Succulents 101: Cacti vs. cactus-like euphorbias

My posts sometimes assume a fairly advanced level of knowledge. Many of you have that, but others might feel a bit left out. To remedy that, I decided to do a series of more basic posts. Hopefully they’ll be useful for readers who are new to succulents without boring folks who are already familiar with the topic discussed.

Today’s post is about cacti vs. cactus-like euphorbias. They are often confused because at first glance they look quite similar even though they are not closely related. They have developed their shared features through convergent evolution—adapting in a similar fashion to survive harsh environments.

Cactus and euphorbia: Cactus far left, euphorbia center right

Cactus and euphorbia: cacti on the left, euphorbias on the right along the main drive at Lotusland

Cactus: Pachycereus pringlei

Here’s a breakdown of their key differences:

❉ Family and origins

Cacti: Belong to the Cactaceae family and are native to the Americas, from Canada to South America.

Euphorbias: Belong to the Euphorbiaceae family and are widely distributed, with many species originating in Africa and Madagascar.

Euphorbia: cactus-like euphorbias at the Fullerton Arboretum

❉ Areoles

Cacti: Have areoles—small, round growth pads from which spines, flowers, and new growth emerge. Areoles are a unique feature of cacti.

Euphorbias: Lack areoles. Their spines and flowers grow directly from the stem.

Cactus: Echinocereus yavapaiensis areoles circled in red

Cactus: Ferocactus herrerae areoles circled in red

Euphorbia: Spines (and flowers) on Euphorbia grandicornis hybrid emerging from the pale horn-like plates called spine shields. In this Euphorbia species, the spine shields form a continuous margin.

❉ Spines

Cacti: Their spines arise from areoles and vary greatly in size and density.

Euphorbias: Spines often occur in pairs directly on the stem and are usually simpler in structure.

Cactus: Mammillaria microhelia

Cactus: Cylindropuntia fulgida

Euphorbia: Euphorbia grandicornis

Euphorbia: Euphorbia grandicornis

Euphorbia: Euphorbia grandicornis

❉ Flowers

Cacti: Produce large, showy flowers with multiple petals, often designed to attract specific pollinators like bees, bats, or hummingbirds.

Euphorbias: Have small, less conspicuous flowers, often surrounded by colorful bracts that mimic petals. These flower structures are called cyathia (singular: cyathium).

Cactus: Echinopsis ‘Flying Saucer’ flowers

Euphorbia: Euphorbia cooperi

Euphorbia: unidentified flowering euphorbia at the Fullerton Arboretum

Euphorbia: Euphorbia horrida

Euphorbia: Euphorbia horrida

Euphorbia: Euphorbia caput-medusae

Euphorbia: Euphorbia grandicornis

❉ Sap

Cacti: Do not produce latex. Their stems are primarily water-storage structures.

Euphorbias: Produce a milky-white latex sap, which is toxic and can cause skin irritation or harm if ingested.

Euphorbia: sap from cut Euphorbia pseudocactus stem

Euphorbia: sap from cut Euphorbia pseudocactus stem

❉ Growth Forms

Cacti: Typically have uniform growth forms such as globular, columnar, or sprawling shapes.

Euphorbias: Highly diverse growth forms. Some mimic cacti, while others look like shrubs, trees, or ground covers.

Euphorbia: weeping form of Euphorbia ingens at Lotusland

Euphorbia: towering euphorbia at Sherman Library and Gardens

Euphorbia and cactus: tree-like euphorbia seen through columnar cacti

Euphorbia: Euphorbia ammak, green and variegated, at Lotusland

Euphorbia: Euphorbia grandicornis

❉ Summary

The primary differences between cacti and cactus-like euphorbias lie in their structural features, sap, and origins. Areoles and spines are key identifiers: if a plant has areoles, it’s a cactus. If it has a milky sap but no areoles, it’s a euphorbia. Recognizing these distinctions can help with proper identification.

Cactus: Trichocereus hybrids

Cactus: Echinocereus sp.

Euphorbia: Euphorbia echinus

Euphorbia: Euphorbia resinifera

Cactus: Mammillaria compressa

Euphorbia: Euphorbia clavaroides

Euphorbia: Euphorbia horrida

Cactus: Toothpick cactus (Stetsonia coryne)

Cactus-like euphorbia the size of a tree in Tony Krock‘s front yard

Cactus: Monstrose form of Myrtillocactus geometrizans at California Cactus Center in Pasadena

Euphorbia: monstrose form of unidentified euphorbia looking remarkably similar to the cactus above. Sometimes, cacti and euphorbias are startlingly similar.

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