Hardening off plants is the process, usually undertaken in spring in the temperate zone, of preparing plants started indoors (as seedlings, for example) -- 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 (that is, you will not be able to feel the difference when you touch the plants with your fingers).
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.
The process of hardening off plants 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 amount 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."
So Do You Need a Cold Frame to Harden Off Plants?
Having a cold frame facilitates hardening off, because it allows you to bring plants outdoors earlier in the spring (a cold frame serving essentially as a miniature greenhouse).
But it is not essential that you have a cold frame. Indeed, you do not need any dedicated structure. When I am hardening off plants in spring, I simply place then on the northern side of some shrubs that I have in my landscape. Dappled sunlight filters through the leaves of the shrubs; it is not strong enough to burn the plants that I am 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.
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. 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 before keeping it outdoors permanently.
Take, for example, tropical plants that you overwinter indoors as houseplants, such as papyrus plants. 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. Arborists assure us that this regimen will make the tree tougher and render it better prepared to cope with winter.