harden off seedlings

How to harden off seedlings

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The first year I started my own seeds indoors, I planted around ten flats of annual flowers and vegetables, growing them on top of my mother’s dining room table (sorry mom!). I was sixteen years old and a fairly novice gardener. When the April showers finally cleared and the bright spring sun came out, I had the brilliant idea of taking those plants – whose only light source had been a modest west-facing window – and moving them outdoors to give them a dose of early May sunshine. Oops! Within an hour, every single plant was fried and I had no idea what I had done wrong. I obviously needed a lesson in how to harden off seedlings. 

Hardening off is a simple, but necessary step for seed starters. Growing your own plants from seed offers many rewards – save money, grow varieties not available at local garden centres, and enjoy a steady supply of top quality seedlings for succession planting. But, as I learned that fateful spring, you need to properly harden off your seedlings before introducing them to the ‘real world’ outside.

How to harden off seedlings to prepare them for the garden!

These orach and amaranth seedlings are ready to be moved to their raised bed.

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Why? Simple! All plants have a waxy leaf cuticle that protects the foliage from sun and wind, and seedlings that have been grown indoors – in a sunny window, under grow-lights or under glass in a greenhouse – have not fully developed their cuticle layer and need a bit of time to build up this horticultural suit of armor. Hence, the hardening off process.

How to harden off seedlings:

Hardening off is not difficult and will take about a week in total.

Step 1 – Place the young plants outdoors in a shady spot.

Step 2 – Bring them back indoors again that night (I call this ‘the spring shuffle’).

Step 3 – Continue giving them daily shade for 3 to 4 days, bringing them indoors at night if the temperature becomes unseasonably cold or frost threatens.

Step 4 – By day 4, begin introducing increasing amounts of sunshine each day, so that by the time a week has passed, the plants have become adjusted to full sunshine.

Do you do the spring shuffle? 

4 Simple steps to hardening off your seedlings!

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3 Responses to How to harden off seedlings

  1. The “spring shuffle” is definitely an annual event. Still trying to get tomato plants out when it isn’t too hot, too windy or too rainy! Haven’t even brought the peppers or Impatiens upstairs to the sunporch. Part of the Begonias are there…they get too tall under lights.The pic is Begonia grandis

  2. Kris Spencer says:

    Spring shuffle??? That’s a lot of in & out in and up & down every day which, depending on one’s home could be the only daunting part of the process. Here’s a different method Alaska’s horticulture expert Jeff Lowenfels recommended: Take your plants outside while its the warm part of day. Place them in a shady spot next to the side of a house and leave them there (yes, overnight) for several days up to a week to get acclimated. The side of the house provides some warmth & protection. If need be, use a tarp, board whatever to cover over the plant as a sun/wind block) note: even cloudy days can have enough radiation to damage tender plants. Next, move the plants to a sunnier location and use the tarp/covering, board etc. to shield from the sun. Remove the tarp to gradually increase exposure to sun. After 1-1/2 weeks the plants should be hardened off and ready to plant. This way, there’s a lot less in and out. I have had much success with this method and its been so much easier on me physically. Winter climates just have to make reasonably certain lowest night time temperatures are beyond any potential frost. Remember, hanging baskets and planters need to be hardened off also!

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