How to Harden Off Your Seedlings and Plants

You’ve sown your seeds indoors. Now what? I’ll walk you through the process of hardening off your seedlings and plants before transplanting them into the garden.

Preparing Seedlings to be Planted Outdoors

If you’ve grown seedlings and plants indoors or in a greenhouse, they will need to acclimate to the outdoor conditions before being transplanted to the garden. This transition period is called hardening off. This is when you gradually expose the plants to outdoor elements such as rain, sun, and wind.

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I’m a self-taught hobby gardener, not a Master Gardener. Everything I share with you on my blog is my personal opinion and things that worked for me along the way.

Hardening off your sensitive seedlings will toughen them up. The process thickens the cuticle on the leaves so they lose less water when exposed to the elements.

This process helps prevent shock to the plants when transplanting them to the garden.

The length of time a seedling needs to harden off depends on the type of plant and temperatures outside. Be prepared to cover the plants or bring them back indoors if a late spring freeze or sudden snowfall is forecasted.

Hardening Off Seedlings

Begin the hardening-off process 7-14 days before you plan to transplant the seedlings in the garden. Most seedlings are typically planted outdoors after the last frost date in spring. However, cold-hardy plants can be planted outdoors 2-4 weeks before the last frost date.

When the outdoor temperatures are above 45 degrees Fahrenheit, set the seedlings in an outdoor location protected from direct sunlight and wind for one hour on day one. Make sure you have watered them thoroughly first. Move the seedlings back indoors when they’ve reached the time limit. Do not put sensitive seedlings outdoors on windy days or when temperatures are below 45° Fahrenheit.

Increase the amount of outdoor exposure by one hour each day to gradually acclimate the seedlings. Remember that gradual exposure to the outdoors is what will keep your seedlings from going into shock when you finally transplant them to the garden.

After a day or two in a protected and shady location, you can place the seedlings in the morning sun. You will be able to gradually expose them to more direct sun, but be sure to take your time doing so. By exposing them too soon can scorch their leaves.

Each day, the seedlings will be able to tolerate more hours of exposure to outdoor conditions. Make sure you water the plants and keep them from drying out.

When to Leave Outdoors Overnight

If temperatures remain at least 50 degrees Fahrenheit in both the day and night, the seedlings should be able to handle increasing amounts of sunlight and can be left outdoors overnight.

After 7-14 days of acclimating your plants to the outdoors, they should be ready to transplant into the garden. Plant them on a cloudy day if possible. Water thoroughly after planting.

Hopefully this has cleared up any questions you may have had about hardening off your seedlings and plants. If not, leave me a comment.

It’s always fun to see what gardeners are doing in other parts of the country. My good friend Stacy at Bricks ‘n Blooms lives in New Jersey and is also starting the process of acclimating her seedlings to the outdoors. Take a look at some tips she has for hardening off her plants.

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