Hardening off means acclimating a plant to a new environment. It refers to toughening up a plant a bit, so that it’s not shocked by direct sunlight, dry conditions, and wind, and it is a term and process primarily used for young transplants.
Plants that are seeded directly outdoors are naturally acclimated from the beginning of their lives, but plants that are started from seed or vegetative cutting in a protected environment like a greenhouse, a kitchen windowsill, or even a wind-protected porch or deck require hardening off before being planted into their final spot in your garden. Hardening them off prevents them from being shocked when it’s time to finally plant in the great outdoors of your garden.
You can begin the hardening off process immediately following germination by using a gentle fan in your protected growing area, “petting” your plants from time to time, and making sure your transplants have differing day and night temperatures.
In the 3-5 days before planting out in the garden, your transplants will benefit from spending an increasing number of hours each day outside the protected area. Try 3-4 hours on day 1 in a partially shaded area, increasing the time by a few hours and the amount of direct sunshine each day until you’re sure they can handle the weather in your garden. In this way you’ll give your transplants the best preparation for a spring and summer of healthy growth and production.