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Matching hardiness zones with succulents

Sept. 5, 2008 – Succulents aren't just for summer, but before gardeners start replacing their yard's thirsty plant assortment with easy-care, cold-hardy succulents, they need to become familiar with the plant hardiness zone map.

Many of the most drought tolerant succulents are not creatures of the desert. The origins of some familiar species – hens and chicks, for example – have been traced to the snowy slopes of the European Alps.

Before replacing your yard's thirsty plant assortment with easy-care, cold-hardy succulents, become familiar with a plant hardiness zone map, which divides the continent into zones based on average wintertime temperature minimums.

Here are some of the most common cold-hardy perennial succulents, along with a few recommendations for gardeners who live at higher altitudes or deep in the Snowbelt:

Hens and chicks, or houseleeks (Sempervivums): Can carpet an entire area with many colours, sizes and textures. But generally, the smaller the rosettes, the slower the rate of growth, so be size-selective if you're looking to cover a large area fast. There are more than 4,000 named varieties, providing many opportunities to mix and match for contrast. “Sempervivum tectorum has incredibly rich red coloration on such a small statured plant that tucks so well into any crevice with soil and light,” said Karl Gercens, conservatory horticulturist with Longwood Gardens, Kennett Square, Pa. Good for zones 3-7.

Ice plants (Delosperma): “Originally hails from the mountains of South Africa, where it covers the rocky slopes with its intensely coloured flowers in the summer,” Gercens said. These tiny ice plants are tough enough to line freeways or highlight alpine gardens, where they produce delicate magenta, yellow, reddish and white blooms. They generally prefer full sun and a consistent water supply. Also important is placing them in a well-drained site. Gercens recommends the Delosperma cooperi, a tough species with traditional fuchsia-purple flowers covering a 15-to-20-centimetre-tall carpet through the growing season. Zones 4-9.

Prickly pears (Opuntia): The spiny Opuntia polyacantha is the most common prickly pear variety in the western Great Plains, growing from Arizona to Alberta, according to Gwen Moore Kelaidis in her book, “Hardy Succulents: Tough Plants for Every Climate” (Storey Publishing, 2008.) “They are similar in size and design value to dwarf shrubs or conifers and they offer year-round interest in foliage and form,” Kelaidis said. Zones 3-9.

Sedum: More than 600 species worldwide. Their evergreen leaves – red, maroon or blue-grey – are the main attraction, though many produce bright blooms. The compact varieties are good companion plants to flowering perennials like clematis and foxgloves (Digitalis), Kelaidis said. Zones 3-8.

Euphorbia: A blend of poinsettia and milkweed; many of the hardy selections have cheery flowers in the spring. “Always avoid the milky sap, which can be an irritant,” Gercens said. Euphorbia 'Robbiae' is a cold-hardy succulent that can tolerate shade.
“Admire the chartreuse flowers illuminating the darkness on a gray day,” Gercens said. Zones 5-8.

Agaves: Short, stout, clump-like plants with thick, pointed leaves that often are spiny. Usually blue-green leaves with blooms ranging from red to cream. These North American natives have many food, fibre and medicinal uses. “Agave parryi is likely the most cold-tolerant agave out there,” Gercens said. “This 18-inch-tall (45-centimetre) Arizona native is reputedly hardy in zones 4-10.”

Leave a couple of sample plants outdoors during a cold spell if you're unsure how winter-hardy a particular variety is, Kelaidis said. “You don't really know the zone (limitations) until you kill them,” she said.