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Hardening Off: Moving Seedlings Outside

April 16, 2024
Written by The Eising Team

Come along on a little journey with me and imagine this… It’s Springtime. The sun shines, and the air finally feels warmer after a long, grey winter. You take a little trip to your favourite garden centre and find that special plant you’ve been reading about and hoping for. Of course, you buy the plant, take it home, and proudly place it on your sunny patio on this beautiful day. Two more sunny days go by, and the plant does not look good at all. It seems to be dying, so you give it lots of water, but that makes no difference. It is in fact dying. So disappointing! How could this be happening?

In our excitement to get outdoors and plant flowers, or add greenery, we forget that the plants we’ve been growing indoors or inside a greenhouse, aren’t used to the elements in the great outdoors. They’ve been sheltered indoors, out of the wind and direct sun, and watered and fertilized regularly with the utmost attention. They’ve been pampered in a plant spa and are quite literally hothouse flowers! Being plunked outside afterwards is the equivalent of us going outside in a bathing suit with no sunblock on for the first sunny, warm days of Spring, exposing our lily-white skin that’s been covered up by a parka all winter long!

Red, sunburned skin is not a great image, but hopefully, this comparison helps to explain the importance of transitioning or hardening off plants. Much like us, indoor plants need to be gradually exposed to the elements and given protection in the process.

The same will be true if you have planted seeds indoors or purchased greenhouse-grown seedlings. Seedlings grown in a greenhouse, a sunny window, or under grow lights in your home need up to 14 days to acclimate to being moved from their cozy indoor space to the great outdoors.

What is Hardening Off?

Hardening off ensures that you will not lose another plant or seedling! It’s a sensible process for all the plants you’re moving from indoors to outdoors. By helping plants or seedlings to gradually toughen to withstand the elements, you prevent transplant shock, which can cause your plants to be stunted or, worse, die.

To ensure success, the process of hardening off takes up to 14 days, so you will want to begin two weeks before transplanting plants and seedlings into your garden. Houseplants may only need 7 to 10 days, but tender, tiny seedlings need the full two weeks to strengthen and get used to their new environments.

When to Harden Off

Cold-weather plants like lettuce, kale, pansies, violas, dianthus, and potted or forced spring bulbs can be hardened off once daytime temperatures consistently reach 5 Celsius, usually in April in southern Ontario.

Perennial plants, tropical houseplants, and heat-loving seedlings like most annual flowers, tomatoes, peppers, and basil should wait until temperatures are around 7-10 Celsius to begin hardening off. For vegetables and annual flowers, start hardening off around May 10 here in Norfolk County if your target is planting seedlings around May 24th.

Successfully Hardening Off Your Seedlings and Plants

Days 1 and 2

Begin on a day that is optimally warm (At least 5 Celsius for cold-weather-tolerant plants, 7 Celsius for heat-loving plants, or 15 Celsius for tropical houseplants) and place your plants or seedlings outside in full shade and protected from the wind.

If you don't have a wind-sheltered area in the shade, putting your plants in an open cardboard box is a good way to protect them from the wind at this stage.

Leave them in this protected area for 1-2 hours before returning them indoors.

Days 3 to 5

Each day, gradually increase the length of time your plants are outside and begin to place them in an area where they will get dappled shade and some wind.

Pay close attention to watering while you’re hardening off your plants as they will require more frequent watering.

Days 6 to 10

On day 6, start to increase your plant’s exposure to direct sunlight by placing them in the morning sun, which is less intense than the afternoon sun.

Begin to leave them outside overnight from this point on unless the temperature is expected to drop below 7 Celcius.

Days 10 to 14

After 10 days of acclimation, it is safe to move houseplants that require more sun to a location that meets their needs.

After 14 days of acclimation, it’s safe to transplant seedlings to their place in the garden or to large pots if you’re growing in containers.

As soon as your plants are transplanted, be sure to water them well! This will help them transition and eliminate air pockets in the soil that can harm the roots.

If the temperature drops and a fluke, unexpected freeze is forecasted after you’ve planted everything, cover your plants with row cover, cloches (you can use plastic water jugs with the bottom cut out, just leave the screw cap off to act as a vent), or cardboard boxes to protect them.

To summarize, when Spring arrives, protect your skin with sunblock, and harden off your plants before you move them outdoors!

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