Hardening Off Seedlings

Seedlings grown indoors need a little time to gradually get acclimated to direct sunlight and wind in a process called “hardening off.”

The Process

To harden them off, expose your seedlings to increasing amounts of direct sunlight and wind each day, typically starting with about an hour of direct sunlight and very good wind protection the first day. Then add about an hour of direct sunlight and decrease the amount of wind protection each day until the seedlings are fine being out for seven-to-ten hours with very little wind protection.

Some gardeners prefer to put their seedlings in dappled shade for a few days before starting this process, similarly increasing the amount of time in dappled shade each day for a few days until exposing them to a full hour of direct sunlight and proceeding as above.

Either way, when their time in the sun and wind is up each day, put your seedlings back under lights indoors.

Another Way

Some especially sensitive seedlings, however—like some varieties of potato-leaved tomatoes—are easily sunburnt and may need an extra step to avoid getting damaged during the first few days. The dappled shade method mentioned above works well, but an especially useful alternative method for all seedlings, especially sensitive seedlings, is to first put them under glass outdoors. This could be in a cold frame or just under an old window propped up for this purpose. After a few days, replace the glass with an old screen. After the same amount of time under the screen, expose the seedlings to increasing amounts of direct sunlight and wind each day as described above, starting with an hour.

Words of Caution

During the hardening off process, be sure to bring seedlings inside any time day or night that the temperature drops below the ideal for each species: 55-60º for peppers, tomatoes, and melons and 40° for onions, broccoli, and other brassicas.

Additionally, if you set up glass to cover plants, be aware that the area under the glass can heat up quickly and dramatically on a sunny day and seriously injure or even kill your delicate seedlings. Give plenty of room (airspace) for heat to escape. A thermometer and a little observation go a long way during the first few days until you get comfortable with your setup.

Additional Benefit of Using Screen and Glass

Using glass and screen has an additional benefit. As long as it’s warm enough, you can leave plants outside and use the screen or glass to provide the protection plants need instead of bringing them back inside under the lights. The plants will love the time under the screen or glass. And you’ll love getting a break from having to carry them in and out all the time.

Even better, you can also set up the screen to be your timer for the amount of sun each day. Simply move the screen so that the sun will shine on the plants for about an hour or so in the morning but so the screen will shade the plants from direct sunlight during the middle of the day and afternoon. Then, just move the screen over a little bit each day to increase their time in direct sunlight until it’s finally time to remove the screen altogether and leave them in direct sunlight for a few days.

All of this is a process, but your seedlings need it, and you’ll be happy you did it.