You cannot simply take your seedlings from their warm, protected place indoors and put them in the ground outside. They’ll die. They need to be introduced to the outdoors gradually.
Hardening off is the process of introducing your young plants to the big, wild world outside. If you have a screened-in porch, this is an ideal place to do it. Starting two weeks before transplanting time, put all of your seedling trays out on the porch for one hour during the warmest part of the day. Then bring them back inside. Continue doing this each day, extending the time by one hour per day until transplanting day.
If you don’t have a screened-in porch, you will need to either construct or buy a cold frame that you will place your trays in outside. A cold frame is basically a large box, with one side several inches higher than its opposite, and covered with glass, plexiglass, or framed thick plastic sheeting. The cold frame should face the south to maximize sun exposure. For a good selection of cold frames to choose from, pay a visit to Mastergardening.com. Free shipping over $100.
On the first day, put your seedling trays in the cold frame (or frames, depending on their size) early in the morning and prop the cover open about 3 inches. Close the cover in the late afternoon hours in order to keep the heat inside around your plants. You can even insulate the cover overnight by placing a thick layer of loose straw or several old blankets on top of it. After the first day and night, depending on the weather, you can either prop the cover open several inches or remove it entirely during the day (and stretch a thin layer of mesh or a piece of thin floating row cover cloth over the top to keep the birds and bugs off while letting air and light in), always remembering to replace the cover in the late afternoon hours so your plants don’t suffer the cold overnight. By the end of the two-week period, they should be ready to plant in the ground.
Ideal transplanting weather is cool, humid, and cloudy. You don’t want your transplants to be scorched by the sun the minute they’re in the ground. They need time to adjust to their new surroundings. If the nights are mild enough by the time you’re ready to transplant, do the transplanting early in the evening after the heat of the day has passed. That way your young plants will have overnight to adjust.
The key to successful transplanting is to make sure the roots remain moist and are disturbed as little as possible during the entire process. Make sure your seedlings are well-watered before planting, and make sure your garden bed is ready. Have your planting rows marked with wooden stakes and strings, have your yardstick handy to measure where the planting holes will go, have your garden trowel (preferably a narrow one specifically for digging planting holes, like the one advertised below), and be prepared to water each seedling and the surrounding soil as soon as you get it in the ground – have a full watering can or a hose nearby.
Using your garden trowel, dig a hole the proper depth. Using your fingers, or even a kitchen fork or spoon in shoehorn fashion, carefully remove each seedling and its surrounding soil from the seed tray. This is why we like the egg carton or modular seed tray method best – it’s easier to remove the seedling while keeping the potting soil attached to its roots. Place the seedling in the hole, fill in around it with the garden soil removed to make the hole, and press down firmly. Water immediately, but carefully, using a fine spray so you don’t inadvertently drown your plant.
Protect Your Transplants
Cuff your larger plants when putting them in the ground (like tomatoes or chili peppers), using toilet paper or paper towel cardboard rolls, to keep the cutworms from attacking the stems. To keep the birds and larger bugs off, be ready with your garden fabric (available for purchase by clicking the link below) or wire mesh cages, and cover your freshly-transplanted vegetables as soon as you are done. Check them daily for weed or bug activity, and make sure to keep your new transplants well-watered – particularly if the weather is warm and sunny.
In addition, you might need to construct something that will artificially shade them from extremely bright, hot sun until they have adjusted and are obviously thriving in their new surroundings. An old bedsheet attached to two tall poles jammed in the ground would work just fine.