How do you harden off plants?

container gardening picture of plants on wagon hardening off
Hardening Off. Photo © Kerry Michaels

If you have ever bought seedlings or plants from a nursery or greenhouse and left them outside in the sun, only to find them limp and withered the next day, chances are it is because they haven't been hardened off. 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.

For plants that have grown up in a greenhouse, wind, sun even rain are a new and stressful. Plants can even get sun and windburned. The gradual exposure to the elements is a 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 by hardening them off.  Practically, this means that you expose your seedlings or plants 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. If you have a lot of plants, and a garage, a good solution and perhaps the easiest way, is to put them in carts or  little red wagons which are easy to find at yard sales.

That way you can easily bring them out and then drag them back into the garage.

  • 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.

Before you buy your plants, ask if they have already been hardened off. Chances are, if they are sitting outside at the nursery, they will have already been acclimated, but it doesn't hurt to ask in case they were just put out that day. However, if you buy plants online or if they are inside, assume that they aren't hardened off.

While hardening off may seem like a pain, it is well worth the effort--your plants will thank you.