Before moving seedlings from the pampered pot, where they received just the right amount of light, nutrients and water, to the garden plot, they need a little toughening up. By gradually letting tender seedlings acclimatize to the harsher outdoor conditions, they'll be better able to withstand the rigors of outdoor living. This strength and endurance training process is called hardening off. It's like boot camp for your seedlings.
As a general guideline, you can start hardening off seedlings ten to fourteen days before the last frost date. While this date varies from one region to another, fortunately, it's easy to find out by consulting publications such as A Gardener's Journal, The Old Farmer's Almanac, or seed catalogs. You can also obtain this information from your local extension service, or by doing a quick search on the Internet. As that date approaches, take notice of your local weather forecasts for a more realistic start date.
When weather conditions are favorable for bringing the seedlings outdoors, choose a partially shaded site near a fence, the side of a house or a hedge to prevent windburn and sunburn. Start exposing the seedlings to the elements for a few hours, and then bring them back indoors.
A drastic change in the weather and temperature can harm the seedlings. If the seedlings are exposed to direct sunlight, wind and lower temperatures too quickly, you may notice signs of distress, such as burning on the leaves or damaged flower buds. Weak plant material may also break in harsh wind.
Little by little, extend the time the seedlings spend outdoors and expose them to a little more sun, ensuring they are kept well watered, but being careful not to over water. Within seven days (and weather still permitting), the seedlings should be able to withstand the entire day outside. After a few days of full exposure to the elements, the seedlings will be ready to be transplanted into their allocated garden setting. This is especially true if you see new growth, sturdier stems and bud formation, as these all show that the seedlings can now survive on their own. In cold climates, seedlings may need the full two weeks to adapt, while in warmer climates, seedlings may be ready for transplant in a week's time.
The hardening-off process is greatly facilitated by the use of frost protectors. You can cover the seedlings with blankets or burlap, or make use of a cold frame, cloches, row covers, or a greenhouse with removable or roll-up sides. They all allow you to gradually introduce seedlings to the outside world without having to bring them back indoors when frost threatens. As the temperature rises during the day, you can remove the covers, and when it drops at night, it is a simple matter to shelter the seedlings. A cold frame needn't be fancy — old windows or Plexiglas® mounted with hinges to a wooden box or frame will do.
For successful hardening off of seedlings, continue to fertilize them regularly with an all-purpose fertilizer or compost tea. This will give them the nutrients they need for healthy and strong stem and leaf growth, flower buds and roots.