Gardening Tips: Seedling Safety

Editor’s Note: This column first appeared on 30 May 2018.


What to do this week

This time of year many people are starting to move their seedlings from the house or greenhouse to the garden. Even if you have decided to run the risk of frost, your seedlings will need a gradual introduction to the outdoors. It is important not to take your seedlings from the house and plunk them right in the garden without any preparation. You have to get the seedlings used to living in the big, wide world. There are a few reasons for this.

One reason is temperature. Even if it were warm outside, if the seedlings have been growing inside your house or greenhouse, they are not used to the fluctuations of temperature during the 24 hour day outside. Frost is part of that. The other thing is the high UV radiation they would be getting outside that they don’t get, even in the direct sun on a windowsill where they are behind glass. Just like people can get a sunburn, plants can get a sunburn. Putting them out without a gradual period of getting used to the UV radiation is a no-no.

Lath house design from "California Garden, Vol. 6, No. 3, September 1914" (1914) via Wikimedia Commons

Feeling ambitious? Lath house design from “California Garden, Vol. 6, No. 3, September 1914” via Wikimedia Commons. (Click to enlarge)

The other change for seedlings which have been moved outside is the wind. Unless you had a fan on them, and jiggled them around periodically, which most people don’t bother with, they will need some acclimatization to the air movement outside. Plants develop stiffening cells called lignins that help them withstand the airflow of the wind. Unless they are exposed to the wind, they don’t develop these cells. This is a gradual process for plants, just like getting tanned is a gradual process for people.

So, you begin by putting your seedlings out for no more than an hour in the early morning or late afternoon when the sun is not so intense. Start off in a location that is also sheltered from the wind. You gradually increase the time that they are out over a minimum of seven to 10 days. You really do need to take your time with this. During this period you gradually move them into a location where they are getting increased exposure to the breeze as well. They will get a little air movement, even in a sheltered spot. Don’t start by plunking them into the middle of the open air when there is a gale blowing. If you hurry things a bit, or start with the mid-day sun, you may notice a bit of leaf damage, but so long as this damage is minor the plants will recover. It is like getting a bit pink from your first sunny day out for a summer walk. With the seedlings, you will know when you have gone too far as the leaves will look crispy.

Another way that you can get around the need to gradually harden off your seedlings is to build what the English call a “lathe” house (and the Americans a “lath” house). I have one. It is a construction, usually of wood, that is like a greenhouse in shape but instead of glass or plastic it has spaced slats that allow the sun in partially, rather like Venetian blinds. A lathe house also partially protects from wind. I can harden off a lot of seedlings at once in my lathe house, although I still have to be careful about frost at night.

If you are away during the day you will have to find ways to avoid having to run home at lunch every day. One option you have is shade cloth, but again this will not protect against frost. You also have to be careful the shade cloth does not touch the plants so that the wind buffets the leaves. Row covers are another way you can cheat a little. This helps, especially if you go to work. You can protect the plants with spun bonded polyester row covers, which you can open when you get home. Another option is to inquire at your garden center for seedlings that they have already hardened off. A good garden center will be able to inform you about this status for their different types of seedlings. You won’t be able to grow your own seedlings from seed if you can’t manage the hardening off process, but with the help of a good garden center you will still be able to garden. Remember, though, that just because the seedlings you buy are hardened off, this doesn’t mean they can withstand frost if they are warm season annuals. You can only push the boundaries so far, but the more you are aware that your seedlings are little living things, the more successfully you will transition them from home (or garden center) to garden.

Featured image: Pinterest-sourced photo of backyard lath house. I could not find the name of the photographer, but the caption read: “South facing wall of Lath House built by my husband.” An accompanying photo showed the house from another angle and noted the door had been purchased from “Barn Boys in Sante Fe.”





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.




Backyard food gardener Madeline Yakimchuk caught the food-security bug in the early ’90s through Cuba’s Urban Agriculture Department, taking her first permaculture course and planting her first garden. She can often be found discussing food security, nurturing a plant-based lifestyle or trying to give away vegetables. Professionally, she is GRYPHON media productions but sometimes uses la bruja in her volunteer work, most notably in managing the garden column, which begins life as a telephone interview.





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