Young, pampered seedlings that were grown either indoors or in a greenhouse will need an adjustment period to acclimate to outdoor conditions before being planted in the garden. This transition period is called "hardening off."
Hardening off seedlings gradually exposes the tender plants to the wind, sun, and rain, toughening them up by thickening the cuticle on the leaves so they lose less water when exposed to the elements. This helps prevent transplant shock, the term used for seedlings that languish, become stunted, or die from sudden changes in temperature and exposure to sunlight.
The length of time a seedling requires to harden off depends on the type of plants being grown as well as the outdoor temperatures. Be flexible when hardening off your seedlings and prepare to whisk them indoors or cover them if a late spring freeze or snow is in the forecast.
Choose either of the following methods to harden off seedlings:
Harden Off Seedlings by Gradually Exposing Them to Outdoor Conditions
- Begin the hardening-off process 7 to 14 days before the date you will plant your seedlings outdoors. In spring, most seedlings are typically planted outdoors after the last frost date. However, cold-hardy plants can be planted outdoors two to four weeks before the last frost date.
- On a warm day when temperatures are above 45 degrees Fahrenheit, place seedlings in an outdoor location that is protected from direct sunlight and wind for a duration of one hour on the first day.
- Move the seedlings indoors when they've reached their daily outdoor time limit, and place them somewhere warm like a heated garage or basement.
- Increase the amount of outdoor exposure one hour each day to gradually acclimate the seedlings to increasing amounts of dappled sun and wind. The key to success is gradual exposure to the outdoors. Don’t put tender seedlings outdoors on windy days or when temperatures are below 45° Fahrenheit.
- After two or three days in a somewhat shaded location, you can place the seedlings in locations that receive morning sun, and over time, gradually expose them to more direct sun. Exposing very young seedlings to direct sunlight too soon can scorch the leaves. Each day, the seedlings will be able to tolerate more hours of exposure to outdoor conditions.
- If temperatures remain warm both day and night (at least 50 degrees Fahrenheit), the seedlings should be able to handle increasing amounts of sunlight and can be left outdoors overnight. Make sure the soil doesn't dry out if temperatures should suddenly turn warmer.
- After 7 to 14 days of acclimating your plants to the outdoors, they are ready to be transplanted into the garden or container. If possible, choose a cloudy day to do so and water well after planting.
- If you decide to gradually acclimate your plants to longer periods of time outdoors, moving them indoors and out is made easier by using a wagon or wheelbarrow.
- Don't forget to protect young seedlings from animals, snails, and slugs. Place them on a table or somewhere animals cannot reach them.
Harden Off Seedlings in a Cold Frame
- Move your seedlings to an outdoor cold frame about 7 to 14 days before your transplant date.
- Be sure the temperature in the cold frame does not go much below 50 degrees or above 80 degrees Fahrenheit. While the seedlings are in this warm, sheltered environment, be sure to monitor soil moisture; don't let the plants dry out.
- Turn off heating cables and/or open the cold frame cover for longer periods of time each day. Start with three or four hours and gradually increase the exposure time by one or two hours per day.
- Close the cold frame cover and resume heating at night if temperatures dip below 40 degrees Fahrenheit.
- Seedlings should be ready to transplant in 7 to 14 days. If possible, do so on a cloudy day and water well after planting.
Hardening Off Vegetable Seedlings. University of Maryland Extension