Aldrovanda vesiculosa, and the only aquatic plant with snap traps. It floats just below the surface as a thin stem 6 to 20 cm (2.5 to 8 in) long bearing whorls of six to nine leaves, spaced along it like the spokes of a wheel. Each leaf ends in a trap 2 to 6 mm (up to a quarter of an inch) long, a pair of hinged lobes fringed with bristles, with 30 to 40 trigger hairs inside. A water flea that brushes the hairs is shut in within about 10 to 20 milliseconds.

The plant has no roots. It grows one to one and a half new whorls a day at the tip while the oldest part of the stem decays, so it stays about the same length; side branches form every few whorls and break away as new plants, which is how it reproduces and how it is propagated. Flowers are rare and seldom set seed. The genus is the sister of the Venus flytrap, but the two traps evolved separately: the waterwheel closes by bending at its midrib rather than flexing the whole lobe.

Where they grow

The natural range spans Europe, Asia, Africa and Australia. It is listed as Endangered on the IUCN Red List: of 379 historical populations in 43 countries, 50 confirmed sites remain, two thirds of them in one region of Poland and Ukraine, and it is extinct in Austria, France, Germany, Italy, Japan and India, mainly through nutrient pollution and falling water levels. Reintroductions are now part of how it is kept alive.

Its sites are specific: shallow, still, brown-stained water over a thick layer of slowly rotting sedge or reed litter, low in nitrogen and phosphorus but high in dissolved carbon dioxide, with abundant zooplankton and no dense stands of other plants or algae. Temperate populations overwinter as turions, dense buds that sink in autumn and float up in spring. The plant is tolerant of each factor alone and intolerant of a bad combination, which is also what happens in a tub.

The range in cultivation

One species, several strains. Green temperate plants from Europe or Japan form proper turions and are what the outdoor methods were developed for. Red subtropical strains from Australia need no cold rest and grow on in a warm greenhouse, but they are prone to algae in a hothouse and are difficult indoors, so they are the second step.

Growing them

Light

Full sun outdoors, shaded to 30 to 60 percent in hot summers, partly with floating plants, which also keeps the water cool and the algae in check.

Water

The water is the whole craft. The figures: pH 6.0 to 7.2 (4.0 to 7.8 tolerated); carbonate hardness unimportant, so tap water is acceptable; dissolved carbon dioxide above 0.1 mmol per litre (about 4.4 mg per litre), from rotting litter rather than a bottle; nutrients low. A depth of 15 to 30 cm (6 to 12 in) works. Let a new tub settle for a week or more before plants go in.

Growing medium

A tub, not a pot. Put a thin layer of pond mud or peat on the bottom, 6 to 8 cm (2.5 to 3 in) of washed sand over it, rooted reed or sedges in the sand, and 2 to 3 cm (about an inch) of sedge litter, soaked first to wash out excess tannin; the litter decays slowly and supplies the carbon dioxide, humic acids and trace of nutrients the plant needs. A simpler version is peat on the bottom, potted sedges for carbon dioxide and a screen over the tub.

Temperature

Growth is best at about 25 to 28 °C (77 to 82 °F); 33 °C (91 °F) is the limit. Temperate strains need a cold winter; hardened turions take frost, but the tub must not freeze solid.

Dormancy

Turions form as the water falls below about 16 °C (61 °F) in autumn and ripen in cold water, when the old shoot rots away. Leave them to sink in a tub deep enough not to freeze through, or keep them in a closed jar of the culture water in a refrigerator at 3 to 5 °C (37 to 41 °F) until April, when they resume growth within days. Never let a turion dry out.

Feeding

Water fleas and other pond animals are both the food and the algae control; a jar from an established pond starts them. Do not fertilise the water. Stalled, pale growing tips are usually boron deficiency; a little clay on the bottom, which releases micronutrients slowly, is the simple precaution.

Common mistakes

Algae is the usual killer, from too much nutrient, too much light or too little zooplankton. The rest: water too clear and poor in carbon dioxide, so plants stop branching; a tub planted the day it is set up; turions left to freeze solid or to dry; and trying the plant indoors in a small container first.

Which to start with

A temperate strain in a tub outdoors in full sun, set up in spring and left to settle. A healthy plant gives 15 to 30 daughter plants in a season, so one piece of stem is enough. Add a red Australian strain once the first tub has run for a year.

Going further

Propagation is simply breaking the stem into pieces, described in the propagation entry, and the same tub serves every aquatic bladderwort. Keep a log: problems show up in the water weeks before they show up in the plants.