Rainbow plants

What they are

Rainbow plants are slender Australian plants, the genus Byblis, whose stems and thread-like leaves are covered from top to bottom in stalked glands tipped with clear glue. In sunlight the drops break the light into colours, hence the common name. Small insects stick and die, and smaller stalkless glands release the enzymes that digest them. For most of the twentieth century no enzymes could be detected and the genus was written off as a “proto-carnivore”; digestive enzymes have since been found in the sessile glands. Neither leaves nor glands move, which, with the colourless glue, is the quickest way to tell a rainbow plant from a sundew.

Eight species are recognised; two are perennials from south-western Western Australia: Byblis gigantea, a branched plant to about 45 cm (18 in), and B. lamellata, a little taller at 60 cm (2 ft), both with pink-purple to white flowers from September to January. The other six are annuals of the tropical north: B. liniflora, small and bushy at 5 to 15 cm (2 to 6 in), B. filifolia at 20 to 60 cm (8 to 24 in), B. rorida to about 30 cm (12 in), B. aquatica, which straggles or floats to 45 cm (18 in) in flooded ground, and the recently named B. guehoi (2008) and B. pilbarana (2014). The flowers are flat, five-petalled and mauve, pink or purple, occasionally white, carried singly from the leaf axils; the petals are 5 to 8 mm (0.2 to 0.3 in) long in B. liniflora and 10 to 15 mm (0.4 to 0.6 in) in B. filifolia, whose cultivar ‘Goliath’ opens flowers nearly 3 cm (just over 1 in) across. In B. filifolia the anthers release pollen only when the flower is vibrated, so-called buzz pollination, and the same is true of the perennials; leaves and flower stalks also move slowly on a swelling at their base called a pulvinus. The family Byblidaceae sits in the Lamiales beside the butterworts and bladderworts; its closest relatives are not settled, and the butterworts are one of several candidates.

Where they grow

The two perennials grow around Perth and north along the sandplains: B. gigantea in sandy-peat swamps and seasonally wet ground, B. lamellata in well-drained white silica sand. Perth has a Mediterranean climate: rain falls in the cool months and the summers are long, hot and rainless. B. gigantea is listed as threatened in Western Australia. The annuals belong to the monsoon tropics of the Kimberley, the Northern Territory and northern Queensland, in sandy soils, seepages and creek margins that are wet through the summer rains and dry out afterwards; B. liniflora keeps to better-drained ground, B. aquatica to depressions that flood. They germinate with the rains, flower from about March to August, seed and die as the ground dries.

For the grower this means two different plants: annuals that want warmth, strong light and a few wet months, after which the aim is seed for next year, and perennials that want cooler nights, no frost, and a sandy mix kept just damp.

The range in cultivation

Byblis liniflora is the one usually offered and the one to start with. It is the smallest species, it rarely branches, its flowers pollinate themselves as they fade, and its seed germinates with no treatment, so a pot can be kept going from its own seed for years; a single line can be kept going for decades, and plants live up to two years under glass. A larger strain was named the cultivar ‘David’ by the Florida grower Brian Barnes in 2010.

Byblis filifolia is the big one, sold under the species name or as the cultivar ‘Goliath’, a heavily branched plant Barnes named in 2009. It is self-sterile and needs pollen from a second, unrelated plant, released by vibration; hold an electric toothbrush against the flower over a sheet of paper and brush the pollen onto another plant. It can be kept from stem cuttings. B. rorida and B. aquatica are grown the same way; B. guehoi and B. pilbarana are seldom seen.

The perennials B. gigantea and B. lamellata are offered as seed more often than as plants. Their seed lies in the soil until a bushfire triggers it, and in cultivation it is treated with smoke or gibberellic acid; the plants build a woody base slowly, and once it is large that base can be divided or its thick roots used for cuttings.

Growing them

Light

Rainbow plants are sun plants. They need at least eight to ten hours of direct light a day, natural or artificial, and the same goes for germinating seed. A dim windowsill produces thin, pale plants with little glue.

Water

Pure water only; the genus is sensitive to dissolved salts. B. liniflora is happy on the tray method with the pot kept wet. Every other species, including ‘Goliath’ and ‘David’, is watered more carefully: add water to the tray only as the pot absorbs it, or let the tray evaporate before refilling it, keep the surface wet but never leave the pot standing in water, and let it dry slightly between waterings. B. aquatica likes wetter ground.

Growing medium

A sandy, nutrient-free mix. Equal parts sand and peat suits most species, with three parts peat to one of sand for B. liniflora; two parts sand to one of peat, with an optional part of pumice, perlite or lava rock, works for everything, and that sandier mix is the one on our propagation page. Put annuals in pots 10 to 15 cm (4 to 6 in) across, since deeper pots prolong their lives, and perennials in 15 to 25 cm (6 to 10 in) pots.

Temperature

The annuals are tropical: 15 to 30 °C (60 to 85 °F) is ideal, plants survive brief spells from 4 to 37 °C (38 to 100 °F), and seed germinates at 20 to 30 °C (70 to 85 °F). The perennials come from a cooler, drier climate and want cooler nights, frost-free, in a cool or warm greenhouse rather than a hot one.

Humidity

Around 50 to 80 percent is ideal, with some air movement; seedlings damp off in still, saturated air.

Dormancy

None in cultivation. The annuals grow, flower, seed and die within a year or two. B. gigantea dies back to its woody base in a dry wild summer, but plants kept watered simply carry on growing and need no rest in cultivation.

Feeding

The plants catch gnats and fruit flies on their own, and nothing is added to the mix. A light monthly misting with dilute fertiliser is optional.

Common mistakes

Sowing untreated seed of anything but B. liniflora and giving up. Untreated seed can take up to a year. The most reliable treatment is a 24-hour soak in gibberellic acid at 10 parts per million, with a small fire of straw or paper over the sown seed as the classic alternative. Much stronger soaks, around one part in a thousand, are also used and have given fast germination, but strong gibberellic acid produces weak, etiolated seedlings, so the weak figure is the safe place to start; the propagation page has the details.

Leaving the big annuals in a full tray, which rots them. Expecting one plant of B. filifolia to set seed alone. Moving annuals: transplant shock kills them, so they are sown thinly in their final pot. And mistaking an annual’s natural death for a failure.

Which to start with

Start with Byblis liniflora from seed in late spring, or at any time under lights: a warm, bright spot, a pot of peaty sand on a tray, and seed scattered on the surface. It flowers within weeks and seeds itself. Move to B. filifolia or ‘Goliath’ once you are prepared to treat seed and keep two plants for pollination. Leave the perennials until you have raised an annual from seed to seed, and preferably grown a dewy pine, which likes the same lean, carefully watered life. A complete beginner should grow a Cape sundew before any of these.

Going further

The genus is small, so going further means the perennials, hybrids and seed work. The Hartmeyers’ cross of ‘Goliath’ with ‘David’ in 2010 produced the first recorded artificial hybrid in the genus, a branching plant with self-pollinating flowers. Seed treatment, pollination, stem cuttings and the perennials’ division are set out in Propagating Byblis. The light and water and media guides cover the two conditions rainbow plants will not compromise on.