How to harden off seedlings

My last frost date is finally approaching, and my seedlings are getting big and strong, just about ready for their more permanent outside homes. For almost two months now, I’ve coddled these little seedlings. I started with the best quality seed mix I could find, put my garden plans boldly down on paper, and calculated how many of each plant I would need. And of course added just a few more, for good measure. On my birthday in early spring, I carefully tucked each seed into its earthy nest and watched and waited for their green little heads to poke out of the soil; transformed by dark, damp, and time.

Over the last 8 weeks, I’ve battled over-watering; mold, mildew, and algae were my foes. I fought back with cinnamon and a more careful watering plan. Some seedlings were yellowing and stunted; I deployed worm castings and fresh soil. I up-potted and up-potted again until my kitchen counter overflowed with a miniature forest of green.

And now, it is almost time to send my little baby seedlings out into the real world. As of yet, they haven’t had to encounter wind, rain, or the sun’s harsh rays. They’ve been kept at a near constant, comfortable temperature under my watchful eye. They’ve had a cushy, luxurious existence up until now. Most years, in the eagerness of spring, I ignore the advice of seasoned gardeners on the internet (what do they know anyway?) and plant my seedlings out straightaway and hope for the best.

This has not produced great results. I have lost many a seedling due to this reckless behavior. And the ones that didn’t immediately shrivel and die were left weak, unhealthy, and stunted. And when you live somewhere with about a 90 day growing season, you do not want stunted seedlings. One of my gardening goals this year is to do things properly. So this year, I am going to take the time to properly harden off my seedlings. I have poured too much of my time and effort into these little plants to let them get scorched or stunted.

What is hardening off?

Hardening off is the gradual process of exposing indoor or greenhouse raised seedlings to the elements that they will experience when planted in the ground. This gives them a chance to slowly acclimate to the harsher conditions outdoors, and build up the proper strength and defenses in order to thrive.

How long will it take?

The entire process will take roughly 2 weeks. Look up the last frost date for your area, and be sure to check the upcoming forecast as your last frost date approaches. Right now I am about a week and a half away from my last frost date, and I’m hoping to plant out most of my seedlings in two weeks time. This is the perfect time to start hardening off your seedlings.

Week 1

For the first week, introduce your seedlings to life outside by placing them somewhere in the shade. Start with 2 hours, then aim to leave them out for an extra hour each day. By the end of the first week, your seedlings should be spending the entire day outside. Continue to bring them indoors at night.

Week 2

During the second week, your seedlings will continue to spend the day outside, but now you are going to slowly introduce them to the sun. Again, start with an hour or two and add an extra hour each day. If your nighttime temperatures are above 50°F you can leave them outside at night as well.

Planting out

Once these two weeks are up, your seedlings will be stronger than ever and ready to sink their roots into the garden. As long as you are past your last frost date and your upcoming forecast is clear of frost (frosts can occur at temperatures of 32°F to 36°F), you are finally free to plant your seedlings out into the garden!

I made a little printable to help you keep track of hardening off your seedlings. The hardest part for me is keeping track of the details- how long they need to be left out each day, how many days they’ve been out, etc. Here’s hoping this will help us stick to a schedule and harden off our seedlings properly!

Until next time,



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