Ethan Fenner, Horticulturist | Southern Africa
Plants have evolved a variety of ways to protect themselves under less-than-ideal conditions. In the Western Cape Province of South Africa, where rainfall is limited to wintertime, species must find ways of making it through long periods of drought. It is advantageous for plants to develop some kind of storage for the long dry summers. Succulents like Aloes and Mesembs store water and nutrients in large fleshy leaves; the Namibian grape (Cyphostemma juttiae) and some species of Pelargonium lose their leaves each year but transport nutrients into expanded stems; Annuals like Ursinia cakelifolia and Dimorphotheca pluvialis die each year and store their genetic materials in the seed.
The other major adaptation is a subterranean storage organ belonging to plants called geophytes – what we colloquially call the South African Bulbs. However, a true bulb is just one example of a geophytic organ – there are also corms, rhizomes, tuberous roots, and tuberous stems. There are more geophytic plants (roughly 1500 species) in the Cape of South Africa than anywhere else in the world. Although some have become naturalized and weedy in the Bay Area, their rapid vegetative growth and short but intense bloom season make these highly desirable garden plants. In spite of the array of geophytic organs and the amazing diversity of species that occur in the Cape, grouping these plants by family allows us to pick out some common characteristics.
Amaryllidaceae: These are some of the largest and most interesting Cape geophytes. The storage organ is a true bulb, a collection of modified leaves called scales (like an onion). It is often the case, especially for the larger Amaryllidaceae, that the bulb sits mostly outside the ground. The papery covering, called a tunic, can be observed on the exposed bulbs of Boophone haemanthoides and Brunsvigia josephinae. Flowers of many genera are held in large, striking umbels that eventually dry up and roll away like a tumbleweed. Most of us are familiar with Amaryllis belladonna which, like many other Amaryllidaceous bulbs from the wstern cape, flowers in the dormant season a month or so before the leaves begin to expand (the term for this is hysteranthous). In March, the Amaryllidaceae bulbs can be seen in a vegetative state with large architectural leaves. Often, species will put out a certain number of leaves per year – one example is the Cape tulip (Haemanthis coccinneus), where each bulb sets out two leaves per season.
Iridaceae: The iris family contains roughly 700 species of Cape geophytes. The storage organ is a corm, a modified stem – if you were to cut into it, it would be a solid mass of tissue as opposed to a collection of fleshy leaves. As opposed to bulbs, corms disappear totally during the growing season, moving all the energy into the leaves and flowers above ground. Left behind is a structure called the basal plate and the tunics. These tend to build up over the years – counting back the basal plates from the corm gives a rough idea of how old the plant is. Flowers are formed after the leaves (synanthous) and come in a variety of colors and forms depending on the pollinator it hopes to attract. One of the most unusual is Babiana ringens, which forms a sterile flower stalk each year that acts as a perch for Malachite sunbirds. Hybrids of Babiana sp. have naturalized themselves in the cracks and crevices throughout the Southern African collection for years- look for these and other impressively-colored irids to open up in the next couple of weeks.
Asparagaceae: Another major family of geophytes, the asparagus family, contains species that were formerly classified as being in the hyacinth or lily family. The collection here at UCBG contains several species of Lachenalia, smaller geophytes with striking flowers. The storage organ is a true bulb, but much smaller than what we see with the Amaryllidaceae. Most are buried below the surface, but some larger bulbs, like Ornithogalum, can be partially exposed. Flowers are held on a raceme or spike, although in some cases these are highly modified and condensed to be nearly level with the ground. Like the Amaryllidaceae, many species put out a set number of leaves per year, although flowering occurs after the leaves have been produced. Look closely for the intricate and often multi-toned flowers of Lachenalia spp. in the collection now.
While these are the three most speciose Cape geophyte families, there is certainly more to be said about the diversity of these plants and their storage organs. You may also observe other plants within the collection that share geophytic characteristics – For example Bulbine and Kniphofia in the family Asphodelaceae, Wachendorfia in the family Haemodoraceae, and Lanaria in the family Lanariaceae all have different types of rhizomes (large, modified roots) as the storage organ – these species typically do not go fully dormant in the summer.