The gradual acclimatisation of seedlings grown indoors or in a greenhouse to outdoor conditions. Involves progressively increasing exposure to sun, wind, and temperature variation over 1–2 weeks before final transplanting.
Hardening off is essential for Australian gardeners because seedlings started indoors or in greenhouses are sheltered from the elements and develop soft, delicate foliage. When exposed suddenly to harsh outdoor conditions—intense Australian sun, dry winds, and temperature swings—they often suffer leaf burn, wilting, or death. The hardening-off process typically takes 7-10 days and involves gradually increasing a seedling's exposure to outdoor conditions.
Start by placing seedlings in a sheltered, shaded outdoor location for 2-3 hours on the first day, gradually increasing exposure each day. By day 7-10, seedlings should tolerate full sun and outdoor temperatures. In tropical zones (far north Queensland), protection from intense midday sun remains crucial even during hardening off. For temperate and cool climate gardeners, begin hardening off in spring (September-November) when nighttime temperatures won't drop below 10°C.
You can harden off seedlings using a cold frame, shade cloth progressively removed, or simply moving pots around your garden. Many Australian gardeners use Bunnings shade cloth products to create a graduated hardening environment. This process is vital for vegetables, annuals, and perennials started from seed indoors. Skipping hardening off often results in transplant shock and poor establishment, wasting the time invested in propagation.