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  • Writer's picture Sophie Jobin

Garden Duties: Hardening Off Seedlings

Updated: Jul 30, 2022

Seedlings growing indoors are giving the perfect environment to thrive. We control the environment from the moment it's planted from lights, heat mats, soil type, no wind, water amount - everything. Exposing those seedlings to outside conditions right away isn't always ideal, it can cause plant shock and leave them vulnerable to plant death.

To help avoid this sad situation, there are a couple of steps to help ensure those seedlings adapt well enough to their new environmental conditions.

Hardening off: by gradually exposing seedlings to the outside elements (wind, sun, and water) it will help stimulate their natural defenses and give them time to acclimate to their new environment.


When happens if you don't harden off plants?

Seedlings are susceptible to certain issues if they aren't hardened off properly.

Sun Damage

If left out in direct sun all day, the leaves could become scorched, curl under, or fall off. This could be a direct correlation with the inability to filter the intensity of the light.

Wind Damage

If caught in some sharp wind - the stem could weaken or break if not strong enough.

Temperature Changes

If they aren't use to different temperatures, like hot days and cool nights, it can cause stress, make them more vulnerable to pests and diseases, or can stunt a plants growth if exposed to cooler temperatures.

When to harden off seedlings?

There's a couple different options when it comes to your seedlings.

Frost-Hardy Seedlings: can harden off a little earlier as they tolerate a light frost. Once hardened off, they can be transplanted into the spring ground workable. Most can tolerate temperature no lower than -5°C, but only a couple of times. Any lower and they could suffer severe damage.

Cold-Hardy Seedlings: can harden off 2 weeks before the last frost date in your area. Making sure to never expose the seedlings to temperatures lower than 3°C.

When it comes to actually knowing when they are ready to be hardened off? When the plants are a couple inches tall and have been potted up twice with their two set of true leaves.

How long do you have to harden off plants?

In basic terms, the process of hardening off seedlings will take:

- Around 7 days, if temperature stays consistently warm.

- Can take up to 14 days, if temperature drops - as they often do in April and May in Canada.

How to Harden Off Plants

Day One

Best time to start is on a cloudy day with sunshine - temperatures around 7-10°C. You never want to place the trays in direct sun or where there is a lot of wind - place them in full shade for a couple of hours and bringing them back inside at night in a warm place.

Day Two

Increase the seedlings exposure by a couple hours and dappled shade with some exposure to direct sunlight. Again bringing them back in for the night.

PRO TIP: Never expose your seedlings to high winds or temperatures lower than 5°C.

Day Three

Place the seedlings in a location that receives morning sun for a couple of hours. This time of day the sun is less harsh than mid-day sun, bringing them back in for the night

Day Four

Repeat Day Three again, leaving them out longer. Each day, the seedlings tolerate longer hours of exposure to outdoor conditions.

During the entire process, make sure you are watering your seedlings well enough to never dry out!

Day Five

This time if temperatures remain warm during the day and overnight (at least around 10°C), the seedlings should be able to tolerate the overnight adventure.

PRO TIP: At any time if the temperatures dip, extend it an extra day. Rainy days count as well. (Hence the reasoning for 7-14 days)

Day Six

Repeat Day Five.

Day Seven

After day seven, if everything went as smooth as pie (in my experience it's never that simple) the seedlings should be able to handle the increasing amount of environmental factors. In the desired area - transplant seedlings into the garden or container.

PRO TIP: Usually recommend planting the transplants on a cloudy day and water well after planting.

In the End

Never try and skip out on this step, it's very important to make sure that things are done properly - whether that be in a cold climate or hot climate.

I know this may seem like a lot to do in one step but it's well worth it. All that will pay off in the end giving you stronger starts and a better growing season.

Let me know if this was helpful! I can't wait to see all your plants. Tag #jobinkitchengarden on Instagram or on Facebook.

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