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  • Writer's pictureMatt Suprunowicz

How to Harden Off Seedlings

Updated: Jul 31, 2023

How can we effectively transition our indoor starts to the outdoor garden?


This Lesson Plan is part the Gardening Module of SustainEd Farms' virtual programming.

Background: Hardening Off Seedlings

Seedlings that are started in an indoor environment have had the advantage of relatively constant amounts of heat, moisture, and light supplied to them throughout their young lives. When plants can no longer be effectively grown indoors (due to nutrient and space limitations), they must be transplanted to an outdoor setting. A sharp transition from the comfort of the inside to the often more harsh and variable climate outside can be devastating for a young plant -- any “swing” in temperature may kill it outright. Thus, it is important to undergo a “hardening off” procedure with indoor seedlings that allows them to slowly adjust to these new outdoor conditions.

Learning Objectives

Students will be able to...

  • Transition seedlings that were started indoors to outdoor environments in a stepwise way

  • Understand the advantages and disadvantages to starting seeds indoors

Academic Vocabulary

cuticle (plants) /ˈkyo͞odək(ə)l/ noun. - the waxy exterior of a plant surface that protects the cells from water loss


1. Gather your materials. You will need the following supplies:

indoor seedlings · outdoor transition space

2. Watch the following YouTube video to see a demonstration of how to harden off seedlings. Then, follow along with the remainder of the lesson by reading the steps below.

3. Evaluate the temperature patterns outside, and identify the average last frost date in your area. In order to avoid damage to the internal structures of the plant, seedlings (and many mature plants) should not be exposed to very cold conditions. However, as the danger of freezing temperatures becomes less prevalent, the hardening off period may commence.

❄🌡What happens to plants in cold conditions? … When exposed to below freezing conditions, water inside of the cell wall of a plant is drawn out into the spaces between cells, causing the dehydration of the cells themselves. While some plants can recover from varying degrees of this, others cannot, and the cellular walls become damaged through pressure and swelling.

4. Each plant species will have different lengths of time for hardening off, so planting the seed and evaluating its progression should be done in a timely way to maximize the growing potential of the plant. Oftentimes, seed packets will indicate how long starts must spend indoors before they are transplanted outdoors. Therefore, seeds planted indoors should be ready to be hardened off towards the end (last 1 - 3 weeks) of their indoor tenure, however long that may be.

5. Choose an ideal outdoor (or semi-outdoor) place for hardening off the seedlings. The place must have differing conditions than the inside environment (more wind, sun, precipitation, etc.), but you may also use a space -- such as an open garage -- that has somewhere between the conditions of inside and outside. Eventually, the transition space should be a complete outdoor environment.

⚘⚘⚘ What is happening during hardening? … The cuticle is a protective layer covering the cell walls on the exterior of a plant. The cuticle helps prevent water loss and infiltration of disease. During the hardening off stage, the cuticle becomes thicker, allowing plants to withstand harsh outdoor conditions.

6. Make a schedule for hardening off your plants. Generally, the hardening off process can begin a week before you intend to transplant the seedlings outside. On the first day, take the seedlings to the hardening off location for one hour. On the second day, do the same but for two hours. Repeat the hardening off procedure for 3 hours on the 3rd day, and so on for up to two weeks (introducing them to outside conditions by one more hour for every successive day, including nights if possible). Return the plants to the indoor conditions when not outside. Sometimes, gardeners will limit the water they give their plants during this time as well to accelerate the hardening off process.

7. Observe the plants every day to see if they appear firmer and more robust. Once hardened, transplant the seedlings into your garden (preferably on a cloudy day to help avoid shock).

Additional Resources:

Download the PDF version of this lesson plan:

How to Harden Off Seedlings
Download PDF • 473KB

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