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How do you harden off plants?


container gardening picture of plants on wagon hardening off

Hardening Off

Photo © Kerry Michaels
Question: How do you harden off plants?

Whether you start your own seeds indoors or buy seedlings, chances are they have been coddled and protected from the elements. After all of this babying, plants need time to acclimate to harsh outdoor conditions. This process is called hardening off. I also think of it as boot camp for plants.

Wind, sun and rain can wreak havoc on delicate seedlings so you need to toughen them up and harden them off. The idea is to expose your plants to the elements gradually. Practically, this means that you expose your seedlings to outside conditions incrementally, over the course of six to fourteen days, depending on your patience, the temperature and the fragility of your seedlings.

The process is more art than science, so the following is just a ballpark schedule that you should modify given the temperature, type of plant and your temperament.

  • Day One: Pick a mild day and put your seedlings outside in a protected area out of direct sun for a few hours. Less is more here, you just want to give them a taste of what is to come.
  • Day two through five: Increase sun exposure gradually, while keeping plants protected from cold and wind. At the same time, also gradually reduce the amount of water you give your seedlings (boot camp isn’t supposed to be fun) and don't fertilize them until they are completely hardened off.
  • Over the next six to ten days: Lengthen the time your plants are outside, until they can stay out all day and night. You may still need to protect your plants even after they are hardened off in the event of high winds, sudden downpours, or freezing temperatures.
  • Hardening off can be a bit of a logistical nightmare. I find the easiest way to do all of this plant schlepping, is to put my seedlings on wagons and garden carts. Then I can simply wheel them in and out of the garage.
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