From our perspective, winter survival is a fundamental question of genetics meeting environment. We, Osteospermum, are native to the warm, sunny, and well-drained landscapes of South Africa. Our very DNA is coded for mild climates with minimal frost. The concept of a hard, freezing American winter is entirely foreign to our biological programming. The USDA Plant Hardiness Zone Map is your guide to our potential survival. We can typically tolerate the cooler nights of zones 10 and 11 with ease. In zone 9, we may survive as semi-evergreen perennials if the winter is mild and a protective layer of mulch is provided. Anything below zone 9, where temperatures dip consistently below 20°F (-6°C), presents a severe and likely fatal challenge to our root systems and cellular structure.
The primary danger of cold is the formation of ice crystals within our cells. When the temperature plummets, the water inside our tissues freezes. These ice crystals are like tiny daggers, physically piercing and shredding our delicate cell membranes from the inside out. Once these membranes are ruptured, the cell's vital contents leak out, and the cell dies. This damage is often irreversible. Even if the aerial parts of our plant (the stems and leaves) appear to die back, there is a chance we could regrow from the crown or roots if they were sufficiently protected from freezing. However, a deep, hard freeze will penetrate the soil, freezing our roots solid and ensuring our complete demise.
Unlike truly cold-hardy perennials, our strategy for handling cold stress is limited. Our best response is to enter a state of greatly reduced metabolic activity. You might observe this as a cessation of flowering and a slowing of foliage growth. In very mild winter areas, we may simply wait it out. However, we lack the sophisticated internal antifreeze compounds (like certain sugars and proteins) that true cold-climate plants produce to lower their freezing point and protect their cells. Therefore, our dormancy is a fragile state, easily broken by a severe cold snap that we are not equipped to handle.
To grant us even a possibility of surviving a colder winter than we are built for, you must manipulate our environment. Excellent drainage is non-negotiable. Our roots fear cold, wet soil far more than cold, dry soil. Soggy conditions lead to rot, which will kill us just as surely as freezing. A thick, dry blanket of mulch (like straw or bark chips) piled high over our crown after the first hard frost is crucial. This layer acts as an insulator, trapping the residual warmth of the earth and buffering our most vital parts from the killing air temperatures above. Planting us in a sheltered microclimate, such as against a south-facing wall that absorbs and radiates solar heat, can also provide those precious extra degrees of warmth we need.
For gardeners in zones 8 and below, our survival in the ground is highly improbable. Our roots, confined in a pot, are exponentially more vulnerable to freezing than those in the insulated ground. The only reliable strategy is to be brought indoors before the first frost. We need a bright, cool (but not freezing) location like an unheated garage, sunroom, or basement window where temperatures stay consistently between 40-50°F (4-10°C). This allows us to enter a proper dormancy without being subjected to lethal freezing conditions, ready to be awakened and rejuvenated by the warmth of spring.
