Types of Cacti Chart: Cactus Identification by Shape

To find out what type of cactus you have, first confirm that it is a cactus at all. The deciding feature is the areole: Cactus Outlet describes areoles as small, fuzzy, cushion-like bumps from which spines, flowers or new stems grow, and Wikipedia's summary of the cactus family describes them as structures unique to cacti. Every true cactus has them, even nearly spineless ones.

Many spiny succulents are not cacti. The quick test for the most common impostor: euphorbias ooze milky white latex sap when cut, while most cacti have clear, watery sap (Cactus Outlet).

This matters beyond the name. The ASPCA lists the pencil cactus (Euphorbia tirucalli), which is a euphorbia and not a cactus, as toxic to dogs, cats and horses because of its irritant sap.

Sort the plant by overall shape first; Cactus Outlet uses the same groups. Each line below gives the shape, the feature to confirm, and an example.

• Columnar, with arms: tall upright stems with ribs. Example: saguaro (Carnegiea gigantea), up to 40 feet with upward-reaching arms and white flowers in late spring (Cactus Outlet).
• Globular or barrel: round, ribbed body. Example: golden barrel (Echinocactus grusonii), dense golden-yellow spines on well-defined ribs, up to 3 feet across (Cactus Outlet). Confirm: distinct ribs, not bumps.
• Globular with bumps instead of ribs: example Mammillaria, the pincushion cacti. Wikipedia describes it as tuberculate rather than ribbed, with an areole split into two parts, one at the tip of each tubercle and one at its base. About 170 to 190 species, mostly from Mexico.
• Paddle or pad: flat, jointed pads. Example: prickly pear (Opuntia), with large spines plus tiny, detachable glochids that Cactus Outlet warns are highly irritating. Bunny ears cactus belongs here.
• Flat, segmented and spineless: the holiday cacti (Schlumbergera), common as houseplants. See the next section.
• Clumping or branching: low mounds of many small stems. Count ribs and look at the spine arrangement, radial spines in a ring plus central spines, to narrow the genus.

These are the cacti most people have indoors, and the stem segments tell them apart. From Wikipedia's account of Schlumbergera:

• Thanksgiving or crab cactus (Schlumbergera truncata): stem segments with pointed teeth, flowers held roughly horizontally, yellow pollen
• Christmas cactus (Schlumbergera × buckleyi): segments with rounded, more symmetrical teeth, flowers that hang down, pink pollen
• Easter cactus (now placed in a separate genus, Rhipsalidopsis): differs in flower structure and flowering time

Many plants sold as Christmas cactus are in fact Thanksgiving cactus, so check the teeth. Flowering is triggered by long nights: Wikipedia cites at least 12 hours of continuous darkness to form buds, and about 8 days of 16-hour nights at 16 C (61 F) as a tested trigger. The ASPCA lists Christmas cactus as non-toxic to dogs, cats and horses.

Shape narrows the group; the details that pin down a species are small. Photograph:

• The whole plant from the side, to show columnar, barrel, pad or segmented form
• A close-up of one areole with its spines, showing how many radial and central spines there are and whether any are hooked
• The ribs or tubercles, from above for a barrel or pincushion
• The flower, if there is one, since many genera are separated by flowers

Handle prickly pear only with thick gloves or tongs because of the glochids. NatureLens gives free scans every day, so you can photograph the plant or sign and get a suggested name in seconds, then confirm it with the checks on this page.