There is nothing more frustrating to a gardener, than to carefully nurture seedlings into fine, healthy plants, only to have them shrivel up and die when moved outside. The way to preventing this catastrophe is to harden off your plants.

Hardening off sounds rather strange when you aren’t familiar with the term. It simply means adapting your plants to outside conditions. When you grow seeds inside, they are used to steady indoor temperatures, no wind, and consistent moisture. Outside growing conditions aren’t so cozy.

By gradually acclimatizing plants to the outdoors, you train them to accept hotter, cooler or windier weather without wilting or their leaves getting crispy.

How Long Should I Harden Them Off For?

There are different timelines to hardening off your seedlings and bedding plants, depending on the area you live in and the plants you are hardening off.

Plan to take your plants in and out over one to two weeks. If it’s quite hot, cool, or quite windy where you live, give your plants longer to adapt. Our spring weather in Alberta is quite mild typically, and the biggest worry is frost.

If you bought your plants from a garden centre and they were outside, it’s likely that the greenhouse already hardened them off for you. It doesn’t hurt to keep them in the shade for a couple of days to see how they do.

Check the Forecast

The first thing to consider is the weather. In Alberta, we aim for May Long weekend, the third weekend in May, to plant outdoors. If frost is forecast on a regular basis for the next two weeks, hold off starting the hardening off process. Once temperatures are consistently above 6 degrees Celsius overnight, then it’s time to start.

Prepare the Location

Set up an area where you can place a table or bench off the ground, away from slugs and other pests, preferably. If you don’t have a table, then on the ground is okay too. The spot should get afternoon shade. Seedlings cannot handle the hot, harsh rays of afternoon sun.

Because it can be a big job moving your plants, try and find a spot that’s the least amount of distance from a safe location. If you have a garage, then you can use it to store the plants overnight, if temperatures aren’t too cold at night. Don’t forget that plants still needed sunlight though, so a dark garage should only be temporary.

Hardening Off

Start with a couple of hours the first day and add two hours the next, and so on. Eventually, weather permitting, you can leave them outside over night. They should be exposed to sunlight so that they harden off adequately.

Once your plants have stayed out overnight, you can move them to a sunnier spot for a few hours during the afternoon, if that’s the type of care they require. Some plants are shade-loving and don’t need to get used to hot sunshine.

Additional Protection

Even with your care, some plants pout when put outside. Cucumbers are difficult to harden off without some damage. They will usually bounce back, if given enough time to acclimatize. If you do see signs of distress, such as the leaves curling up or turning brown, or the plant is wilting, then reduce the time it’s getting in that location.

It may be too windy yet, or the sun may be too strong, or the outdoor temperatures are too cold for the plant yet. Add in some extra protection around your plants and they should get used to the new environment. I added some poly panels to my garden beds to help acclimatize my cucumbers.

Protective Cloths

You can cover your plants with shade cloth or use a floating row cover to help control temperature fluctuations. Not only does it moderate the climate it also protects your plants from moisture loss and insect damage.

Cold Frames

Another option that helps to extend your growing season is a cold frame. This is a mini greenhouse that holds in warmth from the day and blocks harsh winds. They are simple to construct and work really well with raised beds. It moderates the outside temperatures and helps acclimatize your plants. Make sure to open it on hot days though, or you could cook your plants.

Greenhouses

Greenhouses are a much more elaborate structure but provide the same benefits with more space for your plants. Greenhouses can be heated and they trap extra humidity to grow plants that prefer those conditions all summer, like peppers, tomatoes, and any melon family vegetables and fruits.

Planting

Now that you have given your plants a chance to adapt to your yard or patio’s conditions, it is time to plant. Check out the How Do I Grow section for detailed instructions on how to plant your seedlings and bedding plants.