If you grow cactus and other succulents, you probably have a few euphorbias mixed in with the cacti. Source: mashtalegypt.com, montage: laidbackgardener.blog

In the world of succulents, that is, plants that store water in thickened stems or leaves in order to better cope with arid conditions, two different families are often mistaken one for the other: cacti (Cactaceae) and euphorbias (Euphorbiaceae, genus Euphorbia). In fact, many people simply call all succulent euphorbias “cacti,” not realizing there is a difference.

Both cacti and euphorbias tend to be stem succulents, that is, they store moisture in swollen stems, both tend to very spiny and both include a variety of species of all different shapes. They are so similar in many aspects that confusion is understandable, yet they are no more closely related to each other than dogs are to cats.

Dogs and cats are both mammals, are covered in hair, have tails and pointy teeth and are carnivores, yet few people would confuse a dog with a cat. If you’re a gardener, you really should learn to tell a euphorbia from a cactus. Once you know the difference, you’ll never confuse them again.

Convergent Evolution

As a result of convergent evolution, many cacti and euphorbias have evolved to resemble one another. Source: www.kajuard-plantes.com & http://www.uhlig-kakteen.de

The similar appearance of cacti to euphorbias is an example of convergent evolution: the two different families evolved under similar conditions—under extreme aridity—and adopted similar survival tactics, resulting a lot of similarities. Even so, they also have their differences.

Telling the Two Apart

Here are a few tips on telling the euphorbias from cacti. You may well find you’re growing both of them without knowing it!

A cushiony areole, with or without spines, is a sure sign of a true cactus. Source: http://www.krypton.ovh

1. Cacti have areoles. These are cushiony, fuzzy dots from which spines, stems and flowers grow. They can be white or yellow, but are present on all cacti. No other plant has areoles. So, if you look closely at any cactus-like plant and see no areoles, as will be the case with euphorbias, that plant is definitely not a cactus! This is, hands down, the easiest way of telling the two apart. Once you know this, you’ll never confuse a cactus with any other plant again!
   

   Euphorbia thorns, usually single or paired, are part of the stem and hard to remove. Cactus spines often form a circle and are easily broken off. Source: Hernán Conejeros, YouTube.ca & Elton Roberts, xerophilia.ro

2. Euphorbias usually bear single or paired thorns (modified stems), if indeed they have thorns. They’re usually thick and are clearly part of the stem: you can’t break them off without wounding the plant. Cactus have spines (modified leaves) rather than thorns. They often form a circle, although not always, and there may also be longer spines in the circle’s center. However, no matter how they are grouped together, cactus spines always arise from those cushiony areoles mentioned above. They can be snapped off very readily and indeed, some are designed to break off and penetrate the skin of animals to dissuade them from chomping on the cactus. Note that there are both thornless euphorbias and spineless cacti, so the absence or presence of prickles is not necessarily a factor in distinguishing between the two.
   

   Euphorbias produce milky white sap, cacti rarely do. Source:Norman E. Rees, USDA ARS

3. Euphorbias produce milky sap if you wound them. This is a sticky latex, often toxic or irritating to the skin, and almost always white (there is just one species with yellow sap, E. adbelkuri). Most cacti have clear sap, rarely irritating to skin, although there are a very few cacti with white sap, including a few Mammillaria.
   

   Cactus flowers are highly varied and usually colorful. The true flower of the euphorbia above is the center and is unremarkable; it is only made attractive by the bracts that surround it. Source: Florence Rogers, Nevada Public Radio & http://www.backyardnature.net

4. Cactus flowers are usually showy and colorful, with typical flower parts, including petals and many stamens. Take a look at how they are placed on the plant: they always grow from a cushiony areole. Euphorbia flowers tend to be small and yellow, have no petals and are usually quite insignificant, although in some species, such as the crown of thorns (E. milii), they are surrounded by modified leaves called bracts that can be quite colorful. They never grow from areoles.

*Be aware that unscrupulous cactus growers often stick fake flowers on cactus with glue guns. If the flower on a cactus does not grow from an areole and has a glob of glue underneath, you’ve been had!

Many euphorbias, like this Euphorbia trigona, have leaves. Few cactus do. Source: www.plantandpot.nz

5. Cacti are almost always leafless, at least as adults (all bear two cotyledons at germination). There are only a few exceptions (Pereskia, which actually don’t look like cactus at all, and a few Opuntia, for example). On the other hand, many euphorbias with succulent stems nevertheless bear leaves.
6. Cacti evolved in the New World and are found in North, South and Central America as well as in the Caribbean. (There is only one species, Rhipsalis baccifera, that is native to the Old World.) Succulent euphorbias evolved in the Old World, mostly Africa, Madagascar and drier areas of Asia. In the wild, both are still mostly found in their native lands, although some cacti, notably in the genus Opuntia, have escaped from culture to become weeds in other parts of the world.
   

   All these euphorbias are in the single genus Euphorbia, but the cactus shown are in various genera, including Mammillaria, Trichocereus, Astrophytum, Opuntia and Cereus. Source: lonelyplant_ph, deskgram.net & http://www.ebay.co.uk

7. Euphorbia is just one genus in the family Euphorbiaceae. They are often called spurges. There are more than 2,000 species of Euphorbia, including such popular houseplants as mottled spurge (Euphorbia lactea), pencil tree (E. tirucalli) and crown of thorns (E. milii). Cacti are a family, the Cactaceae. The family also contains about 2,000 species, but is divided into about 175 genera, including Opuntia, Echinocactus, Mammillaria and Cereus.

There you go! It isn’t all that hard to tell euphorbias from cacti. Just the lack of an areole on euphorbias should be such an obvious difference that you’ll never confuse the two again!