Utricularia sandersonii

A small plant with a specialized relative group

Utricularia sandersonii is found only in South Africa, in the Eastern Cape and KwaZulu-Natal, where SANBI classes it as a plant of freshwater systems ^[2]. It belongs to section Calpidisca, a mainly African group of about ten small terrestrial species that also includes U. livida ^[8]. Both sit in subgenus Bivalvaria ^[9]. For the genus as a whole, see the bladderwort overview.

Most of the plant is out of sight. Runner stolons only about 0.2 mm thick spread through the soil, and the visible leaves, with stalk and rounded blade together, are up to 15 mm long ^[8]. Rutishauser describes the members of the section as small terrestrial annuals ^[8], while ICPS notes more generally that some terrestrial bladderworts are annuals or short-lived perennials and that even perennial ones die out if ignored ^[3]. Growers keep it for its flowers, which ICPS illustrates ^[3]; flower measurements were not given in the sources used for this page.

How the stolons are built

Rutishauser’s study of cultivated plants at the Zurich botanic garden shows an unusually ordered layout ^[8]:

• Leaves sit in a row along the upper side of each stolon.
• Most traps sit along the sides of the stolons, and some grow from the midrib and stalk on the underside of the leaves.
• Flower stalks, new rosettes and daughter stolons all arise from the upper side.
• Each new rosette sits on the “wrong” side of its leaf, closer to the base of the stolon rather than toward the tip, as if twisted half a turn compared with an ordinary shoot.
• The stolon tips grow straight, unlike the coiled tips of aquatic bladderworts.

The traps are tiny but complete. Their mouths are fringed with radiating rows of gland-tipped hairs, and four-armed glands line the inside wall ^[8]. Płachno’s group found a velum, the soft seal between the trap door and the threshold, in the traps of both U. sandersonii and U. livida, with three distinct zones of glandular hairs in the threshold ^[9]. An earlier study by Westermeier and colleagues had proposed that species in this section lack a velum. Płachno’s team, using light and electron microscopy, found one in all 15 species they examined and suggests the earlier result may come from a method that could not tell mucilage from the waxy cuticle ^[9].

Status in the wild

South Africa’s national Red List assessed it as Least Concern in 2006, with a stable population ^[2]. IUCN has also assessed it; see its Red List assessment ^[4]. Both WCVP and SANBI treat U. treubii and U. sandersonii var. treubii as synonyms ^{[1, 2]}.

Growing it

The ICPS guide to terrestrial bladderworts applies directly. These plants are generally easy, not picky about soil, and grow well in pure Sphagnum or a 1:1 peat and sand mix with the water level near or a little above the surface ^[3]. They need pure water, and too much fertilizer in the soil can kill them ^[3]. See water for carnivorous plants.

Two habits matter in a mixed collection. First, the stolons spread: ICPS jokes that terrestrial bladderworts prefer to live in the pot next to the one you planted them in, so keep pots apart or expect neighbors to be colonized ^[3]. Second, labels drift. Meyers-Rice advised treating bladderwort name tags with healthy skepticism, because plants pass through many hands and the same clone can look different under different conditions ^[10].

Light needs are modest. ICPS rates Utricularia as moderate-light plants, about 8,000 to 15,000 lux of white light, and warns that strong light can encourage algae and cyanobacteria ^[5]. See light.

The species tolerates more cold than its origin suggests. In an ICPS account of a bog garden in North Vancouver, U. sandersonii is listed among the plants that grew outdoors all year; the author adds that many “tropical” carnivorous plants proved surprisingly cold tolerant, but that what worked there may not work everywhere ^[7]. See bog gardens.

To watch the traps, ICPS suggests growing terrestrial bladderworts in a net pot suspended in a glass of water ^[3].

Propagation

• Division: the usual ICPS method is to remove half of the soil and plants from a crowded pot and replace it with fresh mix, which gives two pots ^[3].
• Leaf cuttings: Belanger laid leaves of small species on the usual mix, half covered, at 20 to 30°C and very high humidity, with about half the light given to sundews. Small species showed growth after about six weeks and needed around six months to become established ^[6].
• Seed: sow on the surface of wet to very wet medium, keep the pot in a plastic bag with a little water under lights until germination, then move it to a terrarium; seedlings start slowly ^[3]. See sowing seeds and terrariums and humidity.