Congratulations! You did it. You started from seed and raised all those little seedlings. You gave them the perfect potting mix. You watered, not too much, not too little. You gave them light to grow. You gave them everything they could ever ask for, and now you have trays of future Cherokee Purple tomatoes, Trinidad Moruga scorpion peppers, and Fairy Tale eggplants.
You’re excited to take the seedlings you have grown lovingly from seed, and plant them in your garden.
Not so fast.
You need to harden them off first.
What is ‘hardening off’?
Hardening off is prepping the seedlings for life outdoors. Those seedlings had the life when they were indoors. Their every need was met. They wanted for nothing. (You are a great plant mom!) But now, they need to be outside, in the sun, the rain, the wind, and everything else Mother Nature can throw at them. Hardening off is sort of like sending your kid off to college before they start a life of true independence. It is the transition stage to allow your seedlings to acclimate to the new outdoor life.
Why do seedlings need to be hardened off?
Plants grown in a controlled environment aren’t used to the uncontrolled outdoors. No matter how strong your grow lights are, they are no match for the sun, which is multiple times brighter than your best grow light. There is no wind to tug on their little roots or stems. Rain comes from above and not all of it will be gentle. Hardening off allows the plants time to build up their natural defenses to survive outside.
How do you do it?
- Check the daytime temperature and make sure it’s above 50 degrees F.
- Pick a cloudy day to start. The plants need to form a cuticle layer to protect them from the sun. It’s a waxy coating that acts like a sunscreen and limits evaporation.
- Because they don’t yet have a cuticle layer, water them every time you put them outside.
- Leave them out for a few hours in the shade in a sheltered area.
- Bring them in every night.
- Gradually increase the time outdoors each day during the first week.
- Starting the second week, move them so they get more sun each day but still bring them in at night.
- Check them at the end of the second week. If they look strong and healthy, then they are hardened off and ready to plant.
Do all plants need to be hardened off?
No, plants that are native to your area are generally acclimated to the local growing conditions and do not need to be hardened off if started outside. However, since hardening off is a simple process that doesn’t do any harm, it’s safer to harden off if there’s any doubt. Hardening off properly gives your plants a solid start in your garden and sets them up for a bountiful harvest.
See also:
- Zero-Waste DIY: How to Grow Seedlings Using Newsprint
- The Garden Decoder: What Is ‘Occultation’?
- The Garden Decoder: What Is ‘Permaculture’?
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