How and Why to Harden Off Plants

You started your own seeds or purchased transplants from a garden center. Time to get the seedlings in the garden, right? No! There’s one more step, and it’s an important one.

Until now, those seedlings have led the good life — in your warm house or a heated greenhouse. They’ve been watered and fertilized regularly and protected from harsh weather. They need time to acclimate to the great outdoors.

Whenever you move plants (seedlings, houseplants, etc.) outdoors, they need to be hardened off. Did you know plants can get sunburned? Suffer chill damage? Even plants, like tomatoes and basil, that love full sun in the garden need hardening off. Otherwise…

Tomato seedling with sunburn damageSunburned tomato seedling

Sunburn

Bleached or whitish areas on foliage may indicate sunburn. These basil and tomato plants weren’t ready for the onslaught of full, intense sun, which is far brighter than any indoor lights. Most plants outgrow the damage.

Chill or Freeze Damage

This often shows up as blackened foliage and droopy stems, as it does on this basil. The uppermost leaves bear the brunt of the damage because as cold air settles they offer some protection to the lower leaves. Pinch off the damaged areas and, if it wasn’t completely frozen, the plant should recover eventually.

Chill damage on basil seedlingChill damage on basil

How To Harden Off Plants

Gradually acclimate indoor plants to outdoor conditions over the course of a week or 10 days.

  1. The first few days, place the plants in a sheltered spot out of the wind with light shade.
  2. Gradually move them into a place with a bit more exposure — morning sun, shade in the afternoon.
  3. Next, give them morning sun and dappled afternoon shade.
  4. Leave them out overnight in a sheltered spot such as a porch, if temperatures are in the 50s (or in the 60s for very tender plants like basil and peppers).
  5. Work your way up until the plants are out in full sun and overnight.

Note: Plants exposed to sun and wind will dry out much faster than they did indoors. Seedlings in small containers may need watering several times a day.

Hardening-Off Hack

Don’t have time to be moving lots of seedlings around while they harden off? Here’s a simple solution. Observe the patterns of sunlight around your home. What area gets morning sunlight? Light shade? Afternoon sun? Place the seedlings in a cart, so you can move them to the appropriate spot as they grow more and more accustomed to the outdoors. Roll the cart into the garage at night for the first week or as needed.

Last updated: 07/06/2021