Whether you've been babying seedlings along under grow lights or overwintering a few tender perennials you couldn't let go of last fall, it's time to start getting them outside. But you can't just chuck them out there as soon as the ground begins to thaw. Plants that have been indoors need to be acclimated to the fluctuating temperatures, wind and sunlight. In other words you need to harden them off.
It's a simple process of gradually increasing the amount of time they're outside, usually by taking flats or pots in and out of the house. As plants spend more time outside, stems get stronger and their leaves develop a thicker cuticle, the waxy covering on the surface of a leaf that prevents water loss. this helps them tolerate more varied weather conditions.
WHEN TO GET STARTED
First decide when you want to put your plants in the garden and back up a couple weeks from there to know when to start hardening off. Tender plants' planting date will be determined by your likely last frost date. If you don't know when your last frost date is check with your local county extension agency. Cool weather types such as pansies tolerate temperatures in the 40-degree F range so you can plant a month before your likely last frost date. House plants, tropicals and warm season vegetables, such as tomatoes, prefer days at 70 degrees F (or warmer) and nights in the 50's. Seedlings should have at least four to six sets of true leaves before going out.
WHERE TO PUT PLANTS
Start plants in a protected spot out of direct sunlight and strong wind for an hour or two in mid-afternoon to avoid shock. Examples include under a tree, next to a hedge and near the wall of the house or garage. Bring plants inside for the night. Each day increase the time spent outside by 35 to 40 minutes. If there are a few days of bad weather-driving rain or an unexpected cold snap- leave them inside. Once plants are used to being outdoors all days (and mild overnight weather is forecast), pull them into a protected spot out doors for the night, too.
Full sun plants will need time to get used to direct sunlight. Start them out in part to full shade on the north side of your house or under a tree. Every couple of days edge them into a bit more light until they are basking in full sun. once plants stay outdoors day or night for three or four days in a row, they're ready to go in the ground.
Excerpt from Garden Gate Magazine