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Delospermas in cultivation are more likely to have the wrong name on the label than the right one.
D. congestum is one of the abused names, usually applied to small mat-forming plants with yellow flowers. D. congestum as originally described is an upright shrublet with white flowers and is not known in cultivation.D. nubigenum is a mat-forming species with yellow flowers, often sold as D. congestum or with a cultivar name like 'Gold Nugget'. Sometimes it has a white eye and it is likely that these plants have a hybrid origin. There is also now a 'White Nugget', again an unknown hybrid. There is no other Delosperma that you'd mistake for it, smooth rounded yellow-green leaves forming a mat no more than an inch high.This same plant is also sold as D. basuticum, again a completely different species. The name D. basuticum is also applied to a similar mat-forming plant with yellow flowers, but having sharper leaves minutely toothed along the edges. I'm pretty sure this is also incorrect since D. basuticum is described as a having pink flowers.D. nubigenum is extremely cold hardy and very tolerant of moisture. Mine survive the weather but not the slugs!
Very interesting about the species confusion, Iann.Bolus' original description of D. congestum (in latin) is shown in the attached link within a discussion about the frequently-confused species; the flowers are white, turning lemon-yellow ("alba, marcescentia citrina") as they persist:http://www.fgas-sukkulenten.de/downloads/delosperma_e.htmI couldn't find Bolus' description of D. nubigenum but here's an extremely terse description of it from the South African National Biodiversity Institute (SANBI) - it does note pink petals. (The other specimen described under the same species heading was not in flower but the leaves are said to match Bolus' description.)http://sibis.sanbi.org/faces/SearchSpecimen/SpecimenDetails.jsp?1=1(Amazing what one can find on the internet!