Because 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. 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.
Don't forget to protect young seedlings from animals, snails, and slugs. Place them on a table or somewhere animals cannot reach them. 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.
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 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. 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. Move your seedlings to an outdoor cold frame about 7 to 14 days before your transplant date. At its simplest, a cold frame is a wooden box with no bottom, which is covered in some kind of transparent top that is usually hinged. It's similar to a greenhouse. The biggest difference is the top can be opened or closed as a way of protecting plants and gradually exposing them to the elements.
Be sure the temperature in the cold frame does not go much below 50 degrees or above 80 degrees Fahrenheit. Gradually increase the amount of sunlight the plants receive over the two-week period. The last day or two, the plants can spend 24 hours outside. Reduce the amount of water plants receive, but do not allow them to wilt. Avoid placing seedlings outdoors on windy days. Cold frames are excellent places to harden plants, but another spot that provides protection, such as a porch, will work.
Lois Miklas. Expertise Master Gardener Consumer Horticulture. Why do we need this? Entering your postal code will help us provide news or event updates for your area. Related Products. Introduction to Floral Design Workshops. Wreath and Swag Workshop Workshops. Backyard Grape Growing Articles. Even cold-hardy plants will be hurt if exposed to freezing temperatures before they are hardened. A cold frame provides an excellent environment for hardening off transplants.
After proper hardening, tomato plants can usually tolerate light and unexpected frosts with minimum damage. The hardening process is intended to slow plant growth. If carried to the extreme of actually stopping plant growth, significant damage can be done to certain crops.
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