Bladderworts

What they are

Bladderworts are one of the two largest carnivorous plant genera and the oddest in build. A bladderwort has no roots at all. The plant is a web of thread-like stems that creeps through wet soil, floats in water or runs through moss on a tree, and along those stems hang the traps: hollow bladders, most of them 0.5 to 3 mm (up to about an eighth of an inch) across, with a hinged door at one end. Above ground there are a few small leaves, or in the aquatics a tangle of feathery foliage, and the flowers, which are the reason to grow them. They are two-lipped and spurred, from a few millimetres to about 5 cm (2 in) across, in yellow, white, pink, violet, purple and red, and the best terrestrials flower almost without pause.

The trap is the fastest thing in the plant kingdom. Over about an hour, glands inside the bladder pump water out until the walls bow inward under tension. When a water flea or other tiny animal touches the trigger hairs on the door, the door buckles, opens in about half a millisecond, and the walls spring back, sucking water and prey inside; the door shuts again within a few thousandths of a second. The whole event takes less than a millisecond. The bladder then pumps itself down again, and traps left alone fire by themselves every 5 to 20 hours regardless. Prey is minute: protozoa, rotifers, water fleas, copepods and mosquito larvae in water, and in soil the larvae of fungus gnats.

The genus sits in the Lentibulariaceae beside the butterworts and the corkscrew plants. Counts vary with the treatment, from about 225 described species to at least 235, with higher figures elsewhere. The sundews, at about 250, are usually counted slightly ahead, though some treatments make the bladderworts the largest genus.

Where they grow

Bladderworts grow on every continent except Antarctica and in almost every kind of wet place: waterlogged sand and peat beside pitcher plants and sundews, shallow acid ponds and lake margins, mossy pockets on tropical trees and cliffs, the water tanks of bromeliads and seasonally flooded Australian swamps. Cold-climate aquatics survive winter as dense buds called turions that sink to the bottom; drought-adapted species survive as tubers or seed. The common thread is clean, nutrient-poor water, which for a grower means pure water, no fertiliser in the medium, and for aquatics something like a pond.

The range in cultivation

Terrestrial species are the bulk of the genus and the ones to begin with. In a pot they form a green carpet with the bladders hidden below. Utricularia sandersonii, U. livida and U. bisquamata are the three a shop is most likely to offer, and the whole group is very easy, to the point that many become weeds in a collection. U. subulata, U. dichotoma, U. praelonga and U. tricolor follow; the North American U. cornuta and U. juncea belong in a bog garden.

Aquatic species float free, or anchor loosely in mud, as long stems of thread-like leaves with bladders between them. U. gibba is the small, nearly indestructible one; U. vulgaris, U. macrorhiza and U. inflata are large and need a pond or a sizeable tub. Most cultivated aquatics are temperate and make turions in autumn. They need water chemistry, space and algae control that a windowsill cannot provide.

Epiphytic species from the Caribbean, Central and South America, including U. alpina, U. asplundii, U. reniformis, U. longifolia, U. humboldtii and U. quelchii, grow in moss and debris on trees and cliffs in fog forests, some in the water held by bromeliad leaves. They have the largest flowers in the genus and leaf-like stolons several centimetres long; many have tubers and a resting period. Start with U. alpina and U. asplundii and keep them cool and humid.

Growing them

Light

Bright light to full sun for terrestrials and aquatics; flowering follows the light, and a shaded terrestrial simply stops. Aquatic tubs do well in full sun with the surface partly shaded by companion plants, or under 50 percent shade cloth in hot climates, which also holds algae down. Epiphytes take bright, filtered light.

Water

Pure water only: rain, distilled or reverse osmosis. For terrestrials keep the water level near or a little above the soil surface, which means a deep tray, an undrained bowl or a net pot standing in water; a dry pot can lose the whole colony. Aquatics want still, soft, slightly acidic water the colour of weak tea. Growers condition it with a thin layer of peat, sphagnum or sedge litter on the bottom and let the tub settle for days to weeks before planting, aiming at roughly pH 6.2 to 6.6; a cup of peat to a gallon (about 4 L) of water does the same job. Epiphytes are watered overhead and kept moist in an open mix that drains.

Growing medium

Terrestrials are not fussy. Pure sphagnum, equal parts peat and sand, or three parts peat to one of sand or perlite all work. Shallow pots are fine. For aquatics the medium is the water itself over 2 to 5 cm (1 to 2 in) of peat or leaf litter, in anything from a few litres for U. gibba to a tub of 190 L (50 gal) or a garden pond for the large species. Epiphytes go in equal parts long-fibre sphagnum, perlite and pumice, or live sphagnum, in a net pot or hanging basket.

Temperature

The common terrestrials grow and flower at room temperature all year, roughly 15 to 30 °C (59 to 86 °F). U. livida, U. bisquamata and U. dichotoma take a light frost, and the North American bog species are hardy; the tropical terrestrials and epiphytes are not. Temperate aquatics need a cold winter and tolerate ice on the surface, though not a tub frozen through. Epiphytes prefer cool nights and high humidity; a cool basement or greenhouse suits them better than a warm room.

Dormancy

The windowsill terrestrials have none; they slow in winter light and resume. Temperate aquatics form turions as the water cools in autumn. Leave them to sink in an outdoor pond, or keep them in a jar of the same water in the refrigerator until spring. Many epiphytes die back to tubers once a year and are kept barely damp until new stolons show; U. longifolia is often grown without any rest.

Feeding

Nothing is needed for terrestrials and epiphytes. The bladders take the microscopic animals in any wet medium. Fertiliser in the soil can kill these plants, so give none; some growers mist the leaves at a quarter of label strength about monthly in the growing season. Aquatic tubs do need live food: water fleas (Daphnia) and other pond life, added with a jar of water from an established pond, feed the traps and eat algae.

Common mistakes

Letting a terrestrial dry out, even once. These plants have no reserves.

Putting an aquatic in a jar on a windowsill and expecting a pond. Without settled, peat-conditioned water, companion plants and live food, algae win. Change part of the water when it fouls, never add an algaecide, and keep duckweed out.

Leaving a colony alone for years. Terrestrials exhaust a pot; divide into fresh medium every two or three years.

Sharing trays. U. bisquamata and U. subulata seed themselves into every pot within reach, and U. bisquamata is very difficult to eliminate from a large collection.

Trading restricted species. Some aquatics are listed as invasive weeds; U. inflata cannot legally be sold or moved in Washington State.

Which to start with

Utricularia sandersonii is the usual first bladderwort because it flowers almost continuously at room temperature and needs nothing but a pot that stays wet. U. livida is grown the same way with taller sprays of white to violet flowers; U. bisquamata is easier still and tiny, so give it a tray of its own. Among aquatics, U. gibba is the one to try: it lives for years in a bowl of peaty water with no dormancy and thrives in containers as small as 100 ml (3.4 fl oz). The large aquatics and the epiphytes come later, when you have a pond or a cool, humid growing space.

Going further

Bladderworts lead in three directions: the epiphytes, from U. alpina and U. asplundii to the tepui species, for growers with a cool terrarium or greenhouse; the large aquatics and the unrelated waterwheel plant for a pond keeper who enjoys water chemistry; and the corkscrew plants, grown exactly like terrestrial bladderworts. All bladderworts multiply by division, as the propagation entry sets out, and the water and media guide covers the water they all depend on.