Before the young plants can be placed outside in the outdoor garden, they have to acclimate themselves to the outside environment. If you simply put them in the outdoor garden, they will go into shock and either die or become stinted in their growth. For this reason you must gradually introduce them to the increased light and lower temperatures of the outdoors. Hardening off simply means slowly exposing young plants to the outdoor environment. What you must do is slow the metabolism of the plant, thereby interrupting its vigorous rate of growth, and force it to begin to store food, in the form of carbohydrates, and to conserve and store water.
The first thing you must do is to decrease the water the plant receives. The second is to stop fertilizing. Why? Because we want the plant to stop its vertical growth cycle and begin a horizontal growth cycle. We want the plant to start growing thicker and stouter, not taller and lankier. This will make their stems less susceptible to being kinked or bent by harsh Spring winds and will aid in their coming battle with insects.
You should now begin to take then outdoors for an hour or two at a time in a shaded, wind-protected location. Remember that this is a tremendous shock to your young plants and you must monitor, and be concerned about, their progress in adapting to their new environment. If some plants begin to wilt, leave them inside for a day or two to perk up and then try it again. Tough plants, like the cole crops (broccoli, cabbage, Brussels sprouts, cauliflower) can normally handle the transition well. The more delicate ones (eggplant, peppers and tomatoes) handle the transition less well, so be especially vigilant for these varieties. If a cold or windy day is forecast, perhaps cover the eggplants, tomatoes and peppers with a light cloth or clear plastic for that day.
Now increase the time you leave them outside by adding an hour to their outside time each day or two. The outdoor winds will dry out the plants quickly, so check daily and water if necessary, but only enough water to prevent wilting. Try not to promote vertical growth with lots of water and fertilizer. The object is to keep the plants small, thick and tough, not tall, lanky and thin.
Some gardeners recommend that you place your plants in an outside cold frame, which is okay if you have one large enough. The bad thing about the cold frame is that it might protect the plants too much, especially if you close the glass cover. Temperatures rise and exposure to wind is limited. You dont want to allow your plants to experience warm temperatures during the hardening off process. In fact, just the opposite is true: you want exposure to lower temperatures and increasing tolerance to wind dynamics. Simply placing them in a sheltered north-side area next to the home we find works best. Then gradually switch the plants to a partial-sun, and then a full-sun, area as time passes.