To answer when it is safe for us to be planted outside, you must first understand where we come from. We, Osteospermum, are native to South Africa. This means our genetic programming is for warm, sunny climates with well-draining soil. We are perennial in very warm regions (USDA zones 9-11) but are most often treated as annuals in cooler areas. Our root systems and foliage are highly susceptible to damage from cold, wet soil and frost. Our internal dormancy cycle breaks only when soil temperatures consistently reach a level that signals the true end of winter, allowing our roots to actively grow and establish themselves instead of just sitting dormant and rotting.
While you gardeners often watch the air temperature, we are more concerned with what is happening below the surface. Our root systems are the foundation of our health. Planting us into cold, soggy soil is a primary cause of failure. For us to thrive, the soil must be workable and, most importantly, warmed. The ideal soil temperature for planting Osteospermum outside is consistently above 60°F (15.5°C). You can measure this with a simple soil thermometer. Cold soil shocks our systems, stunts our growth, and makes us vulnerable to root diseases like pythium and phytophthora (root rot). Warm soil, however, encourages rapid root expansion and establishment.
We must communicate this with utmost urgency: Osteospermum are not frost-tolerant. A single light frost can severely damage our tender leaves and stems, and a hard freeze will be fatal. Therefore, the timing of outdoor planting is entirely dependent on the passing of this danger in your specific region. You must wait until after your area's average last frost date. This is a statistical date, not a guarantee, so always monitor the local short-term forecast for unexpected cold snaps. For us, "spring" does not begin on a calendar date; it begins only when the threat of freezing nights has completely passed.
The vast climate differences across the United States mean there is no single planting date. Our planting schedule must be tailored to your garden's zone.
Zones 9-11: Here, we can often be planted in early spring, as early as March or even late February, as the risk of a hard frost is very low.
Zones 7-8: The target window is typically mid to late April. The soil has had more time to warm, and frost becomes increasingly rare.
Zones 5-6: Planting should be delayed until mid to late May. Winters are longer, and soil takes more time to lose its chill.
Zones 4 and below: Wait until late May or even early June. It is crucial to be patient, as a late frost is a real possibility here.
If you have raised us indoors from seed or purchased us from a warm greenhouse, we are not prepared for the harsh realities of the outdoor world. Our tissues are soft and susceptible to sunscald and wind damage. You must gradually introduce us to these conditions over 7-10 days. This process, called hardening off, involves placing us outside in a sheltered, partially shaded location for a few hours each day, gradually increasing our exposure to sunlight and wind, and bringing us back indoors at night. This allows us to develop tougher, more resilient cell walls and prevents severe transplant shock, ensuring a much stronger start once we are in our permanent home in your garden.
