Skip to Content

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.



Disclaimer: This post may contain affiliate links. See my disclosure policy for more information.


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.)

How to harden off seedlings in hot summer climates.
How to Harden Off Seedlings

How do I harden off seedlings?

How to harden off seedlings in hot summer climates.

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.

How to harden off seedlings in hot summer climates.

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. 
Wilted seedling that was not hardened off properly

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.
Bleached out leaves on a seedling that wasn’t hardened off properly

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.

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.
Sun burnt leaves on a seedling that wasn’t hardened off properly


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. 

How to Harden Off Seedlings
How to Harden Off Seedlings
  • 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. 
How to Harden Off Seedlings
How to Harden Off Seedlings

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. 

How to harden off seedlings in hot summer climates.

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.


Once the seedlings are hardened off and ready to plant, read this post, “How to Plant Transplants,” for tips and information about planting transplants in your garden. 


If this post about how to harden off seedlings was helpful, please share it:


Samantha

Wednesday 24th of May 2023

I’m new to the hardening off process, and I thought I was doing good until today. All my plants that looked healthy and happy are droopy and wilted. Will they bounce back or did I just kill off all my plants?

Angela Judd

Thursday 1st of June 2023

Sometimes they wilt and then bounce back - other times that means we have to back off a bit and take it more slowly. I hope yours recovered!

Elsa

Wednesday 15th of March 2023

Thanks for providing such informative content—I’ve watched so many of your vids trying to get ready for this season.

I grew transplants from seed this season for the first time in all my few years of gardening. Hardened them off for about a week before transplanting into my garden beds. It’s been about 10 days now and I feel like a lot of my peppers, tomatoes, basil, and eggplants are just… stuck. They’re not dying or wilting, but they’re not really growing in height or bearing new leaves. Fed them fish emulsion a few days ago, deeper infrequent watering, but nothing. I figure okay, a week of root growth underneath the surface… are they just in shock? Is there a way to overcome this or do I consider this a loss??

Elsa

Thursday 16th of March 2023

@Angela Judd, Thanks! I guess I just have to keep waiting then—while day temps were in the 70s I think night temps have still been in the upper 40s/low 50s. I’m in Tucson, so a little cooler than Mesa.

Angela Judd

Wednesday 15th of March 2023

My guess is they will take off soon, especially as temperatures are consistently warm. (all of the crops you mentioned are happiest with nighttime temps above 50) I'm rooting for them to grow for you, best of luck!!!

Joe

Saturday 18th of February 2023

Hello, I planted chili tepin seedlings in an aerogarden. They are ready to be planted. How do I go about planting them in a pot and hardening them? First time doing anything like this. I live in southern Arizona.

Angela Judd

Tuesday 21st of February 2023

This post may be helpful as you pot them up: https://growinginthegarden.com/when-and-how-to-pot-up-seedlings/ and then follow this blog post's steps about hardening them off.

Amie

Saturday 23rd of July 2022

Just what I needed. Thank you!