Harden off seedlings to toughen them up for transplanting outdoors. When you harden off vegetable and flower seedlings that began life indoors (thanks to you or a commercial grower), you help the seedlings acclimate gradually to the brighter light and cooler temperatures of the outside world. This process slows plant growth, causing the plants to store more food internally and increase the thickness of their outer leaf layers.
To harden off your transplants, follow these steps:
A week or two before you intend to set plants out in your garden, stop fertilizing and reduce the amount of water you give them.
Give plants just enough water so they don't wilt.
Take your plants outside for a short time.
Give the plants a half hour of filtered sunlight (light shade) — setting them under an arbor or open-branched tree — during the warmest part of the day. If the weather is windy, put the plants in a spot where they're sheltered, or construct a windbreak out of pieces of wood.
Gradually increase the amount of time that the plants spend outside and the intensity of the light that they're exposed to.
You want to increase the amount of time your plants are outside so that by day seven, they're out all day. Move them into progressively sunnier locations during the week so they get used to their future condition in the ground. However, make sure that you bring the plants in every night.
Another option is to move your plants to a cold frame, and then you can open the cold frame more each day and close it at night. Plants that are raised in a cold frame from the time that they're young seedlings will need much less hardening off.
Don't overharden your plants. Certain crops, such as cabbage and broccoli, can quickly bolt if seedlings older than three weeks are repeatedly exposed to temperatures lower than 40 degrees for a couple of weeks.