Plural of “cactus”

The plural of cactus is cacti (some dictionaries also accept cactuses).

Why

"Cactus" comes from Latin (which borrowed it from Greek "kaktos," originally referring to a spiny plant native to Sicily, not the American succulents the word describes today). Because it entered English scientific vocabulary through Latin taxonomic naming in the 18th century, it kept the Latin second-declension plural pattern, -us becoming -i, giving "cacti" — the same pattern behind "fungus/fungi" and "nucleus/nuclei."

"Cactus" is also a genuine case of a word regularizing in real time: "cactuses" has become the more common plural in everyday, non-technical writing over the last several decades, while "cacti" remains dominant in botanical and horticultural contexts specifically — dictionaries list both as fully correct rather than treating one as an error, reflecting the same frequency-driven drift toward regular -s plurals seen across most Latin-derived words once they become common outside academic use. (Note: the genus name itself, capitalized "Cactus," refers today only to a single narrow group of species within the much larger cactus family Cactaceae, so the everyday lowercase "cactus" covering all spiny desert succulents is broader than the strict scientific genus the word originally named.)

Usage note

Neither form is wrong in general writing, but context matters: a botany textbook or a nursery's plant tags will more often use "cacti," while a casual blog post or conversational sentence about houseplants is just as likely, and just as correct, to use "cactuses." If you're ever unsure which register you're writing in, "cactuses" is the safer default for a general audience, since it needs no explanation.

Spelling checkers and style guides at most major publications now accept both plurals without flagging either, which wasn't always true — older 20th-century usage guides were far more insistent on "cacti" as the only correct form, a prescriptivist stance that has softened considerably as "cactuses" became entrenched in mainstream usage over the last few decades. The word also has an unusually well-documented birth date for an everyday noun: Linnaeus formally established the genus name "Cactus" in his 1753 taxonomy "Species Plantarum," and the common English noun followed directly from that scientific naming rather than drifting in gradually through everyday speech the way most plant names do — one reason its plural still carries a faint scientific-Latin flavor even in casual gardening writing today.

Common real mistakes

The one genuine error to avoid is treating "cacti" as singular ("a cacti") — it's a plural form only; the singular is always "cactus," regardless of which plural you choose to pair with it. A separate, more casual error is using "cactus" itself as an uncountable mass noun in speech ("there's a lot of cactus out here"), treating the whole plant type the way you'd treat "grass" or "sand" — understandable in relaxed conversation about a landscape full of them, but nonstandard in any writing that's naming or counting individual plants.

The wider pattern

This is the same pattern behind "nucleus/nuclei," "radius/radii," "alumnus/alumni," "stimulus/stimuli," and "syllabus/syllabi" — a whole family of Latin second-declension masculine nouns that entered English through science, medicine, and academia and largely kept their original Latin plurals for as long as they stayed within those specialist registers, with each one showing its own degree of drift toward a regularized -s plural depending on how common the word has become in everyday speech outside its technical origin.

Examples

• The desert garden was full of cacti of every size.
• She bought three small cactuses for her windowsill.
• Cacti store water in their thick stems to survive drought.
• Each cactus in the collection had its own hand-written label.
• The botanist studied over forty species of cacti native to the region.
• One cactus was enough to brighten up his desk.
• A row of cacti lined the front walkway.
• He photographed cacti across three different deserts.
• That particular cactus blooms only once a year.
• The nursery sells both potted cacti and cactus seeds.