Cotula Genus

Cotula is a genus of flowering plants in the family Asteraceae (the daisy or sunflower family), placed in the tribe Anthemideae and the order Asterales. Commonly known as water buttons or buttonweeds, Cotula is the largest genus found in the Southern Hemisphere within its tribe, comprising roughly 80 species distributed across southern Africa, Australia, New Zealand, the Subantarctic Islands, and parts of the Americas.

The genus presents unusual taxonomic challenges: its species vary so extensively in habit, leaf division, involucre structure, receptacle form, and achene morphology that they cannot be reliably grouped by these features alone. Instead, the genus is defined by the structure of its corollas. Most species are disciform — meaning they lack the ray florets (the strap-like "petals") characteristic of many daisies — and produce tubular, reduced, or even absent corollas. Flower heads are borne solitarily on a peduncle.

Carl Linnaeus first described the genus in 1753 in the first edition of Species Plantarum, listing four species. In 1867, George Bentham subdivided Cotula into three sections based on chromosome number and geographic distribution. Section Cotula, the largest, contains about 40 species centred on South Africa and includes the cosmopolitan species C. coronopifolia (brass buttons). Section Strongylosperma encompasses around eight species distributed across warmer parts of Africa, Asia, Central and South America, and Australia. Section Leptinella holds the remaining roughly 30 species, primarily from New Zealand, the Subantarctic Islands, and South America; it is distinguished by inflated pistillate corollas not found in the other sections.

Several former genera, including Cenia and Otochlamys, have been subsumed into Cotula. The genus has found practical use in New Zealand, where low-growing species serve as ground cover on bowling greens.