How To Harden Off Seedlings

Have you planted seedlings outside that looked healthy when growing inside (or in the greenhouse), only to have them burn up, wither, and die? You may not have hardened them off properly before planting. Hardening off seedlings is the crucial last step of growing seeds indoors. Learn how to harden off seedlings before you plant with these tips.

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What is hardening off? 

“Hardening off” is the process of gradually exposing seedlings to outdoor conditions. During this process, a change takes place in the seedlings. The cell walls harden, and the plant’s cuticle thickens to protect the plant from UV rays and against moisture loss. Once seedlings are hardened off, they are better able to tolerate outdoor conditions. 

A plant’s cuticle is the outermost layer of a leaf that acts as a protective film around the leaf. The cuticle is primarily a water permeability barrier. (Similar to human skin.)

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How do I harden off seedlings?

Begin the process of hardening off seedlings 7-10 days before planting. 

• On the first day, take seedlings outside and leave them in the shade in a protected location. Leave them outside for an hour or two and then bring the seedlings back inside.
• The next couple of days, gradually increase the time outside. Do not expose them to direct sunlight or windy conditions for the first few days. Allow the seedlings to be in filtered sunlight and morning sun for increasing amounts each day. 
• After several days, allow the seedlings to be in the sun for longer periods of time each day. Monitor the plants for signs of stress (see below).
• By the end of 7-10 days, the seedlings should be ready to be in the sun throughout the day and you can transplant them in the garden.
• Protect young seedlings from birds with tulle or netting if necessary.
• During this process, keep the transplants watered. 

A special note for hot summer climates:

Do not put new seedlings in direct afternoon sun during the hottest months of the year. Provide afternoon shade for newly-planted seedlings for the first week (or more) as needed.

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Signs that you need to harden off seedlings more slowly 

Pay close attention to your plants during the hardening-off process. It doesn’t take much sunlight or wind to damage young seedlings unaccustomed to outside conditions. 

Wilting: A plant that wilts (not from a lack of water) is not adjusting well to the new environment. This may happen during extreme temperatures. You may need to take it a bit slower. 

Burnt or bleached out leaves: Leaf bleaching and burn are typical on plants accustomed to indoor lighting and placed in sunlight too quickly. Prevent burning leaves by exposing the plant to dappled light and morning sun before it gets bright afternoon sun.

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When do I leave the seedlings outside all night? 

If temperatures are above freezing, allow plants to stay outdoors overnight after several days of hardening them off. 

• For warm-season crops, bring them back indoors if temperatures are cold. If it is warm outside, you can leave them overnight after the first few days. 
• For cool-season crops, if nighttime temperatures are at least 45° once the seedlings are accustomed to being outside all day, leave them outside at night. 

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Do I need to harden off nursery-grown purchased transplants? 

Often, plants purchased from a nursery are hardened off; however, they may benefit from an abbreviated hardening off. For example, put them in part shade for a day, then full sun for a day, and finally leave them outside and plant them on the third day. 

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Learning how to harden off your seedlings correctly will benefit your garden in the coming months. Healthy transplants will become healthy plants that are much less susceptible to pests and diseases.

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