After being babied indoors, too much sun and wind can wreak havoc on transplants until they've "hardened off", gradually building up a tolerance to the outdoor elements.
Think as if you were starting a physical fitness regimen; you shouldn't try to run a marathon on the first day! Plants are much the same, as they need a conditioning program (hardening off) to develop the strength necessary to withstand outdoor conditions.
Following, is the outdoor hardening schedule that I used for the tomato transplants pictured above. This is typical of the type of hardening regimen that you might employ for most vegetable transplants:
Day 1 - 45 minutes, sheltered from wind, indirect sunlight (shade)
Day 2 - 1 hour, minimal wind exposure and dappled sunlight
Day 3 - 2 hours, mild wind exposure, full sun
Day 4 - 3 hours, normal wind, full sun
Day 5 - 5 hours, wind, full sun; started earlier in the morning to expose plants to cooler morning temperatures
Day 6 - ditto of prior day, for 6 to 7 hours
Day 7 - plants can tolerate a full day of exposure, and are ready for the garden after today!
Even after the plants have been successfully hardened, a mild overcast day is ideal for planting. If your only option is planting on a hot day with full sun, you may wish to provide temporary shade to lessen the stress on the plants, or plant later in the day when the sun is less intense.
Simple Tip: A cardboard box is your friend for hardening transplants! You will be making multiple trips in and out with your transplants. So, put them in a box with sides tall enough to keep the transplants from tipping over and spilling out all over your floor. Also, you don't want those transplants to be knocked over by the wind, so just keep the plants in the box when you put them outside.
In the photo above, you'll notice that the box I had was a little too big, so I added another friend of the gardener... a brick!
Blog entry by Mark Murphy, CMG