Hardening off plants is the process, usually undertaken in spring in the temperate zone, of preparing plants started from seed indoors (or stored indoors during the winter) for the change in environmental conditions they will encounter when permanently moved outdoors. The term refers to plants becoming "harder" in the sense of being tougher or hardier, not in the sense of being harder to the touch. Without allowing plants to harden off, they will be negatively impacted by the sudden shock of exposure to the daytime's sun rays, the nighttime's coolness, and any high winds or pounding rains to which they may be subjected.
How Long Does It Take to Harden Off Plants?
It takes one to two weeks to harden off plants. For most gardeners, the process involves a transitional period in which plants are left outside during daylight hours only and in an area where they can be shaded and protected from wind and rain. They will then be brought back indoors to spend the nighttime. The number of hours that they stay outdoors will be increased gradually over time, as will the amount of direct sunlight they are allowed to receive (assuming that they are plants intended to be grown in full sun). Indeed, the operative term for the whole process of hardening off is "gradual."
When you are hardening off plants in spring, you can simply place them on the northern side of some shrubs that you have in the landscape. Dappled sunlight filters through the leaves of the shrubs; it is not strong enough to burn the plants that you are hardening off. Being outdoors in this sheltered area allows them to begin to acclimate to conditions to which they were not exposed while indoors. Gradually, the plants are allowed exposure to an increasing amount of sunlight and allowed to stay out later and later into the evening. But do not leave tender plants outdoors overnight until the last frost date for your region has passed.
The other two methods are less frequently used for hardening off plants. One involves an on-again-off-again regimen for watering seedlings prior to transplanting them into the garden. Stop giving them water until you detect the first signs of wilting, after which you will quickly revive them with water. Repeat. Keep this up for two weeks, then transplant.
The third method for hardening off plants involves having them grow in a cold frame.
What Is a Cold Frame?
A cold frame is defined as a bottomless, boxlike structure, usually covered with glass or transparent plastic, used for protecting plants from cold temperatures. It can be heated or unheated. Think of it as a miniature greenhouse. It is not essential that you have a cold frame to harden off plants. Indeed, you do not need any dedicated structure to do so. But having a cold frame allows you to bring plants outdoors earlier in the spring because of the shelter it provides. If the cold frame is heated or stays warm enough, you can even leave plants inside it overnight.
Cold frames can be fancy or simple. If you lack carpentry skills, keep it simple. In fact, the sides do not even have to be made of wood. If you feel more comfortable mixing concrete than sawing wood, you can make the sides out of concrete. Some keep it even simpler and build walls of stacked cinder blocks or hay bales.
It is best that the back wall be taller than the front, so as to achieve an angle of 25 to 30 degrees. Orient it to the south. An old storm window could supply the cover. You must be able to lift and prop up the cover so as to let out excess hot air.
Is Hardening Off Just for Plants You Start Indoors?
When most people hear talk of "hardening off," they probably think of seedlings started indoors. Indeed, many gardeners in the North start vegetable plants or annual plants such as red salvia flowers from seed on a window sill in spring, then transplant them into the ground in summer. You should, moreover, even harden off seedlings that you buy from the garden center if they had just recently come from the greenhouse where they were raised because greenhouse conditions are very different from outdoor conditions.
But there are other cases, too where you will want to harden off a plant in spring before keeping it outdoors for the summer. Take, for example, tropical plants that you overwinter indoors as houseplants, such as papyrus plants (Cyperus papyrus). Such plants have grown accustomed all winter long to the relatively low light levels in your house. Putting them outdoors in the intense sunlight all of a sudden would be shocking to them.
Occasionally, you will also hear "harden off" used in reference to winterizing trees in the fall. Used in this sense, you "harden off" a plant by reducing watering in early fall. You follow this up by watering the tree again later in the fall. This regimen will make the tree tougher and render it better prepared to cope with winter.