Euphorbia
Euphorbia is one of the largest genera of flowering plants — a vast, cosmopolitan group in the family Euphorbiaceae that includes annual weeds, garden perennials, tropical trees and, of greatest interest to succulent growers, a huge assemblage of drought-adapted species. Many of these succulent euphorbias have evolved to look strikingly like cacti, with swollen, ribbed, often spiny stems, yet they are only distantly related. Every species shares two defining features: a caustic, milky latex that bleeds from any wound, and a tiny, highly specialised flower-like structure called a cyathium.
Description
Euphorbia is enormously diverse, but the succulent members share a recognisable set of traits. Most store water in thickened green stems that take on globular, columnar, candelabra, medusoid (a central head crowned with radiating arms) or many-angled cactus-like forms. Leaves are frequently reduced, shed in drought, or present only briefly on new growth, leaving the green stem to do the work of photosynthesis.
What look like "spines" are not the same as a cactus's areole-borne spines. In Euphorbia they may be hardened flower stalks (peduncles), paired stipular spines, or modified structures — but there is never the felted areole that defines a true cactus. This is one of the easiest ways to tell the two apart.
The flowers are unlike almost anything else in the plant world. Individual blooms are so reduced that they are gathered into a cup-shaped false flower, the cyathium, which typically surrounds several naked male flowers (each a single stamen) and one central female flower. Many species carry colourful nectar glands or petal-like bracts around the cyathium, so what appears to be a "flower" is really a small inflorescence. The genus as a whole is mostly monoecious, but a good number of succulent species are dioecious, with male and female cyathia on separate plants — worth knowing if you hope to set seed.
All parts exude a white latex when cut. This sap is irritating to skin and eyes and toxic if ingested; in some species it is severely caustic. Treat every euphorbia with respect and see Common problems and safety below.
Distribution
The genus is genuinely cosmopolitan, found on every continent except Antarctica. The succulent species, however, are concentrated in the arid and semi-arid regions of the Old World — above all continental Africa, Madagascar, the Arabian Peninsula, and parts of India. Southern Africa and Madagascar are particular hotspots of succulent diversity, home to everything from ground-hugging cushions to tree-sized specimens. This Old World distribution mirrors the New World home of the true cacti, and the resemblance between the two groups is a classic example of convergent evolution — unrelated plants arriving at similar solutions to the same dry conditions.
Notable species
A few widely grown succulent euphorbias give a sense of the range:
- Euphorbia obesa — the "baseball plant", a neat, near-spherical ribbed body prized by collectors.
- Euphorbia tirucalli — the "pencil cactus" or "firestick", a shrubby, near-leafless plant with slender green stems and notably caustic sap.
- Euphorbia trigona — the "African milk tree", an upright, three-angled houseplant grown for its architectural columns.
- Euphorbia milii — the "crown of thorns", a spiny, long-flowering shrub from Madagascar popular for its persistent bracts.
- Euphorbia lactea — often seen as the crested "coral cactus" and in ghostly variegated forms.
- Euphorbia ingens — a large candelabra tree euphorbia of southern Africa.
Cultivation
Most succulent euphorbias are undemanding once their basic needs are met. Grow them in a very free-draining, largely mineral mix and give them the brightest light you can — many colour up and stay compact in strong sun, though thin-stemmed and freshly imported plants appreciate a little shade at first. Water thoroughly when the soil has dried out, then allow it to dry again; err on the side of dryness, as root rot from a cold, wet mix is the commonest cause of loss.
Warmth is important: the great majority are frost-tender and are happiest kept above about 10 °C, with a drier, cooler rest in winter. See Repotting for general technique, and take particular care handling spiny or brittle species.
A word on the latex. The milky sap is the single most important thing to know about growing this genus. It can cause painful skin irritation and is genuinely dangerous in the eyes. Always wear gloves and eye protection when cutting or repotting, keep the sap away from your face, wash any splashes off promptly, and keep plants out of reach of children and pets. See plant safety for more.
Hobby and cultivar notes
Succulent euphorbias are a deep and rewarding speciality. Collectors particularly prize crested (cristate) and monstrose forms, in which the growing point fans or proliferates into strange sculptural shapes — the so-called "coral cactus" is a crested Euphorbia lactea, usually grafted onto a sturdier rootstock to keep it vigorous. Variegated clones, which carry less chlorophyll, are also popular and generally want a touch more shade and warmth than their plain-green parents.
Because many species are dioecious, seed is not always easy to obtain, so a great deal of propagation is vegetative — an important practical difference from most true cacti.
Propagation
Succulent euphorbias are usually increased from stem cuttings, which is straightforward for the shrubby and branching kinds. When a cutting is taken, dip or rinse the cut end to stop the flow of latex, then let it callus for several days to a couple of weeks before setting it in a barely-moist, gritty mix to root. Solitary, non-branching species such as Euphorbia obesa rarely offer cuttings and are raised from seed instead — which, given the separate sexes, means having both a male and a female plant in flower. Choice crested and variegated forms are frequently grafted.
Always take cuttings wearing gloves and eye protection, for the reasons given above.
Common problems and safety
- Caustic latex — the foremost hazard of the genus. Skin contact can burn or blister, and sap in the eyes can cause serious injury. Gloves and eye protection are essential; wash off any contact at once and seek medical advice for eye exposure.
- Toxicity — the sap and plant tissue are toxic if eaten. Keep euphorbias away from children and from cats and dogs.
- Rot — soft, discoloured, collapsing stems almost always signal overwatering or a mix that stays wet and cold.
- Etiolation — too little light causes weak, pale, stretched growth that loses the plant's natural form.
- Pests — mealybugs (white fluff in crevices and at the roots) and spider mites (fine webbing, stippled or bronzed stems) are the usual troublemakers; see Pests and diseases.
See also
- Euphorbiaceae — the spurge family
- Euphorbia obesa · Euphorbia tirucalli · Euphorbia trigona · Euphorbia milii · Euphorbia lactea
- Grafting · Soil and potting mix · Watering · Propagation - cuttings · Propagation - seed · Pests and diseases