Sowing Seeds Vegetables

Start Hardening Seedlings Off Early

Where I live, this spring has been exceptionally cold and I’m nowhere near acclimatizing any seedlings to outdoor conditions, but I mustn’t forget that many other temperate climate gardeners are enjoying warmer days. If so, and if lugging trays of plants indoors and out is not something that bothers you, it can be advantageous to start hardening off your seedlings, even when nights are still cold.

Days over 55 °F (12 °C) are warm enough for most seedlings you started indoors (annuals, vegetables, perennials, etc.) to begin enjoying outdoor conditions. I’d wait, though, for 65 °F (18 °C) days before putting the tenderest seedlings, like tomatoes, peppers, eggplants, cucumbers, melons, begonias and impatiens, outdoors. It’s highly likely that night temperatures will still be too cold for them to be outdoors, but you might not have to carry them far: you can often simply put them in a garage or shed overnight: temperatures there are bound to be warmer.

This gradual exposure to cool air and sun (called hardening off or acclimatization) will give shorter, denser, tougher plants and may possibly boost productivity later in the season.

Greenhouses are ideal for hardening off seedlings. Open them on warm days, seal them shut on at night if temperatures drop. They remain warm even on cold nights. Photo:

Start by acclimatizing the seedlings in a shaded spot that only a few sunny rays can reach, then, after 2 or 3 days of shade, move them to partial shade. Yet another 2 or 3 days later, they’ll be ready for their first taste of full sun.

If day temperatures begin to dip below 55 °F/12 °C (65 °F/18 °C for tender seedlings), temporarily end your experiment with hardening off and move the young plants back to their indoor location. But even a short period outdoors will have benefited the seedlings. You can start acclimatizing a few days later, when temperatures warm up again.

Obviously, don’t plant your seedlings outdoors permanently until there is no longer anyrisk of frost and the soil has warmed up. That usually follows the first warm days of spring by several weeks. 

Larry Hodgson is one of Canada’s best-known garden communicators. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He has written for many garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 60 other titles in English and French. He is a past president of the Garden Writers Association (now Garden Communicators International) and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. He resides in Quebec City, Quebec, Canada.

3 comments on “Start Hardening Seedlings Off Early

  1. Yes, those of us in temperate climates don’t have much to move around anyway, because so much gets sown directly. Would you believe that we are expecting rain on Wednesday?! Frost is long gone of course.

    • Here, frost still occurs regularly, but that should end soon. And we get rain (or snow) all year long, often far too much. All our gardens are planned for better drainage! Quite a difference!

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