Euphorbia trigona 'Rubra'
Euphorbia trigona' 'Rubra is a red-tinted cultivar of the African milk tree, a fast-growing columnar succulent of uncertain wild origin, generally attributed to Central Africa. Also sold as Euphorbia trigona' 'Royal Red, it differs from the plain green species chiefly in colour: as the plant matures and receives strong light, its three- to four-angled stems, ridge teeth and small deciduous leaves flush a deep wine-red to burgundy, making it one of the most striking easy-care succulents in the hobby.
Care follows the parent species, Euphorbia trigona; see that page for full detail.
Description
Like the species, Rubra builds upright, candelabra-like stems with three or four prominent ribs, edged with paired short thorns and a running row of small, spoon-shaped leaves along the ridges. The defining trait is pigment: new growth emerges green but develops red to purple-red tones across the stems and leaves, deepening in bright light and cooler conditions. In low light the colour fades back toward green, so the cultivar is often grown specifically to bring out and hold its blush.
The plant branches freely from the upper stem to form a dense, tree-like column, and in time can reach well over a metre indoors. As with all Euphorbia, it is not a true cactus; the paired spines and milky latex distinguish the genus from the cacti it superficially resembles.
Every part of the plant exudes a white latex when cut or broken. This sap is an irritant to skin and eyes and is toxic if ingested, so gloves and eye protection are sensible when pruning, and the plant is best kept away from pets and children.
Cultivation
Grow Rubra as for the parent species, Euphorbia trigona. It wants a very free-draining, mostly mineral mix, a bright position, and thorough watering only once the soil has dried through, tapering off in winter when growth slows. It is not frost-hardy and should be kept comfortably above freezing.
The one point that sets this cultivar apart is light. The red pigmentation is light- and stress-dependent, so the strongest colour comes from the brightest position the plant can take — a sunny window or, once acclimatised gradually to avoid scorch, protected time outdoors in the warm season. Too little light leaves the plant green and etiolated; the tall stems are also somewhat top-heavy, so a weighty pot and occasional support help keep a large specimen upright. See Repotting for moving on rootbound plants.
Because Rubra is a selected colour form rather than a hybrid, it comes true from cuttings but not reliably from seed, and it is propagated vegetatively (below).
Propagation
Propagation is by stem cuttings, the standard method for the species. Cut a healthy branch, rinse or blot away the milky sap, and let the cut end callus dry for several days to a week before setting it in a gritty, barely-moist mix to root. Handle cuttings with gloves and eye protection, as the latex flows freely from fresh cuts. Colour on rooted cuttings develops as the new plant establishes and receives good light.
See also
- Euphorbia trigona — the parent species
- Euphorbia — the genus overview
- Soil and potting mix · Watering · Propagation — cuttings · Repotting · Pests and diseases