Delosperma, the Hardy Ice-Plants
One of the up-and-coming stars in the plant world are the Delosperma, commonly called hardy ice-plant. A little over a decade ago, this plant was virtually unknown to North American gardeners. Their introduction to western gardeners was primarily due to one individual, Panayoti Kelaidis, a world-renowned gardener, lecturer and plant collector. Panayoti is a long-time employee of the Denver Botanic Garden and designed its extensive Alpine Garden. He is currently the Senior Curator and Director of Outreach. A little over a decade ago, Panayoti made a plant exploration trip to the Drakensberg Mountains of South Africa, looking for new plants that might prove hardy for American gardeners. Delosperma cooperi and D. nubigenum were the first two hardy Delosperma introduced by Panayoti, species that could survive to zone 4! A couple of years later he returned to South Africa and introduced two more species, D. floribundum and D. herbeum. He has made several more trips to South Africa and continues to introduce new plants from the wonderful flora of that region. In honour of his discoveries, a spontaneous hybrid Delosperma that arose in the Denver Botanic Garden collection in 1998 was given the cultivar name ‘Kelaidis' (known under the trade name ‘Mesa Verde'). Many of our now available Delosperma are thanks to Plant Select, a cooperative program administered by the Colorado State University and the Denver Botanic Garden (not surprising, Panayoti was one of the leaders in this project.)
Panayoti has been a long-time member and advocate for the North American Rock Garden Society (NARGS). Much of the excess seed he collected in South Africa was offered to NARGS members for us to try in our gardens to help test their hardiness (I am the chair for the Newfoundland Chapter of NARGS.) I was fortunate to receive some of that seed, including Delosperma cooperi. I grew these plants as houseplants for years before I was brave enough to try them outside in my super-wet and snowy climate. No one was more surprised than me to discover that D. cooperi could survive Newfoundland winters! After the first winter, the plant looked a little sodden and ragged but a few tips plumped up and started into growth and soon the plant was a trailing mat again.
At this point, a little background on the genus might be helpful. Delosperma are members of the Aizoaceae, a family containing about 135 genera, mostly confined to the South African region. Many genera are commonly called ice-plants since several produce calcium crystals on their leaf surfaces that given the appearance of frost. Their leaves are succulent and rounded to tongue-like in shape. The flower are typically daisy-like with a satiny texture. For northern gardeners the most familiar genus is Mesembryanthemum, a fantastic trailing annual species with a long blooming season. The genus Delosperma has about 100 species, mostly with low, mat-like habits. All are native to the South African region. Most are not hardy in northern climates but the list of those that can survive zone 6 and colder seems to be increasing. Besides Panatoyi, Joseph Halda, a plant collector from the Czech Republic, has also made trips to South Africa looking for hardy plants and he too has introduced some surprisingly hardy Delosperma species such as D. deeleeuwiae.
Mesembryanthemum are essentially annual versions of Delosperma
In regards to cultivation, these sun-loving plants are more tolerant to wet than most succulents (surviving my 60 inches of rain a year will attest to that!) However, they still prefer sharp drainage, particularly in winter. For gardeners in wetter regions, scree conditions suit them best or alternatively, grow them atop retaining walls. Needless to say, they are wonderful xeric plants and with the increase in water restrictions these days, they will no doubt gain more popularity. While you would think the hardy ice-plants could withstand heat, most hail from mountain regions, therefore actually dislike excess heat. Some, like D. basuticum and D. nubigenum, bloom mainly in late spring while others, like D. cooperi, blooms all summer. They are proving to be star performers in the Rocky Mountain States but are also proving their worth in northern areas like the Okanagan Valley of British Columbia and southern Ontario. My garden contacts in Norway and Sweden are also singing the praises of these hardy Delosperma. Although many species are rated for zones 5 to 6, these are only reliably hardy in reasonably dry winter regions. Only three species survive outdoors in my wet zone 5b winters: D. cooperi, D. congestum and D. basuticum. The others I grow in pots that are overwintered in a cold-frame or frost-free basement.
The hardy ice-plants are generally either hot pink or yellow shades, but there a couple of exceptions. Delosperma cooperi is among the most trailing of the hardy ice-plants with masses of brilliant pink flowers from late spring through summer and even into the fall. In suitable climates, it makes an admirable ground-cover. It is rated for zone 5 and is among the most heat-tolerant of the hardy ice-plants. Very similar in appearance is D. deleeuwiae, but this one is slightly more tender.
Details of D. cooperi
Two similar species are D. ashtonii and D. sutherlandii, both with relatively large, hot-pink flowers with paler centers. While florally, they look identical, they do differ slightly in their foliage. Both are rated hardy to zone 7, maybe zone 6, but make lovely pot-plants which may be overwintered in a cold basement (that's how I keep mine). Also with large magenta, white-eyed flowers is D. floribundum, known in the trade as ‘Starburst'. It is rated to zone 6, possibly 5.
Above are the species D. ashtonii, D. sutherlandii and D. floribundum
Among the hardy yellow-flowering species are D. nubigenum, D. congestum and D. basuticum. All are rated to zone 5 and I know of D. congestum growing and surviving in Calgary, Canada, a zone 3 region! The former species is another suitable ground-cover due to its trailing habit but I find the look-alikes D. congestum and D. basuticum to be more tufted in habit. There is a white-flowered version of D. basuticum called ‘White Nugget'. These species bloom primarily in late spring.
Above are the common yellow-flowered species D. nubigenum, D. congestum and D. basuticum
Delosperma herbeum has white or pale lavender flowers all summer and grows a little taller than the other species. With orange-peach flowers is the species D. dyeri. There is a red-flowered selection called ‘Psdold' (aka ‘Red Mountain'). For interesting foliage you can try D. daveyi, which has bronzy foliage dotted with small white flowers. For troughs, the best species is the mini D. sphalmanthoides with tiny, upward-pointing foliage and magenta-pink flowers in spring. All of these are rated for zone 6.
Above are D. herbeum, D. dyeri 'Red Mountain' and D. sphalmanthoides
Of possible hybrid origin are the selections ‘John Proffitt' (aka ‘Table Mountain') with reddish-pink blooms, ‘Kelaidis' (aka ‘Mesa Verde') with peach flowers and the newest introduction (to be released in 2009) called ‘Psfave' (aka ‘Lavender Ice') with lavender flowers. These all bloom from spring through fall and are rated to zone 5.
Above are the hybrids 'Kelaidis' and 'John Proffitt'
For gardeners in cold but dry-winter areas, keep your eyes open for more upcoming hardy ice-plants as no doubt more will become available in the near future.
A sincere thank-you to the following people whose pics helped beautiful this article: Ally_UT (D. nubigenum), altagardener (D. congestum), growin (D. cooperi (mass flowering) and D. sphalmanthoides), JamesCO ('Red Mountain'), LawrenceM (D. herbeum), Nurbanworld (D. floribundum), QCapen ('Kelaidis') and TomH3787 ('John Priffitt')