Expert’s Tip: Hardening Off Seedlings

By Eileen Nelson, UW-Extension Herbaceous Ornamental Specialist

As the wind blew and the snow swirled you patiently planted precious seeds in starter trays, all the while visioning the blooming flowers and fresh produce of summer.  Now, those seeds have sprouted, the plants have sized nicely and the garden bed awaits.  You don your gardening clothes, pick up a trowel and head out, plants in hand.

NOT SO FAST!  Your precious plants may look ready to go into the garden, but stop and consider what you are asking them to do.   They just aren’t ready to go  directly from their nursery to the grown up world, without a chance to toughen up. They need to be toughened up – and in the world of growing seedlings that is known as Hardening-off.

Hardening-off is the process of gradually moving tender seedlings from their protected environment to an outdoor setting.  It is done in stages so the impact of strong sunlight, cool nights and less frequent watering doesn’t shock their system.  Use the following steps as a guideline.

  1. On a mild day (temperatures above 40 F), set your plants in a sheltered location with 1-2 hours of sunshine.
  2. Each day increase exposure to sunlight a few additional hours at a time, keeping them out of harsh winds and cold rains in early stages.
  3. As they strengthen they will also be ready to withstand stronger breezes and cooler rains.
  4. Gradually reduce the frequency of watering, but don’t allow the seedlings to wilt.
  5. If temperatures below the crops minimum are forecast, bring the plants indoors or cover them to protect them from damage.
  6. After transplanting to the garden, use a weak fertilizer to get them growing again and help avoid transplant shock.
  7. Utilizing a cold frame can minimize moving plants indoors, but, the guidelines for gradual increases in sunshine, exposure to temperatures and wind should still be followed.

The process of hardening off doesn’t have to be perfect.  Just remember that tender plants need tiny steps as they acclimate to their new environment.  They won’t survive the extremes of the outdoors without your gentle guidance.