How To Harden Off Seedlings Before Transplanting

Hardening off is an important process for seedlings of all types grown indoors. Hardening off acclimatizes the tender seedlings slowly to the natural elements in the outdoor environment. This step is essential for new seedlings grown indoors or in any protected environment, and will help reduce stress to the plant during the transplanting process. Learn how to harden off seedlings to prepare them for life in the garden.

lavender seedlings outside hardening off

Hardening Off Definition

Hardening off is a process which occurs in a plant while it is acclimatizing to outdoor elements. It ideally should happen gradually so that the plant does not experience stress. As the plant gradually acclimatizes to changes in temperature, wind, sunlight and rain, it begins to undergo a physical change in it’s structure and metabolism. This change creates a stronger more durable plant that is able to endure the outdoor environment more easily.

Some of the changes that occur in the plants that are hardened off include:

Cells in the leaves become thickened, creating a sturdier and more durable leaf. You can literally see the difference after the plant has been hardened off. The leaves become stiffer and more firm to the touch.

There is a change in the water storage within the leaf structure. This storage is reduced, and therefore reduces the risk for freezing if exposed to subzero temperatures.

Here is a great article from Nebraska Extension, Institute Of Agriculture and Natural Resources, on the physical changes which occur to the plant during hardening off process.

a tray of lavender seedlings on a wooden bench outside — hardening off lavender

The Importance Of Hardening Off

It is very important to harden off seedlings, regardless of the type.

The reason for this is that all seedlings, even cold hardy annual and perennial seedlings, are tender if they are started and grown under lights indoors.

If the seedlings were to be taken directly outdoors and planted into the garden without hardening off, you will risk injury to and possible death of the tender young seedlings.

Exposure to sunlight, without hardening off, will risk scalding and sunburn of the leaves. If all the leaves are burned the plant will die. If some leaves are burned, the plant will be stressed.

Exposure to wind and cool temperatures may cause damage as well. The plants may wilt and injured leaves will perish.

Hardening off allows the plants to gradually get stronger, over a period of 7 to 10 days. After this time, you can be rest assured that they will be able to tolerate the great outdoors.

hardening off small lavender seedlings outside — hardening off lavender on the porch

Do All Seedlings Need To be Hardened Off?

Yes, all seedlings should be hardened off.

Even the hardiest of seedlings are tender when planted indoors.

How Big Should Seedlings Be Before Hardening Off?

Size does not really make a difference in the hardening off process.

Even large healthy seedlings are still tender if they have been grown indoors under lights, or planted in a protected structure like a greenhouse.

Hardening Off Plants Wilting

Here is an example of a large tender seedling wilting:

green leaves of yarrow wilting, showing how to harden off seedlings — yarrow wilting after exposure to sunlight during hardening off

I placed some yarrow seedlings outdoors to harden off. Yarrow is an extremely hardy plant. It is as tough as nails when grown outside, and a hardy perennial in zone 5b.

I placed the seedlings on my covered porch, first day outside. The seedlings were large, at eight weeks of age, very green and very healthy-but still tender.

Within one hour I discovered that some of the leaves were starting to wilt. They had been exposed to the noon day sun shining through the porch on an angle.

How quickly this injury can happen when the plants are still tender, no matter the type of plant, or the size of the plant.

The plants were brought back inside, and are still recovering. I am sure they will bounce back, however there will be some leaf loss.

Keep an eye on your plants when you put them out in the beginning, so that you can intervene early if they start to show signs of stress.

green plants in trays outside on a porch — plants hardening off on the porch

When Should You Start Hardening Off Seedlings?

You can start to harden off your seedlings at any time when the temperatures start to get warmer and the plants are not exposed to harsh outdoor conditions.

Many people start to harden off in preparation for transplanting out.

If you are scheduling a time to transplant your seedlings into the garden, check your last frost date and count back 7 to 10 days. This would be a good date to start the hardening off process (weather permitting).

How To Harden Off Seedlings-The Process

Seven to ten days before planting your seedlings into the garden it’s time to start the hardening off process.

Remember it’s important to slowly acclimatize them.

Pick a spot in your yard that offers some protection from sunlight, like a covered porch, or a large tree.

Up against the north side of the house is also a good location.

Start the hardening off process on a day that is fairly mild, not too windy or cold.

Placing the plants on a structure rather than on the ground is a good idea , as it reduces damage by animals and also keeps the plants off the cold ground.

young plants in trays on the front deck — seedlings hardening off

Hardening Off Day 1

Place the plants out on the first day in the protected area for 2 hours, then bring them back inside.

If it is really nice out, I will often leave them out a little longer, but will check on them.

I will also rub my hand over the leaves, simulating the wind and getting the leaves ready to start toughening up.

Bring the plants back inside when the time is done.

I usually don’t return them to the grow lights, just place them in a handy location where I can easily get them the next day to bring them back outside.

Hardening Off Day 2

Place the plants back outside and double the exposure time.

Keep them in the shade.

Keep an eye on the sunlight and notice if it encroaches upon the seedling location.

Move the seedlings to the shade if they start to become exposed to the sun.

Bring back inside the house after their time is done.

green seedlings on a porch outside showing how to harden off seedlings — trays of seedlings on a porch getting hardened off

Hardening Off Day 3

On day three increase the exposure time again by several hours or more.

From here on in it is generally a rinse and repeat process.

In my area, if the weather is good I may leave the plants outside overnight on my covered porch after day 3 or 4.

You really just have to gauge it depending on your location and weather.

If I see that the temperatures will be very low at night I will likely bring the seedlings back inside if I’m not sure if they have completely hardened off.

You will be able to tell when they are hardened off based on the changes that you observe in the sturdiness of the plant.

Hardening Off Day 4 through 10

Continue to protect the seedlings with gradual exposure to the sun once they have been hardened off.

I usually leave them on the covered porch for 7 to 10 days total before planting my seedlings out.

They get early morning sun, which is less harsh and a good introduction for them.

Some people harden their plants off within three days by leaving them outside during a stretch of cloudy weather. This way they have some protection from the overcast skies.

From my experience it’s still possible to burn the plants even on a cloudy day, so watch your plants carefully.

green seedlings on the deck being hardened off — hardening off seedlings on the porch

What Happens If You Don’t Harden Off Seedlings?

If seedlings are not hardened off, the tender structures within the plant cells are at risk for injury from the outdoor elements.

Cold will cause wilting and injury.

Sun will cause burning and scalding of the leaves. Even on cloudy days there is a risk of sunburn.

Strong winds will cause wilting and damage and dehydration.

By protecting your seedlings and hardening them off first before planting them out, you are giving them a great head start for the growing season,

By acclimatizing your plants before transplanting you will have reduced any stress on the plants, so that the plant’s energy can go into growth rather than repair.

After Hardening Off What Is The Next Step?

lavender plants in a tray outside on a railing — lavender seedlings hardening off

After your plants are hardened off they will be able to be planted into the garden.

This is such an exciting time.

If they have been hardened off successfully you should be able to notice a difference in their quality. It’s amazing to see and feel the changes in these tiny seedlings that we have grown from seed.

The plants will feel sturdy and firmer. They have undergone structural changes to prepare them for life out side.

Your seedlings are now acclimatized, and your gardening adventure is just beginning!

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I hope that you enjoyed this post on how to harden off seedlings before transplanting.

Any questions or comments are always welcome!