If you started your vegetable seedlings indoors a few weeks ago as we suggested, it’s almost time to get those babies outside. When and how you do this depends on the crops you planted, the outdoor light levels, and the average temperature. However, even if the conditions are right, you can’t just plonk those tenderly-raised little shoots outside without warning; they need a chance to experience the process known as “hardening off.” Without this phase, exposing those pampered plants to an outdoor environment with wide fluctuations in temperature, light, and wind can seriously weaken or kill the seedlings.

How to “Harden Off” Seedlings

Seedlings have sprouted in a variety of containers and grow boxes filled with soilFor most plants, it’s safe to start hardening off about 7 to 10 days before the final frost date for your area (which, for the Capital Region region, is May 2nd according to the Old Farmer’s Almanac.) Once your seedlings are big enough to plant outside and the temperatures are milder, place the plants into something that will make them easy to transport all together, such as a shallow cardboard box. (The box is for you, not the plants; you’ll be moving them around quite a bit during this process).

Carry your seed babies outside and place them in a sheltered, shady area. Leave them there for a few hours and then bring them back inside before evening. Repeat this process every day, leaving the box in its sheltered, shady spot a little longer each day. This gives the plants time to get accustomed to gentle wind and temperature variations without exposing them to shock. It’s a good idea to do this for about three days, at least.

Once the box is staying outside for the entire day, you can start introducing the plants to direct sunlight. For a few hours each day, move the box from the shaded area to a sunny one. Gradually increase the length of time the box stays there as well. 

A woman inspects her tray of growing seedlingsEventually, your plants should be tolerating full sun for the whole day without burning, and most likely will be strong enough to stay outside overnight as well. (They grow up so fast, don’t they?) This whole process shouldn’t take longer than ten days, even if you’re being overcautious. If it’s past the final frost date, your seedlings can be safely planted outside.

Tips and Tricks

  • Try to keep the soil moist at all times; Dry air and spring breezes can result in rapid transpiration.
  • Cool-season crops such as lettuce, onions, beets, and peas don’t need as much hardening off. These crops can be planted in the ground as soon as the soil is no longer cold and wet, and is about 50 degrees Fahrenheit.
  • To go the extra mile, take a pre-hardening off step and strengthen your seedlings against wind while they’re still indoors. Simply run your fingers gently over the foliage a few times a day, or you could place a rotating fan on low nearby.Gnarled hands planting a young tomato plant into rich soil
  • If you got a late start sowing your seeds, that’s ok! Planting seedlings outdoors when they’re still quite young — as soon as 3 to 4 weeks after sowing — will help them get established quicker than if they’ve become root-bound in their containers.

All of this lugging plants back and forth may seem like overkill, but you’ll be thankful you did it when you’re harvesting all those veggies later on this summer. The gradual hardening off of seedlings leads to stronger, more resilient plants, and will help to ensure that all your hard work will pay off.

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