Somewhere in a garden centre right now, a customer is buying a “cactus” that is not a cactus. It happens millions of times a year: Euphorbia trigona sold as “African milk tree cactus”, Euphorbia tirucalli labelled “pencil cactus”, Euphorbia lactea ‘Cristata’ marketed as “coral cactus”, Euphorbia horrida sitting on the cactus shelf next to the Ferocactus. The confusion is understandable — many euphorbias look almost indistinguishable from true cacti. But it is not harmless. Euphorbias produce a toxic, milky latex that can burn skin and blind eyes; true cacti do not. Treating an euphorbia like a cactus — or vice versa — means getting the watering, the handling safety, and sometimes the legal protections wrong. Knowing which family your plant belongs to is not a pedantic detail; it is a practical necessity.
This article explains why two unrelated plant families look so similar, gives you a simple, reliable method to tell them apart in seconds, and summarises the key differences in care, safety, and biology.
Why They Look the Same — Convergent Evolution
Cacti (family Cactaceae) evolved in the Americas. Succulent euphorbias (family Euphorbiaceae) evolved primarily in Africa and Madagascar. The two families are separated by approximately 100 million years of evolutionary divergence and, for most of that time, by the Atlantic Ocean. They are not closely related — cacti belong to the order Caryophyllales, while euphorbias belong to the order Malpighiales.
Yet they look alike because they faced the same problem — survival in hot, dry environments — and arrived at the same solutions independently. This phenomenon is called convergent evolution, and it is one of the most spectacular examples in the plant kingdom.
Both families independently evolved: thick, water-storing stems that can photosynthesise without leaves; reduced or absent leaves to minimise water loss; spines or thorns to deter herbivores; ribbed or angular stem cross-sections for structural strength and expandability (the ribs accordion outward when the plant absorbs water and contract during drought); and CAM photosynthesis (stomata open at night to reduce daytime water loss).
The convergence is so precise that individual species from each family have near-perfect counterparts in the other :
- Euphorbia obesa mimics Astrophytum asterias.
- Euphorbia horrida mimics Ferocactus.
- Euphorbia ingens mimics Pachycereus.
- Euphorbia meloformis mimics Echinocactus.
In each pair, the plants are virtually indistinguishable at a glance — yet they are as unrelated as a dolphin is to a shark.
The Five-Second Test — How to Tell Them Apart
You do not need a botany degree or a DNA lab. Four simple tests, any one of which is sufficient on its own, will identify whether your plant is a cactus or a euphorbia.
Test 1 — The latex test (the most reliable)
Scratch the surface of a stem or break a small piece of an expendable branch. If a milky white sap oozes from the wound, it is an euphorbia. True cacti produce clear, watery sap — never milky.
⚠ Safety warning: if the sap is milky, do not touch it with bare skin and keep it away from your eyes. Euphorbia latex is a potent irritant that can cause skin burns, blisters, and temporary blindness if it contacts the eyes. Wash immediately with running water if contact occurs.
This test is nearly 100% reliable. Milky sap = euphorbia. Clear sap = cactus (or another non-euphorbia succulent).
Test 2 — Areoles (the definitive cactus character)
Look at where the spines emerge. On a true cactus, spines grow from areoles — small, cushion-like, often woolly or hairy structures on the surface of the stem. Areoles are unique to the Cactaceae and are found in no other plant family. They are the defining morphological feature of the cactus family.
On a euphorbia, spines (when present) grow directly from the stem surface, often in pairs, sometimes from a small, shield-shaped structure called a spine shield — but there is no woolly, cushion-like areole. The spines may also be modified stipules or hardened peduncles rather than true spines.
Areoles present = cactus. No areoles = not a cactus (possibly euphorbia, or another succulent family).
Test 3 — Spine arrangement
Cactus spines typically emerge in clusters of multiple spines from a single areole — a central spine surrounded by radial spines in a starburst pattern is classic.
Euphorbia spines typically emerge in pairs — two spines side by side, often along the rib margins. This paired arrangement reflects their origin as modified stipules (the small appendages at the base of a leaf).
Single cluster from a woolly pad = cactus. Pairs along the ribs = euphorbia.
Test 4 — Flowers
If the plant is in bloom, the difference is unmistakable.
Cactus flowers are large, showy, with many petals, many stamens, and a single pistil. They are immediately recognisable as “flowers” in the conventional sense — often spectacular in colour and size.
Euphorbia flowers are not flowers in the conventional sense at all. They are cyathia — tiny, cup-shaped structures containing several reduced male flowers (each just a stamen on a stalk) surrounding a single reduced female flower (just a stalked ovary). What looks like “petals” on a euphorbia (for example, the red “flowers” of Euphorbia milii or the red bracts of a poinsettia) are actually modified bracts or glands, not true petals. Cyathia are unique to the genus Euphorbia and found in no other plant.
Summary Table — Cactus vs Euphorbia
| Character | True cactus (Cactaceae) | Euphorbia (Euphorbiaceae) |
|---|---|---|
| Sap | Clear, watery | Milky white, toxic |
| Areoles | Present (woolly pads) | Absent |
| Spine origin | Clusters from areoles | Pairs from spine shields or stem |
| Flowers | Large, many-petalled | Cyathia (tiny, cup-shaped) |
| Native range | Americas (one exception: Rhipsalis baccifera) | Africa, Madagascar, Eurasia, Americas |
| Toxicity | Generally non-toxic (some psychoactive) | Latex toxic: skin irritant, eye danger, gastrointestinal burns |
| Leaves | Absent (except Pereskia) | Often present when young, then deciduous |
| Family | Cactaceae (order Caryophyllales) | Euphorbiaceae (order Malpighiales) |
The Most Commonly Mislabelled Species
These euphorbias are routinely sold as “cacti” in garden centres, online shops, and home décor stores:
Euphorbia tirucalli — sold as “pencil cactus”. Not a cactus. Dense masses of smooth, green, pencil-thin branches. Produces copious toxic latex. The cultivar ‘Sticks on Fire’ (coral-red branches) is particularly popular.
Euphorbia trigona — sold as “African milk tree” or sometimes “African cactus”. Not a cactus. Upright, three-ribbed, segmented columns with small leaves. The dark-purple cultivar ‘Rubra’ is especially common in the houseplant trade.
Euphorbia lactea ‘Cristata’ — sold as “coral cactus”. Not a cactus. A crested (fan-shaped) mutation grafted onto a straight euphorbia rootstock. The wavy, coral-like crest is visually striking but entirely euphorbiaceous.
Euphorbia horrida — sold alongside barrel cacti. Not a cactus. Ribbed, spiny, columnar South African species that closely resembles Ferocactus. The paired spines and milky sap betray its true identity.
Euphorbia obesa — sometimes sold as a “living baseball” alongside Astrophytum. Not a cactus. A near-perfect sphere with no spines. Extremely popular with collectors.
Euphorbia milii — sold as “crown of thorns cactus”. Not a cactus. A spiny, woody Malagasy shrub with colourful bracts. One of the few succulents that flowers almost continuously.
Why the Distinction Matters in Practice
Safety
This is the most important practical difference. When you prune, repot, or accidentally break an euphorbia, the white latex flows immediately. Without gloves and eye protection, you risk painful skin burns and — in the worst case — days of temporary blindness from eye exposure. True cacti pose a mechanical injury risk from their spines, but their clear sap is not chemically dangerous. If you think your plant is a cactus and handle it without gloves, but it is actually an euphorbia, the consequences can be serious.
Watering
While both families are drought-tolerant, their watering needs differ in detail. Many African succulent euphorbias come from semi-arid tropical environments with seasonal summer rainfall and do not tolerate the extreme, prolonged dryness that some desert cacti endure. Conversely, many cacti from the Sonoran or Chihuahuan deserts survive months without any water — a regime that would weaken most tropical euphorbias. Knowing the family helps you calibrate your watering schedule to the plant’s actual ecology.
Legal protection
Many succulent euphorbias are listed under CITES (Convention on International Trade in Endangered Species) — particularly the South African globular species (Euphorbia obesa, Euphorbia meloformis, Euphorbia symmetrica) and the Malagasy species. Similarly, many cacti are CITES-listed. The legal frameworks differ by species, and misidentification can create compliance problems for collectors and sellers.
Temperature tolerance
Most tropical succulent euphorbias are frost-tender (minimum 5–10 °C for the majority of African and Malagasy species). Many cacti — particularly those from high-altitude deserts in the American Southwest, Mexico, or the Andes — tolerate significant frost (–10 °C to –25 °C for species like Opuntia humifusa, Echinocereus triglochidiatus, or Escobaria vivipara). Assuming that your “cactus” is frost-hardy when it is actually a tropical euphorbia can kill it on the first cold night.
The One Exception — Rhipsalis baccifera
There is exactly one cactus species native to the Old World: Rhipsalis baccifera (mistletoe cactus), which occurs in tropical Africa, Madagascar, and Sri Lanka in addition to the Americas. Its presence outside the Americas is thought to result from ancient bird dispersal across the Atlantic. It is an epiphytic, spineless, trailing cactus that looks nothing like a typical columnar cactus — and is sometimes confused with trailing euphorbias. The test is the same: no milky sap, and tiny areoles present on the stems = cactus.
Frequently Asked Questions
Is pencil cactus actually a cactus?
No. The “pencil cactus” (Euphorbia tirucalli) is a euphorbia — a member of the family Euphorbiaceae, entirely unrelated to the true cacti (family Cactaceae). It produces a milky white latex that is highly toxic and irritating, which no true cactus does. Despite the misleading common name, it should be handled with gloves and eye protection, particularly when pruning.
Is coral cactus a real cactus?
No. The “coral cactus” is Euphorbia lactea ‘Cristata’ — a crested (fan-shaped) mutation of a euphorbia species, grafted onto a euphorbia rootstock. It is entirely Euphorbiaceae, with no relationship to the Cactaceae. Like all euphorbias, it produces toxic milky latex when damaged.
How can I tell if my plant is a cactus or euphorbia?
The fastest test: scratch the surface of a stem. If milky white sap appears, it is a euphorbia (handle with care — the sap is toxic). If the sap is clear and watery, it is a cactus or another non-euphorbia succulent. You can also check for areoles — small, woolly, cushion-like pads from which spines emerge in clusters. Areoles are unique to cacti; euphorbias have paired spines growing directly from the stem or from spine shields, without any woolly pad.
Are euphorbias poisonous?
Yes. All species in the subfamily Euphorbioideae produce a milky white latex containing diterpene esters that cause severe skin irritation, eye damage (including temporary blindness), and gastrointestinal burns if ingested. Some species are co-carcinogenic. The toxicity varies by species — Euphorbia tirucalli and Euphorbia poissonii are among the most dangerous — but all should be handled with gloves. True cacti, by contrast, have clear, generally non-toxic sap.
Can euphorbias and cacti be grown together?
Yes, in the same collection or garden — they share similar needs for bright light, well-draining substrate, and moderate watering. However, be aware of two differences: most tropical succulent euphorbias are frost-tender (minimum 5–10 °C), while many cacti tolerate significant frost; and euphorbia latex is toxic, so position euphorbias where they will not be accidentally damaged near paths, play areas, or pet zones.
Sources
- Plants of the World Online (POWO), Royal Botanic Gardens, Kew — Euphorbiaceae and Cactaceae. powo.science.kew.org
- Horn, J.W. et al. (2012). Phylogenetics and the evolution of major structural characters in the giant genus Euphorbia. Molecular Phylogenetics and Evolution, 63(2): 305–326.
- Anderson, E.F. (2001). The Cactus Family. Timber Press, Portland.
- Court, D. (2000). Succulent Flora of Southern Africa. CRC Press.
- Carter, S. & Eggli, U. (2003). The CITES Checklist of Succulent Euphorbia Taxa. Federal Agency for Nature Conservation, Bonn.