Princess of the Night

Thick blue stems and night flowers

Selenicereus pteranthus earns both its names. To gardeners it is the princess of the night, for the enormous white flowers it opens after dark, and the snake cactus, for the way its thick, angular stems coil and lean through whatever they can reach. Those stems are stout and bluish, four to six sided, with low, waxy ridges and only short, weak spines, quite unlike the slim, prickly canes of some of its climbing relatives. The stems reach about 3 m long and 5 cm thick, and the waxy ridges often flush purplish in strong light. Along their length grow aerial roots that grip bark, stone or a moss pole, helping the plant climb towards the light instead of standing as a column.

It comes from the warm scrub and dry forest of Mexico, where it clambers over rocks and up trees instead of growing in open desert. In Mexico it is known from Hidalgo, Oaxaca, San Luis Potosi, Tabasco and Veracruz. The flower is the main attraction: on a summer night a mature plant opens a bloom the width of a saucer, pure white within a fringe of tawny outer petals, with a heavy sweet scent for moths and other night fliers. It is among the largest in the genus, up to about 30 cm long and 20 cm across, opening around ten at night and fully spread by midnight, and the epithet pteranthus means winged flower. The flowers generally need pollen carried from another plant to set fruit. By morning the flower has collapsed.

Warmth and bright light

This is a tender plant of warm country, so hold it above about 10 deg C the year round and never let it stand cold and wet at once. It needs bright light with a little soft sun early or late in the day, with shelter from the fierce midday glare that bleaches and scorches the fat stems. A place beside a bright window, or good dappled light under glass, suits the leafless stems. A spot too dark draws the growth thin and usually prevents flowering.

Watering

Water it more generously than a desert cactus, but never let the roots sit wet. Through the warm growing season, water once the top few centimetres of the mix have dried, let the surplus drain, and keep the plant in even, moderate moisture. As light and warmth fall towards winter, cut back and let it rest cooler and drier; that rest helps set the buds it will open the following summer. It appreciates a little humidity around the stems, so misting through hot, dry spells can help.

An open, airy root run

Pot it in a loose, quick-draining medium instead of dense soil, a mix of a peat-free or loam-based compost lightened with orchid bark, coarse perlite and grit so air reaches the roots and water passes straight through. Give the heavy stems something solid to climb, whether a moss pole, a stout trellis or a wire hoop, and let the aerial roots grip it. Alternatively, let the plant trail from a tall pot or a basket. Move it into a larger container in spring once it has filled the old one, easing the tangle of stems onto its new support with care.

Feeding

A tomato feed rich in potassium, or a dilute cactus feed, given monthly through the growing season, supports both the heavy stems and the buds once they form. Keep it well diluted, and stop over the cooler winter rest when the plant is idle. A freshly repotted plant can go a few weeks on its new mix before feeding starts again.

Propagation

The climbing cacti root readily. Cut a length of stem in warm weather, stand it in a shaded, airy spot for a few days until the wound seals, then lay or set it on a just-moist open mix, where the aerial roots quickly anchor it. A rooted cutting reaches flowering size faster than a seedling and keeps the parent’s habit, so a piece from a plant that has already bloomed is the most reliable route. Handle the fat stems gently to avoid snapping them at the joints.

Flowering

Each bloom lasts only from dusk to dawn, so it is worth watching the buds closely. A fat bud swells over several days and takes on a pointed, upturned shape in the last day or two, which shows that opening is close. A plant on a bright sill often opens in the early evening and holds until the small hours. Move the plant somewhere you will notice it that night, and the heavy scent will usually tell you it has opened. A mature specimen may set several buds across a summer, so flowering can happen more than once even though no single flower survives the morning.

Problems

Blackening, soft patches on the stems or a collapse at the base are rot from cold, wet compost, worst through a chilly winter; cut out the ruined tissue, dry the plant off and move it somewhere warmer and drier. Mealybugs tuck white cottony blobs into the rib grooves and areoles; wipe them away with a swab of dilute alcohol. Pale, spindly stems with long gaps and no buds point to too little light, or to a plant that is too young or kept too warm in winter to flower.

Why won’t it flower? The large white blooms appear only on a plant that has reached maturity, and generally only after a cooler, drier spell over winter. Kept young or warm all year, it grows instead of blooming.

How big are the flowers? Very large, often the width of a saucer, white within a ring of tawny petals and strongly scented, open for a single summer night before they fade.

Does it need support? It does best climbing a moss pole, trellis or frame, gripping with its aerial roots, though it will also trail from a tall pot or a basket.

Is it safe around pets? It holds no poison; the only caution is physical, since the stems carry small spines, so keep it where a pet will not brush against it.