Hardening Off Seedlings and Young Plants
Hardening off is the process through which seedlings and other young plants are acclimated to outdoor weather. Here’s how to do it.
Plants that have been growing indoors—be they seedlings or annuals bought from a greenhouse—should go through a process called hardening off before they're planted in the garden or outdoor containers in spring. (You may skip this step if the garden center confirms their plants have been hardened, but if you're not sure, play it safe and do it yourself.)
Hardening off entails gradually acclimating the plants to the harsher light, wind and temperatures of the open environment. Hardening off takes a week or two.
For cold-tolerant vegetables and flowers, which can sustain temperatures slightly below freezing once fully hardened, the process should begin when daytime temperatures remain above 40 degrees (F).
For warm-season vegetables and flowers, wait until nighttime temperatures stay above 50. Give seedlings you've grown yourself a full week or more, while storebought cool-season annuals, like pansies, generally need less time.
During the hardening off period, avoid moving your seedlings or plants indoors at night. Instead, put them in an unheated space, like a garage, shed or enclosed porch; or cover them with a cardboard box or heavy sheet. This will ensure there's not too much of a temperature flux between night and day.
Hardening off timeline
The first day, set the seedlings or plants in a shaded, sheltered position, out of the wind, for a few hours. Over the next few days, leave them out in this spot a bit longer, until they're sitting there for half the day.
By day four or five, begin leaving them in a spot where they'll be subject to some wind and direct sun for part of their outdoor time.
Increase the length of exposure to sun and wind over the next few days, until the plants are spending most of the day in these conditions.
By this point they should be ready to plant.
If heavy wind, rain or freezing temperatures occur at any point in the hardening off process, keep the plants in their nighttime shelter instead of setting them out.
If the weather turns cold after you've planted them out, cover them with an old sheet or upturned nursery pots until temperatures recover.
