How To 'Harden Off' Your Seedlings

What is Hardening Off?
Hardening off is the process of gradually exposing indoor-grown seedlings to the outdoors, allowing them to adjust to the harsher conditions outside.

When plants are started indoors, they are in a controlled environment with consistent temperature, light, and moisture. However, the outside environment is vastly different, with fluctuating temperatures, wind, and varying levels of sunlight. If seedlings are not hardened off properly, they can become stressed and potentially die. Hardening off allows the seedlings to develop thicker, stronger stems and roots, making them better equipped to handle the challenges of outdoor life.

How to Harden Off Seedlings:
Begin the process of hardening off 7-14 days before transplanting outside, making sure the average daily temperatures are above 50F, and that it is after the last frost date for your area. Check your last frost date here.

Start by placing your seedlings outside for a few hours each day in a sheltered location that receives bright, indirect sunlight, such as a covered porch or under a tree. Begin with just a few hours in the morning when the sun is not as strong.
Gradually increase the time the seedlings spend outside each day, always keeping an eye on the weather. If there is a sudden cold snap or strong winds or heavy rain, bring the seedlings inside.

After a few days of gradually increasing outdoor time, start exposing the seedlings to direct sunlight for short periods, gradually increasing the time they spend in the sun. Do this during the morning hours when the sun and temperatures are not as intense, so that your seedlings do not get sunburned.

Decrease watering by small amounts to encourage root growth and resilience; however, be careful not to let the growing medium dry out completely.

After about 10 days, if nighttime temperatures remain above 50F, it is ok to leave your seedlings outside overnight, provided they are in a location safe from weather conditions and wildlife.

After approximately 14 days of the hardening off process, your seedlings should be ready to be transplanted to their final location. If possible, choose a day with mild or overcast conditions and be sure to water thoroughly after planting. Check seedlings daily until established.

Here are a few additional things to keep in mind when hardening off your seedlings:

  • If you have pets or wildlife that may damage your seedlings, take steps to protect them.
  • If you are in a location where you are unable to gradually harden off your seedlings, you can use a shade cloth or other cover to protect them from the sun and wind.
  • Growing medium will dry out more quickly when exposed to outside conditions, so be sure to check moisture levels frequently and adjust watering schedule as needed.