Euphorbia polygona
| Light | Bright light to gentle full sun; a few hours of direct sun keeps the ribs tight and the body compact |
|---|---|
| Water | Moderate in the warm season; let the mix dry fully between waterings, and keep nearly dry through winter |
| Soil | Fast-draining, mostly mineral mix (see Soil and potting mix) |
| Temperature | Keep above freezing; happiest in USDA zones 9b–11 |
| Propagation | Seed, or cuttings taken from offset heads |
| Toxicity | Toxic — the milky latex is irritating to skin, eyes and mucous membranes, and harmful if ingested by people or pets |
Euphorbia polygona is a many-ribbed, columnar succulent from the Eastern Cape of South Africa that mimics the look of a true cactus, forming clustering greyish-green heads lined with dark, closely-set ribs. It is best known in cultivation through its ghostly white Snowflake form, whose chalky, powder-white body has made it one of the most sought-after ornamental euphorbias. Despite the cactus-like appearance, it is a member of the genus Euphorbia and, like its relatives, is entirely unrelated to the true cacti — a classic case of convergent evolution.
Description
Euphorbia polygona forms an erect, cylindrical stem that clusters with age, branching from the base into a dense colony of parallel heads. Each head carries many narrow, prominent ribs — often a dozen or more — running vertically down the body and edged with small, hardened tubercles. The skin ranges from dull green to a striking blue-grey, and in the celebrated Snowflake selection the whole plant is overlaid with a pale, almost white bloom.
Like most succulent euphorbias, the plant is dioecious, meaning individual plants are either male or female. In the growing season the rib margins produce small cyathia (the specialised euphorbia "flowers") on short stalks, typically deep purple, sometimes maroon, adding a subtle contrast to the pale stems. Old ribs may retain the dried remnants of these flower stalks as short, spine-like persistent peduncles.
The closely related Euphorbia polygona var. horrida (the "African milk barrel") is often treated under this name and shares the same ribbed, cactus-like habit, differing mainly in its stouter body and stiffer spine-like flower stalks.
Distribution and habitat
The species is native to the Eastern Cape Province of South Africa, where it grows on rocky slopes and in scrubby, semi-arid veld. In habitat it endures strong sun, lean mineral soils and long dry spells, contracting and relying on its ribbed, water-storing stems to ride out drought. The ribbed body is a textbook adaptation: the pleats expand and contract as the plant takes up or loses water, and the vertical grooves help shade the stem and channel the scant rainfall toward the roots.
Cultivation
Euphorbia polygona is a tough, rewarding plant for anyone who can resist overwatering. Grow it in a gritty, free-draining mix in a pot only a little larger than the root system, and give it the brightest position you can — plenty of light keeps the ribs tightly spaced and, in the Snowflake form, preserves the pale, powdery coloration. Water thoroughly during the warm months once the mix has dried out, then reduce sharply as temperatures fall; the plant should be kept nearly dry and frost-free through winter to prevent rot.
The single most important cultivation note is the sap. All euphorbias bleed a milky white latex when cut or bruised, and in E. polygona it is a genuine irritant: wear gloves, keep it away from eyes and mouth, and wash off any splashes promptly. See Watering and Repotting for general technique.
Propagation
The species can be raised from seed, though seed is only produced when both a male and female plant are in flower together. More commonly it is propagated by removing one of the clustering offset heads and rooting it as a cutting. Because of the copious latex, cuttings need special handling: rinse or blot the cut surface to stop the flow of sap, then let the wound callus and dry for several days to a week or more before setting the cutting in a dry, gritty mix. See Propagation — cuttings and Propagation — offsets for full walkthroughs.
Cultivars
By far the most popular selection is the Snowflake form, prized for its dense, chalk-white bloom over blue-grey ribs. Well-grown Snowflake plants look almost frosted, and the effect is strongest in bright light and lean conditions. The var. horrida complex has itself given rise to numerous grower selections distinguished by body colour and the density and colour of the persistent flower stalks.
Common problems
- Rot — the usual cause of loss, almost always from overwatering, a slow-draining mix, or cold wet conditions; heads soften and discolour from the base.
- Loss of bloom / etiolation — too little light causes stems to stretch, ribs to loosen and, in Snowflake plants, the prized white coating to fade to plain green.
- Pests — mealybugs (white fluff tucked between the ribs and around the roots) and, less often, spider mites are the main offenders. See Pests and diseases.
- Latex burns — not a plant problem but a grower one: always handle cut plants with care, as noted above.
See also
- Euphorbia — the genus overview
- Propagation — cuttings · Propagation — offsets · Soil and potting mix · Watering · Repotting · Pests and diseases