Introducing Them to the Outside World: Hardening Off Your Seedlings for Planting

If seeds are a promise of what’s to come, seedlings are the path in which it will take to fruition.

So you’ve got all of your happy little seedlings growing and thriving in your growing room, greenhouse, light hut, or whatever it is that you grow them in, and spring is nearing faster than you can say “13 days until spring!” It’s time to get your seedlings acclimated to the outdoors in preparation for moving them to their new home, their final destination—your garden.

Hardening Off Seedlings for Planting - Stone Family Farmstead

Why Not Just Plant Them in the Ground?

Your indoor seedlings have lived somewhat of a charmed life. Whether you have used a light hut, a seed starting container and heat mat, a greenhouse, or even an indoor windowsill, your seeds have never experienced the harshness of direct sunlight, winds, or other elements that make the outdoors more dangerous to the life of these baby plants.

Your protection over the first few weeks of their lives does little to strengthen them to live outdoors and in your yard’s micro climate. Hardening off is the process that will ensure that your plants get the best opportunity to make the transition from indoors to garden comfortably.

When Are They Ready?

Your seedlings are ready when they have a few sets of “true leaves”, which are the leaves that they grow after their very first set. Often these “true leaves” will look completely different than their first ones, so you should be able to tell when you’ve got a few sets.

Often when I plant my seeds indoors, there is at least one transplant that needs to take place before my seedlings grow a few sets of true leaves. Be prepared for that if you are using peat pellets or any other seed planting containers that would confine growing roots to a very small space. Keep your newly transplanted seedlings in the same location as before until they develop their few sets of true leaves.

Seedlings - Stone Family Farmstead

The “true” leaves are the first different looking leaves that grow on your seedlings after the first two.

How It’s Done

To harden off your seedlings, you will want to start slowly, setting your plants outdoors each day for a short time. Burpee suggests leaving your plants outdoors starting with at least an hour a day, adding one extra hour each successive day for a week. I also believe that in order to get your plants used to sunlight, they need to experience it, little by little, each day. You might use a schedule like this one:

• Day 1: 1 hour outdoors, in the shade
• Day 2: 2 hours outdoors, 30 minutes in the sun, the rest in the shade
• Day 3: 3 hours outdoors, 1 hour in the sun, two in the shade
• Day 4: 4 hours outdoors, 2 hours in the sun, 2 in the shade
• Day 5: 5 hours outdoors, 3-4 in the sun, 1 in the shade (if needed)
• Day 6: 6 hours outdoors, 5-6 in the sun, moving to shade (if needed)
• Day 7: 7 hours outdoors in the sun, outside overnight
• Day 8: All day in the sun, outside overnight

When choosing an area to set them out, keep in mind that wind and long periods of direct sunlight can dry tender seedlings out, causing them damage. Choose a semi-shaded area that will be easy to move your plants from shade to sun (and back, if need be), and an area that is blocked from high winds. While some sunlight and a small breeze can be helpful as it builds resilience, as well as strength in the structure of your plants, early exposure to constant sunlight and higher winds will be of detriment to your tender seedlings until they are completely acclimated to living outdoors.

You will want to keep an eye on your seedlings and how they are doing outside, especially in the sun. If they begin to look droopy while they are in the sun, move them into the shade. If their soil is on the dry side, give them a drink to moisten the soil. Personally, I like to make sure that my seedlings’ soil is kept moist while outdoors, but Steve Aegerter of the Colorado State University Cooperative Extension suggests that it is best to cut back on watering your seedlings during hardening off, which will allow plants to toughen and be better prepared to be transplanted.

Serrano Tampequeno Seedling - Stone Family Farmstead

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Kristi Stone

Kristi Stone

Honestly? I'm the author of this blog...sometimes. My family and I live in Sunny Southern California on a one acre hobby farm where we are working diligently to one day produce our dairy, eggs, and produce. That takes A LOT of work, so if you don't hear from me as soon as you'd like, give me a shout out in the comments and I'll chat back as soon as I can between the garden, goat kids, chickens, husband, human kids, and playing with my grandboy, Kieran. And in the mean time, if you'd like to see more frequently what goes on at our farm, please feel free to join my Facebook group (see the right column) where I am more active. I'd love to get to know you! 🙂
Kristi Stone

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