Gardening Tips: Hardening Off

Editor’s Note: The Spectator is reaching into Michelle Smith’s gardening column archive for some weekly advice that is as relevant now as when it was first written. This column was first published in June 2017.


What to do this week

This week you should be hardening off your precious seedlings. Take your plants from their sheltered environment, whether it is the house or cold frame or greenhouse, and gradually get them used to the real world where they are going to grow. This is not just about temperature, although that is certainly one factor. It is also about wind and the UV radiation of the sun. These are the three things that will kill your little plants if you move them from their shelter out into the open from one day to the next. Plants get sunburned and windburned, just like people do. You would not lay on the beach the entire day of your first day of Caribbean vacation in the middle of February, no matter how nice a day it is. If you have, you’ve learned about UV radiation shock.

Seedlings. Image via YouTube

Seedlings. (Image via YouTube )

Your plants can also get cold stress from unexpected cold temperatures when they are not used to it, especially warm season annuals like tomatoes. So what you have to do is move the plants gradually, for small amounts of time, followed by longer amounts of time each day. If you do it too quickly, you are going to kill your plants.

If you have a manageable number of plants, you can put them on a tray and move them out, starting with a half hour, and not starting with the noon sun. You should gradually increase their time in the sun, wind and cooler air for about a week before planting.

If you have a lot of plants, like me, you would spend the entire day moving them inside and out for a week, so you might want to build a little halfway house. The English call it a “lathe house.” It is like a greenhouse but without the glass. I cover the structure with something called shade cloth. It is a polyethylene mesh, and partially protects your plants from full sun. The sides of this structure are high, so the plants are also buffered from the full force of the wind. I have quite a large lathe house, but you can make a smaller one.

You also have to harden off your plants from the greenhouse. Plants under glass or plastic in a greenhouse need to get used to UV radiation. (The glass and plastic shifts most of the the short-wave ultraviolet sunlight into long wavelength red, which we mostly feel as warmth. If you remember high school science, you will know that the glass or plastic does this not by slowing light down, because the speed of light is constant, but by stretching short little waves into long ones. Okay that was really nerdy, even for me.) Greenhouse plants must also be prepared for wind. I sometimes put a fan in the greenhouse prior to the hardening off process, to give the tender seedlings a bit so their stems start to toughen up. If they go from no wind, no direct sunlight and constant warmth to nature, it is a total shock.

Sometimes I run out of room in my lathe house, or I want to start acclimating plants on the site where they will be planted, so I have the plants on their trays and cover them with the material used for floating row covers. It is spun-bonded material that lets in some of the elements, but not all. You can find this material at a good garden store. Probably the Co-op on Keltic Drive would have it, but you likely wouldn’t find it at your neighborhood grocery store garden center. Using this cloth is a little cheat, but it is what I am doing now. I have a bottleneck of seedlings because we have had such cool weather I haven’t been able to move things outside or to plant on my usual schedule. So I am using a lot of this floating row cover and making lots of mini-lathe houses all over the farm.

You really have to do this. You have to harden off your seedlings. Before I understood that, I killed a lot of seedlings. Now that we surely will be getting beautiful days, it is very tempting to put the seedlings out, but they will crisp up just like you would on that beach in February. Don’t do it.

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Market gardener, farmer, workshop leader, seed-saver, political candidate and mother, Michelle Smith has spent over 30 years coping with the challenges of our bioregion and in the process has built a store of practical and technical knowledge. The Inverness resident has served on the board of Seeds of Diversity Canada and represented Alternative Producers with the Federation of Agriculture but can do nothing about her hair. She is pictured with a head of Club Wheat, a seed that shares her approach to hairdressing.



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