"Hardening off" may sound like some sort of difficult preparation for a college exam. Take heart. It isn't. It isn't even hard!
Hardening off refers to the process of acclimating plants from indoor temperatures to the outdoors. Because plants usually are grown in greenhouses (or, as seedlings in our kitchen window!), they've been pampered. They need to be introduced slowly to the elements of wind and intense sun.
Initially, you will put plants outdoors only for short periods of time, perhaps for a couple of hours. You'll want to set them in a semi-shaded area of the yard. Gradually, you will increase the time plants are kept outdoors; you also will gradually increase their exposure to sun. After 6 to 8 days, these plants will be ready for the outdoor life.
As part of acclimating the plants to the outdoors, you also will cut back on watering. This will allow plants to toughen and will prepare them for being transplanted.
It's a good idea to transplant on a cloudy day, when the plants won't get full exposure to the hot sun on their first day in the ground. When you transplant, arrange soil to provide support for stems and water with a weak solution of fertilizer. Let the water do most of the work of settling soil around the roots.
In this area, the average last day of frost is about May 10. That can vary considerably, however, especially at higher elevations. Therefore, be aware of the possibility of frost and be prepared to protect tender transplants. The use of floating row covers, water-containing "walls 'o water," or similar other barriers can help with this problem.
Photograph courtesy of Judy Sedbrook.
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010