by Bruce Brethauer
Delosperma cooperi, one of the so called purple ice plants, has long been heralded as one of the most reliably hardy of the Delosperma species, tolerating temperatures down to -20 degrees Fahrenheit, and (given a site with sharp drainage and full sun) it will tolerate a surprising amount of rain and snow. It is an excellent plant for hot, very sunny sites with very sharp drainage - where few other garden plants will succeed. Where it is happy, it is a rapid grower, quickly growing to a height of about 8 inches and spreading to about 2 feet or more. It is a succulent plant with long semi-succulent stems and narrow, succulent foliage growing to over 2 inches in length and to about 1/4" wide at its widest. The leaves and stems are covered with papillose tissues which reflect light in such a way as to give the surface of the plant the appearance of being covered with dew or frost, which probably accounts for the common name of "Ice Plant". The stems have very long internodes, giving this plant a rather leggy, almost etiolated appearance - but this is typical for this species, and is not cause for concern - but for those of us who prefer a more compact looking plant, this can detract a little from its general appearance - especially when the plant is not in bloom. An established plant will flower throughout the summer months, producing an abundance of pink to vibrant magenta/purple daisy-like flowers, and will often flower until frost. It is one of a very few perennial plants which produces such a long and abundant floral display. Because it is such a rapid grower, it is probably best to grow this plant away from some of the slower growing drought tolerant plants (such as sempervivum, lewisia, and many of the lower growing hardy cacti), as the rampant growth of this species would soon overwhelm these plants. Some growers state that the growth of this species is so aggressive, that it is able to out-compete some garden weeds - a statement that I would treat with a grain of salt, as I frequently have to remove the odd weed from my plant.
Delosperma cooperi is widely grown as a groundcover, and is especially popular in the states of the high plains and the mountain west, where its cold hardiness and drought tolerance makes it an especially popular plant in the landscape. But this plant is available at practically every nursery in the contiguous states, and true to its reputation, it has proven to be remarkably adaptable, and is very cold hardy - surviving to temperatures of -20 degrees Fahrenheit, at least in some regions: but here in Ohio, it has frequently proven to be a bit less than hardy - despite having winter temperatures which fall well within the cold tolerances of this plant. Other Ohio growers share my experience: many growers here have reported difficulty in maintaining their plants through an Ohio winter. I suspect that our combination of winter wet, and temperatures which fluctuate irregularly though the winter months contributes to its difficulty in surviving an Ohio winter, as it is a more reliable survivor in regions with more stable winter temperatures - even regions with colder winters than ours. This is not to say that this species will not survive a winter here - given a protected site and a somewhat drier and warmer microclimate (or alternately, covering the plant to keep it drier through the winter months) it shows significantly improved survival rates through our winters - but even then, survival may not be guaranteed through our winters.
Propagation is easy - I recall that I had once grown this plant from seed, and there are accounts of plants self sewing in greenhouses, and in garden beds - but this species is usually grown from cuttings, as this is often the best method to propagate especially choice clones - those which are especially free flowering, or with particularly large and vibrant flowers or unusual flower colors, and possibly even clones with greater survival rates in a given region. I have sometimes inserted cuttings directly into the soil immediately after taking them, and I have also set cuttings aside for a week or so to allow the cut ends to heal before inserting them into the soil - both methods will work well, but cuttings immediately inserted into the soil will have a slightly higher failure rate due to rot. I insert the cut end into the soil or potting medium until the node just touches the soil - the nodes are generally where the new roots will be produced. Cuttings readily take, and will frequently set roots within a week or two when taken early in the season - cuttings taken late in the season will tend to be slower to root. Cuttings taken early in the season, and planted out in the garden will usually produce significant growth - especially if the plantlets are given some extra water and fertilizer as they become established. Established plants will also benefit from some supplemental watering - otherwise, these will generally require little additional care.
In general, the mesembs do not require much fertilizer, but because this species has such a rapid rate of growth, it will probably benefit from a few applications of fertilizer during its peak growing season. Probably the best fertilizer would be a slow release bulb fertilizer, or bone meal applied near the roots of the plant in spring just as this plant is showing signs of growth. You may also make several applications of a water soluble fertilizer such as Miracle Grow or Peters fertilizer - a lower nitrogen formulation is best - such as a bloom booster - but even then, dilute the fertilizer to 1/3 to 1/4 the recommended strength - beyond that there is little else to do aside from some periodic garden maintenance - otherwise, sit back and enjoy the marvelous display of flowers.
Delosperma cooperi is a magnificent plant, producing a vibrant floral display though the summer months, surviving, and often thriving in hot, sun baked, conditions where many other plants would have difficulty getting established. Properly sited, this species has proven to be reliably hardy in greater portions of the United States, frequently surviving temperatures down to -20 degrees. Even though this species has proven to be marginally hardy through Ohio's wet winters - it should nevertheless be more widely grown here as it is a great plant for problem places where few other plants will thrive - its vibrant flowers and long flowering season would make it a good selection for any annual border, and perhaps it should indeed be treated as an annual. But occasionally, even here in Ohio, it sometimes will survive a winter - providing one of those serendipitous surprises when new growth is detected in the spring.