Hardening Off Seedlings (& Avoiding Transplant Shock)

Hardening off seedlings is an important step when raising new seeds to grow in the garden. And because it’s important, I wanted to share with you exactly what it means to harden off plants and how you can harden off seedlings to avoid transplant shock for greater growing success in your edible garden at home.

Hardening Off Tomatoes

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What Does Hardening Off Mean?

Hardening off plants is the process of conditioning them to a new environment. Seedlings raised indoors, in a greenhouse or just in a sheltered position are slowly introduced to their new growing conditions before planting. These new conditions usually include greater exposure to the outside elements of sun, rain, wind and fluctuating temperatures.

The hardening off process is done slowly, usually over the course of 7-10 days.

Importance of Hardening Off

So what happens if you do not harden off your seedlings? Why is it important?

If seedlings are taken from indoors or a sheltered position and planted directly in the garden, they risk transplant shock. Seedlings can simply collapse and die.

Sudden exposure to the sun can cause wilting, burning or both. And young seedlings can snap and break in the wind and rain.

Sudden exposure to fluctuating temperatures can also stress the seedlings and ultimately cause them to fail.

Hardening off seedlings is their best chance to ensure they are strong enough to withstand the environmental elements they will grow in.

How To Harden Off Seedlings

Hardening Off Seedlings

To harden off seedlings, start by placing the seedlings outside in morning sun for 2 hours. Morning sun is important as it won’t be as strong as midday or afternoon sunlight.

After 2 hours, move the seedlings back into indirect light for the remainder of the day and return them to their previous growing environment for the night.

The following day, increase the direct sunlight hours by 1-2 hours and continue to do so over the next few days.

Try to start the hardening off process in optimal conditions, that is, a mild temperature, gentle sun, light or no winds, and little or no rain.

Then as the hardening off process gets underway, allow the seedlings some gentle exposure to these harsher conditions. After all, they will soon be growing in these conditions full-time.

This of course, does not mean leaving the seedlings out during a serve storm or extremes in temperatures. Seedlings will still need some protection from these types of conditions.

The hardening off process should take 7-10 days with some gardeners preferring to extend the process to around 2 weeks.

Once the seedlings have been hardened off, they can be transplanted into their final growing position in the garden or outdoor pot.

When to Harden Off Seedlings

How To Harden Off Seedlings

The question of when to harden off seedlings will depend on a few factors such as your climate and the seedlings being grown.

One important time hardening off should be practiced is when seeds are started indoors to get a head start on the growing season.

If the seeds are being grown for spring planting, for example in the case of hardening off tomatoes or other warm-season crops, then the hardening off process can begin after all chance of frost has passed.

Also, consider the size of the seedling. Seedlings should be almost big enough to be transplanted out in the garden. So start the hardening off process about a week or two before you intend to plant them out in the garden.

For U.S. readers, you can check your first and last expected frost dates here.

Related: Regrow Vegetables From Kitchen Scraps | Are Coffee Grounds Good for Plants?

Hardening Off Plants Tips

Planting Hardened Off Seedlings - Pepper

Remember to check the soil moisture of the seedlings regularly as outdoor conditions and exposure to the elements can mean they will dry out quicker. Keep the seedlings moist and ensure they do not dry out.

Once the hardening off process has been completed, consider holding back a few seedlings from transplanting outdoors. So if a few seedlings fail after transplanting, you will have replacement seedlings ready to be planted.


Hardening off seedlings is a simple process and well worth undertaking to avoid transplant shock and ensure strong and healthy plants in the garden. And given the right start, healthy edible seedlings will turn into productive plants with delicious harvests.

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Further reading:

How To Harden Off Seedlings - Tomatoes

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