Hardening off may sound like some sort of difficult task, but relax, it's not very hard.
Hardening off is the process of acclimating nursery seedlings to the outdoors. Because young plants are grown in greenhouses or under lights in our homes or on a sunny windowsill they have been pampered. They need to be introduced to the elements that mother nature provides like wind, temperature fluctuations and hot sun... gradually.
Start by moving young plants outdoors to a shady or semi-shaded spot for three to four hours a day. The area should be somewhat protected like a porch or under a large tree. It's wise to plan to initially bring the plants in at night, especially in early to mid-May when temps can dip into the freezing range. You need to be watchful and prepared to quickly rescue your plants if temperatures become very cold or hot or if heavy rain is predicted. Gradually increase the time outdoors for your plants by one to two hours a day. After a few days move them to an area with morning sun, but return them to shade in the afternoon. After about a week the little plants should be able to handle all day sun and can stay out at night when the temperature stays at around 50 degrees.
Watering needs to be decreased also in order to acclimate the plants. This will toughen them and prepare them for transplanting. One source I found recommended letting the plants begin to wilt before watering them. Do this for about two weeks prior to transplanting. That seemed a little harsh, I have never done that...not on purpose anyway.
Transplanting is best done on a cloudy day which decreases the stress caused by full sun. Make sure to water plants in well with a weak fertilizer solution. The water will help to settle the soil firmly around the roots.
Cold frames can also be used to harden off young plants. Some of the small greenhouses that are available now can serve as a cold frame. The process is essentially the same. You must have a heat source to use at night, either cables under the flats or a small space heater with a temperature control. During the warmer daytime hours doors and flaps should be opened to gradually introduce outside elements much the same as the basic hardening off process.
Another consideration when hardening off plants is to think about plants individually. In other words, tomatoes, squash, melons and cucumbers are some of the warm weather plants and are more susceptible to cold. Take special care with them when hardening off. Whereas, plants like broccoli, cauliflower and other cole crops can take the cold better. They can even tolerate temps close to freezing if they have been acclimated properly.
If the moving in and out process sounds tedious and time consuming, try putting your plants in a wagon or wheelbarrow. That way you can just wheel them into the garage at night.
The University Of Minnesota Extension Service website-www.extension.umn.edu/garden/yard-garden/-offers researched information on transplanting and basic gardening as well as many other horticultural topics.