Hardening Off Seedlings for Transplanting

Seedlings. They’re your babies. You’ve nurtured them for weeks, pampering them with warmth and light and plenty of water. As they grew, you may have lovingly transplanted them to larger containers in that warm, light-filled place. But spring is almost here, and it’s almost time for the babies to move into the garden. (At least, down South it is. For you Northerners, you have several more weeks of doting left.) Well, just as you wouldn’t push your child out the door without preparation for the world, you can’t transplant your seedlings without first preparing them for their world in the garden.

The world we create for our seedlings is one of environmental perfection – at least, to the best of our ability. Perfect soil and temperature for seed germination, plenty of humidity and moisture for growth, lights that best mimic sunshine – all as it should be. But the outside world is not like this. If we were to take our seedlings and transplant them directly outside where they would experience the heat of the actual sun, wind, fluctuating water levels, and inconsistent temperatures, they would experience transplant shock. And that would lead to susceptibility to pests, disease, and even death.

Plants build cells and structures based on their environment. In the cozy conditions of our grow rooms or greenhouses, the plants grow large leaves with thin cuticles (the “skin” that protects against dehydration), as well as thinner stems. They experience little to no stress, so no protections have been created. However, in the outside garden, the plants grow smaller leaves with thick cuticles, as well as thicker stems, to protect itself from dehydration, scorching, wind, etc. To transition from one type of cellular structure (for the cozy inside environment) to another (the real world in the garden), a plant needs time and an easing into the change in environment. That process is called “hardening off” and is begun 10-14 days before transplanting to the garden.

Peppers ~ 6 weeks post planting

Peppers ~ 6 weeks post planting

Transplanting to the outside should be done after the last frost, which is March 25th in our area. Even with that estimated date, you must watch the forecast carefully before transplanting. A surprise frost will kill tomatoes, peppers, and other tender plants. One of the first things we can do for our plants is to introduce them to wind. An electric fan or ceiling fan on the lowest setting can mimic wind. I turned on the ceiling fan in my bedroom-turned-grow-room just today. However, I read an article today that stated you can turn on the fan as soon as your plants develop their first true leaves. This will encourage the plant to grow more sturdily and be better prepared for the outside. (I will try this next year! For now, my plants will get at least two weeks of “wind” before I harden them off outside.)

Ten to fourteen days before you plan to put them into your garden, you will slowly acclimate them to the outside world:

  • Place your seedlings in a sheltered, shady spot outside for 2-3 hours. Increase the amount of time by 1-2 hours/day for about 3 days. (Bring them inside at night!)
  • After 3 or 4 days in the shade, move them to a sunny spot in the morning, putting them back in the shade in the afternoon. (Bring them inside at night.) Do this for another 3 or 4 days.
  • After 7 or 8 days, place them in the sun all day and leave them out at night if the low temperature will be above 50 degrees.
  • Throughout this process, be sure they don’t dry out! Wind and sun can dry out the soil more quickly than in the humid grow room or greenhouse.
  • After 10-14 days, if they show no sign of distress, transplant them into the garden, preferably on a cloudy day. Water very well.

Seedlings. Your pride and joy. Prepare them well, and they will not only survive in the outside world, but also make you proud by producing delectable fruits and veggies for your summer table!

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