The Portulacaceae family contains 20 genera and an estimated 500 species worldwide, particularly in arid regions of the southern hemisphere. The genus Calandrinia contains 150 species, followed by Portulaca with 100 species and Talinum with 50. Genus Portulaca includes both annual and perennial species. Growth habit is generally prostrate but sometimes erect, with succulent red or purple stems that adventitiously root at nodes. Portulaca includes Portulacella, an Australian and Pacific Island subgenus.
Annual Portulaca complete their life cycle rapidly. The flower produces nectar and is self-pollinated by insects. Portulaca fruits are calyptra (dry capsules), which dehisce (separate from the plant) and release one to 50 seeds that are distributed by rainwater or by animals. The seeds may germinate immediately or can rest dormant. Seedlings thrive in full sun. At the seedling stage, Portulaca avoid shade by sensing and growing away from far-red spectrum light reflected by other plants.
Portulaca oleracea (purslane) is a noxious weed affecting 45 crops in 81 nations with tropical and temperate climates. In some cultures, purslane is cultivated as a salad green. Oleracea is salt tolerant and thrives in open fields and disturbed sites. The species originated in northwestern India and southern Russia, spread with crops into Europe prior to the Greek civilizations and reached the Americas prior to Columbus. Carbonized oleracea seeds recovered from Salts Cave, Kentucky, were dated to 3,000 years ago. Any typical temperate-climate corn field contains 220,000 Portulaca oleracea seeds per square meter.
Portulaca grandiflora (Moss Rose) is a commodity of the international horticultural trade. Grandiflora originated on the South American pampas, the arid plains of Uruguay, southern Brazil and northern Argentina. Scottish physician and botanist John Gillies first collected its seeds in the 1820s, along Rio Desaguadero on the east face rain shadow of the Argentine Andes. Gillies described purple and orange blooms in the wild. By 1851, Joseph Breck's seed and tool warehouse in Boston, Massachusetts, advertised rose, scarlet, yellow and white varieties. Cultivated grandiflora have escaped to the wild in Florida and California.
Tropical Africa Portulaca
East Africa is a center of diversity for the Portulaca subgenus, including both annuals and perennials. For example, Portulaca decorticans of Kenya and Somalia is erect, almost shrublike, with stems a centimeter in diameter. Perennial Portulaca massaica establishes itself in the stony arid soils of Kenya and Tanzania with a wide, branching root system. Portulaca centrali-africana can replicate at rooting nodes along creeping stolons. Portulaca somalica has a fleshy taproot similar to a tuber. Stems of Portulaca decorticans exfoliate papery bark.
Australia and the Pacific Islands' Portulacella subgenus is the oldest Portulaca. It retains the dichasia flower cyme, a single terminal flower with a central stem that develops two lateral stems for additional flowers in a cluster. The capituliform flower (flat and terminal, like a daisy) of other Portulaca evolved later. Seedlings of Australia's unique Portulacella bicolor begin life as tubers and mature plants grow flush on the surface gravel of desert pavement.