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How to harden off seedlings

Plants in pots on a wooden deck

One of the first big mistakes I made as a beginner gardener was not taking the time to properly harden off my seedlings. In the rush to get things growing, I took the small plants I had been carefully nurturing indoors and stuck them straight in the garden. In the process, I lost plants that could have easily thrived and set others back by several weeks; time that is critical when you garden in an area (like mine) with a short growing season.

What does ‘harden off’ mean, exactly?

The term ‘harden off’ is gardener speak for gradually exposing tender plants to full sun, wind, and temperature variations. For the impatient gardener, it’s fussy and annoying but those small seedlings really need time to acclimatize to living outdoors.

It’s the equivalent of starting to exercise after a long sedentary period. You want to start small and work your way up, to avoid tearing a muscle or harming your body unintentionally. The same goes for your plants – they need a short ‘warm-up’ period before they start their intensive growth spurt.

How to harden off successfully

Ideally, choose an overcast day or a slightly shaded area in which to start the process. You don’t want your babies to get burned by the sun!

Put the plants outside for just one or two hours at first, then bring them back inside. Each day, add another hour to the time they sit out.

If you’ve started in a shady area, gradually move them into a bit of sun each day. Continue the process for about a week before transplanting to the garden.

Keep your seedlings on a tray to make all this moving a bit easier.

Make sure the soil is kept moist – don’t let all the sun and wind dry out your baby plants.

When you are ready to move the plants to their permanent home in the garden, choose an overcast morning to prevent transplant shock. If possible, cover the seedlings with row cover (or even a plastic milk jug with the bottom cut off) for the first few days.

I even harden off cold-hardy seedlings like beets and broccoli, though some might argue this isn’t necessary. What do you think?

Do you have any advice on the best way to harden off plants? Leave your best tips in the comments – we love to hear from you.

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