Don’t move plants outdoors until the temperatures are warm enough for them to handle. According to Growing Gardens, it’s important to know how “hardy” the seeds you’re planting are, that is, what temperatures they can take. Hardy plants like broccoli and onions can move outside when the temperature is regularly above 40 degrees F (4 degrees C), half-hardy plants like celery and lettuce can move out at 45 degrees F (7 degrees C), and tender plants like melon and corn need temperatures in the 50s and 60s F (10 to 16 C).
If it’s too cold to move your plants outdoors, but you want to begin the process of strengthening them, Burpee has a neat trick: Place a fan near the area indoors where you keep your seeds. Make sure it blows gently, though; too much direct wind could dry out your plants.
When you are ready to start moving your plants outdoors, Burpee further recommends that you bring them to a protected spot for an hour the first day. Add an hour each additional day for a week. Once they can hack it outside for seven hours, they’re ready to transplant.
One of the biggest hassles involved in hardening off plants is repeatedly moving them in and out of doors. Proven Winners suggests placing your plants in a wheelbarrow or wagon so that you can move them easily between your yard and a covered patio or garage.
Jeremy Dore of GrowVeg recommends moving plants first to a greenhouse or cold frame. This way, you can open the structure to the elements more and more each day, increasing your control. Dore acclimatizes his plants to an unheated greenhouse over a period of two weeks. After that, transition to full outdoor living is a weekend’s work.
So you’ve gradually exposed your plants to the elements and it’s time for them to spend their first full day outdoors. The Living Farm stresses the importance of keeping them watered that first day and recommends placing them in a shallow dish of water if you don’t have time to monitor them.
Here’s a fun and useful idea from Veggie Gardening Tips for hardening off tomatoes. About two weeks before moving them outside, run your fingers lightly over the seedlings each day. This will help their stems to grow thicker, strengthening them for outdoor living. Source.