How To Tell Euphorbia from Cacti

I was on a plant walk the other day, and one of the group didn't know how to tell Euphorbia from Cacti. Although, like so many things in life, there are always exceptions to the rule, here are the main key things to look for:

1. Euphorbia(Euphorbiaceae) - although they are a succulent and spine-bearing, they have no areoles.

2. The Cacti (Cactaceae) - is showing large, white spines borne on the areoles, the key diagnostic feature of the family Cactaceae. Only the family Cactaceae bears structures called "areoles." An areole is a pad structure that is seen at the base of the spines. Even if a cactus has no spines it will have areoles.

3. As a last resort, if you really need to know, the sap of the plant will tell you. Carefully make a cut, and if the sap is milky white and sticky, then it's a Euphorbia. If the sap is clear and watery, it's a cactus. Just be careful because Euphorbia sap can cause skin irritation, so if you're going to try this, wear gloves to be safe.

Not to confuse the issue, but many plants (such as Euphorbiaceae, Rosaceae, Punicaceae, etc.) produce spines, but no other plants bear areoles, only Cactaceae.

Hope this helps you identify cacti from euphorbia - at least most of the time!