Barry Rice

- doing science on a terrestrial planet -

Focusing on U. prehensilis--an Inquisitive Plant
(Carnivorous Plant Newsletter, 1994, 23:2, 34.)

This is the second in a sequence of articles showcasing Utricularia species. In this series I integrate the information in Peter Taylor's taxonomic monograph with my own carnivorous plant horticulture experience. Since Utricularia in carnivorous plant collections are so often misidentified, I will carefully describe the species being highlighted so you can determine if you are growing it yourself. I warn you against trusting the veracity of your plants' name tags, even if they came from seemingly reliable commercial sources (who may be more confused about Utricularia than you!). Unfortunately this is probably also the situation with Drosera, especially rosetted species. The subject of this article is U. prehensilis--a popular plant that is easy to grow and fairly free flowering, although not as floriferous as U. sandersonii or U. dichotoma. Its species name comes from an interesting feature you usually do not associate with carnivorous plants--but more about that later.

Once I showed a pot of a non-flowering Utricularia to my family. When I did, they looked at it and asked questions like "When will it germinate?" and "Died on you, did it?" When I explained to them that the tiny leaves were what I was growing ("No, not next to the 'tiny moss,' it is the 'tiny moss'"), they shook their heads and walked away muttering. I knew then I was out of the will. With U. prehensilis this would not have happened--even out of flower it is unmistakable something is growing in the pot. The leaves are lime green and moderately large--in cultivation they are commonly up to 2.5 cm long and Taylor reports plants with 10 cm leaves. They are only about 3 mm wide, strap shaped, and terminate with a blunt rounded tip. The leaves are multiveined with a central nerve running along the middle of the leaf and many smaller secondary nerves that branch to the leaf margins. If grown in strong light the nerves and other parts of the leaves are often reddish. They usually lie flat on the soil surface (but can be semi-erect), and as in many small Utricularia do not form a rosette--instead they emerge above ground in unpredictable spots. The underground bladders are up to 1.5 mm across, and smaller bladders are often sporadically formed on the undersides of the leaves, especially if the plant is kept very moist.

Cared for properly, a small clump of U. prehensilis will quickly colonize its entire growing area. When the leaves are so densely packed they lie on each other in tangled confusion, the plant may produce a green or golden brown scape about 1 mm thick and round in cross-section. It grows vertically until it is 15 cm or more tall and then it starts to twine. As it grows, the upper several cm waves and wanders through the air in search of something to spiral around. The motion is slow--it takes a few hours for it to move appreciably--but when it finds something it winds tightly about it. Strangely, sometimes the next day you may discover that the plant has unwound itself and wound onto something else. This prehensile nature is the origin of the specific epithet prehensilis. Only the most recently developed 5 cm or more of the scape is mobile, so as the tip continues to grow, the length of non-twining scape increases. The scape always twines to the right when viewed from the side. In other words, a scape winding its way around a stick as it climbed it would wrap itself around the stick counter-clockwise as viewed from above. About twenty Utricularia species may twine, and all grow in this direction except the African plant U. appendiculata which grows in the other direction. Each U. prehensilis inflorescence produces one to several odorless flowers. They are spaced by up to several cm, and mature slowly. When each flower bud is ready to open, the portion of the peduncle it is attached to is no longer twining and has stabilized. No matter what orientation the peduncle might have gotten itself into, even straight down, the pedicel twists around so the flower is borne level.

thumbThe corolla (see Figure to right) is pure yellow and is remarkably similar to yellow-flowered species of Linaria, commonly known as toad-flax or Butter-and-eggs. In my previous article (Focusing on U. calycifida) I defined some botanical terms useful in describing Utricularia flowers and you may find it helpful to refer to my drawing in that issue. The lower lip is about 1 cm long and 1 cm wide. It is approximately obovate--sometimes with a suggestion of having two or four lobes--and its edges often slightly curl towards the ground. A striking feature of the lower lip is its large inflated palate bulge located near where the lower lip emerges from between the calyx lobes. Three grooved indentations parallel to the long axis of the flower divide the bulge into four swollen lobes. The upper lip is a few mm wide and up to several mm long. Its shape varies, and can be strap-like or spathulate with an end that is rounded or, as often the case with my plants, forked to a depth of a few mm. It usually curves over the inflated palate bulge. The spur is conical, sharply pointed, and is as long as or longer than the lower lip. The spur points away from the lower lip by nearly 180 degrees. My flowers average about 1.5 cm long, and this agrees with Taylor's stated range of 0.8-2.0 cm. The two calyx lobes are both ovate or elliptical, and the upper calyx lobe is always at least a little larger than the lower lobe. When the flower is open the lobes are about 5 mm long, but when a fruit is developing they can enlarge to twice this size. Each flower is attached to the peduncle by a pedicel 1 cm long (Taylor reports a range of 0.3-1.8 cm) that is distinctly flattened in cross-section. Where it attaches to the peduncle is a small (1.5-2.0 mm) tapered bract and two shorter and much narrower bracteoles.

Utricularia can be variable in many ways, especially flower color. But as a rule yellow flowered species like U. prehensilis are almost always consistently yellow flowered. In contrast, species that have flowers colored with whites, pinks, lilacs, and purples are prone to great variation in flower coloration (i.e. see the discussion of U.calycifida in the last installment of this series). Of course there are some exceptions to this, for example those yellow flowered plants which can produce small white or reddish cleistogamous flowers (e.g. U. subulata), or two species that are normally purple but with occasional yellow forms (i.e. U. spiralis and U. tortilis). Still, it is a useful rule to remember.

thumbCultivation of this species is easy. It will grow in pure milled Sphagnum or a 2:1 peat-sand mix. I grow mine in moss and my plants flower well. I do not advise live Sphagnum, especially species with coarse and large tufts, because they will overwhelm the Utricularia. As with all my carnivorous plants, I use water purified by distillation or reverse osmosis. I keep the water level 2-5 cm beneath the soil surface but some growers raise the water level to the soil surface or even submerge the plants when they become established. I have found this to be a successful method but if you grow your pot of U. prehensilis in a deep tray of water with other pots of plants, stolons from the plant will quickly grow out of the pot and invade the neighboring pots. This plant enjoys warm temperatures but is not picky, 15-32°C (60-90°F) is fine. If temperatures are too high, the flower scapes may abort, even if they are more than 30 cm long. I grow my U. prehensilis in terraria under fluorescent lights and in the greenhouse under 50% shade cloth. Its cultural needs are easily met, so the only challenge the plant offers is when it flowers. Left on its own, the questing scape will quickly find other nearby plants--including other U. prehensilis scapes--and will wind around them. I forgot about this once and when I checked on the plant a few weeks later, one scape had found my D. regia and the other a large clump of D. binata dichotoma--when I finished untangling the mess I was thoroughly slimed. The easiest way to restrain the plant is to insert a vertical stick into the pot and let the scapes wind around that. I use chopsticks, and when they reach the top I train them back to the bottom and let them climb up again (Figure 2). Since crawling pests such as wingless aphids use toppled scapes as bridges from one pot to the next, training the scapes can decrease the occasional insect problems that inevitably occur in greenhouses.

In the wild, U. prehensilis grows in tropical and South Africa, and in Madagascar. In this range it grows in bogs and often shallow water. It typically flowers during the wet season, but in permanently wet conditions it flowers all year. I keep my plants constantly wet and they flower year round, but most heavily during the late winter and spring.

Growing and studying carnivorous plants is fulfilling for many reasons. We may marvel at their beauty and form, be fascinated by how they fill difficult ecological niches, enjoy the challenges posed by growing them, or even take ghoulish delight in how they devour their prey. But as I watch the curiously probing scapes of U. prehensilis thrash around, fitfully searching the air one day and then sliding against a terrarium wall which offers no foot-hold (and I use that term uneasily) the next, I know it is the only carnivorous plant that makes me laugh.


10 November 2007