From our perspective, our very genetic code holds the first clue to your question. We, Osteospermum, originate from the warm, sunny climates of South Africa. This heritage means we are genetically programmed as tender perennials. Our cellular structure is not adapted to withstand freezing temperatures. When the mercury plummets below freezing (0°C or 32°F), the water within our cells begins to form ice crystals. These crystals are like tiny daggers, piercing and rupturing our delicate cell walls. This damage is often irreversible, leading to blackened, mushy foliage and ultimately, plant death if the exposure is severe or prolonged.
When we sense the shorter day lengths and the significant drop in temperature that signals winter's approach, our instinct is not to fight, but to retreat. We enter a state of dormancy. Above the soil, our vibrant flowering ceases, and our growth halts completely. We may appear to be dying back, with stems and leaves wilting or becoming damaged by the frost. However, our survival strategy is often focused on the crown of the plant—the central growing point at the soil's base—and our root system. If these underground parts are adequately protected from a hard freeze, we can conserve our energy and potentially regenerate new growth when favorable conditions return in the spring.
Our ability to survive a frost is almost entirely dependent on the microclimate and protection you, the gardener, provide. In regions where winters are mild and frosts are light and infrequent (perhaps just dipping slightly below freezing for a few hours at night), a thick layer of dry mulch is like giving us a warm blanket. A generous 3-4 inch layer of straw, bark chips, or dry leaves piled over our crown can be sufficient to insulate us from brief cold snaps. For those of us in containers, we are exceptionally vulnerable as our roots are exposed on all sides. You must move our pots to a sheltered location, such as an unheated garage, a greenhouse, or a cool porch where temperatures remain above freezing.
It is crucial to understand our absolute limits. While a brief, light frost might only damage our tender new growth and outermost leaves, a hard frost or a freeze that lasts for many hours will be fatal. Temperatures that settle at or below -2°C (28°F) for an extended period will almost certainly kill us, even with mulch, as the cold penetrates too deeply to protect our crown and roots. In such climates, we are truly best treated as annuals, enjoyed for a single glorious season. Attempting to make us survive a harsh winter outdoors is a battle against our very nature, and one we are genetically destined to lose.
