Harden Off Seedlings Before Planting

When you head out to your garden on a chilly morning, it takes you some time to acclimate to the weather. After you’ve done some digging or weeding for 15 minutes or so, you might peel off a layer of clothing as your body warms up. The outside temperature probably didn’t change much in 15 minutes, but your body adapted to your surroundings and even toughened up to take them as your blood pumped into muscles and your heart rate rose.

tomato seedling

Short-season tomato seedling growing in starter soil.

Your tomato and other vegetable and herb seedlings need the same sort of acclimation before you take them from under your grow light, from a greenhouse, or from the comfort of their commercial nursery home. Give them a little bit of time to adapt to their new outside location. It’s called “hardening off,” and here are a few tips:

  • Take a minimum of a few days when you can, and ideally up to a week, to harden off seedlings. Count backward from your planting date – usually your average last day of frost – and start hardening off seedlings about eight days before that date.
  • Ease your plants into their new environment. That means it’s not a good idea to take them from a greenhouse from dawn to dusk the first day. Start with a few hours of outdoor time and gradually increase it each day. Cut the time short if the wind really picks up.
  • Speaking of wind – and sun – keep your plants in a fairly protected location. Start by putting them out mostly shade the first day and moving them every few days to gradually increase their sun exposure. You may need to protect them from critters, too, so consider local bunnies or other munchers if they often visit the area where you set out the seedlings and place them up on a table.
Seedlings and cuttings hardening off

Tomato seedlings and sage cutting hardening off under the shade of a glass patio table. The table offers some shade and wind protection.

  • Gradually increase time spent out late in the day as well. Your plants need to learn to spend the night outside, but don’t leave them out the first few nights. And be sure to bring them inside if there is any chance of freeze. On the first night out, try to put them in a well-protected location where they won’t get too much wind and receive a little warmth from your home or a south-facing wall.

Finally, cut back on watering as you begin hardening off the plants. They also need to learn to toughen up before transplanting. Of course, once you plant them, water a little extra until established, then water consistently per the plant’s needs.

bell pepper seedling

Bell pepper seedlings need to harden off and toughen up before the soil and daytime temperatures are hot enough for us to plant them.

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