Monadenium is a genus in the plant family Euphorbiaceae. These African native succulents may be bushy and upright like a tree or trailing, depending on the species. Approximately 50 species make up the Monadenium genus group, and cacti lovers collect many of the species. The flowers take the form of a cyathia, a flower composition that is unique to the Euphorbia family.
A cyathium inflorescence, or flower, has a cup-like ring of bracts, called involucre. In the involucre, the bracts often overlap, forming a ring around the base of the flowerhead or group of flowerheads. In most Euphorbia cacti, this involucre ring surrounds a single pistil and several male flowers, each bearing a single stamen. During the summer, these petalless flowers bloom during the daytime and close at night.
Many of the Monadenium plants have very small involucres. The M. montanum, or rubellum, has pink involucres that may be up to one-eighth of an inch (about 0.3 cm) across. The involucres of the M. lugardiae are generally yellow or orange-brown and often are 0.25 inches (about 0.6 cm) in diameter, but still significantly small. The yellow-green involucres of the M. ellenbeckii generally are 0.5 inches (about 1.3 cm) across.
The Monadenium stem growth pattern varies from species to species. Some of the species retain their fleshy stems throughout the year; others produce new growth each year. In most of the plants, this new growth sprouts from a subterranean tuber, or caudex, which is a swollen stem base.
Often botanists classify these cacti in one of three categories based on the stem growth. One classification includes plants that grow semi-succulent leaves at the tips of fleshy stems or at the growing points. In another category, the plant has substantial, bushy stems that have many semi-succulent leaves growing along the stem. The last category contains plants that have both tuberous roots and succulent stems. Growers may want to research their cactus in order to give it the proper care for its growing habits.
The leaves of the Monadenium cacti usually are deciduous, falling off during the dormant season. The leaves generally are fleshy and succulent or scaly. The size varies with the species, variety, or cultivar. Typically, the leaves of M. lugardiae may be obovate, scalloped, or toothed and reach lengths of 3.5 inches (about 9 cm). M. montanum, or rubellum, leaves often are lance-shaped and only 1 inch (2.5 cm) long.
Gardeners may use Monadenium cacti as garden specimens, depending on the climate. Typically, they prefer temperatures above 65°F (about 18°C), although some of them, like M. montanum, or rubellum, will tolerate colder climates.
Many of the plants are manageable as houseplants. M. ritchiei tolerates cool temperatures, but not freezing ones. Unlike some cacti, it usually survives lots of rain during the growing season, but needs a dry dormant season. Many growers considered it a good winter windowsill houseplant and an outdoor summer plant. As it ages, it may reach heights of 2 feet (about 60 cm) and spread to be quite bushy.