How to Harden Off Seedlings

What you need to know before transplanting your seedlings outdoors

Did you start your vegetable, herb and flower seeds indoors? Before you plant them outside, taking the time to introduce them to the outdoor elements — sunlight, wind and temperature variations — will ensure success. 


“Hardening off” helps seedlings adapt before transplanting. Although indoor grow lights are very bright and warm, they don’t compare to the intensity of direct sunlight. Plants that have not been introduced gradually to direct sunshine may have damage to their leaves (curling under or falling off) and transplant shock can even kill many types of seedlings.


Start this process 10-14 days before transplanting into the garden plot or containers. 


Choose a warm, sunny day to leave the plants outdoors for a few hours, extending their exposure by a few hours each day until you are leaving them out for the entire day. Bring them back in at night, especially on evenings when there is a frost warning. After a couple of days and nights outdoors, they will be ready to transplant. 

Mark Cullen placing trays of seedlings in his cold frame.


Building a cold frame is another great way to harden off plants as you prepare them for the garden. The off-season is a great time to build a new cold frame, which is an amazing season extender: earlier out in the garden, later in. You use it to get an early start in spring and to make late autumn more productive. It’s also great for overwintering dormant perennials. Essentially, any box with an angled top and a clear lid works well. Mark used a series of single-pane glass windows for the “solar panelling” of his cold frames. Whatever recycled glass or plastic you can get your hands on can set the dimensions for the base, which should be about 30 cm (12 inches) tall on the front and up to 50 cm (20 inches) on the back. The angled transparent top maximizes exposure to the sun, perfect for hardening off seedlings in the spring and protecting greens in the fall. 

Follow these steps to maximize your odds of success in the veggie garden — something worth getting your hopes up for.

Young vegetable plants in the cold frame.


How to start seeds indoors

Plan your best garden ever


Mark & Ben Cullen
Mark & Ben Cullen

Mark Cullen is an expert gardener, author, broadcaster and tree advocate
and holds the Order of Canada. His son, Ben, is a fourth-generation
urban gardener and a graduate of the University of Guelph and Dalhousie
University in Halifax. Follow them at, @MarkCullen4
(Twitter) and @markcullengardening (Facebook) and look for their latest book, Escape to Reality.

Follow them at, @MarkCullen4, and biweekly on Global TV’s national morning show, The Morning Show.

By The Same Author:

Posted on Wednesday, January 19th, 2022
Harrowsmith Magazine | Spring 2023 | Now Available

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