Hardening Off Seedlings

Photo of trays of seedings
Four trays of seedlings waiting for their time in the sun!
With a month to go until Boulder County’s average last frost date, it’s time to start thinking about hardening off cool-season crops! If you have season extenders like cold frames, row covers, or Wall-o-Waters, you can even harden off a few tomatoes and peppers too.

Why Harden Off?

Plants are wildly adaptable, which is part of the reason they grow all over the planet, inside and out. But that means that the structure of the plant itself (from the thickness of its stem to how open its pores are) is different based on whether it was started outside in the sometimes harsh and variable conditions of early spring or indoors where light, moisture, food, and wind are constant. You can replicate some outdoor conditions for your seedlings by directing a fan at the seedlings as they grow and by putting them somewhere where the temperature is a bit variable, but they’re still in for a shock when they move to your garden bed.

To minimize that shock, you have to do what is called hardening off the seedlings. Hardening off means gradually exposing the plant to longer and longer periods of outdoor conditions and should take place over two full weeks for best results. I know it’s tough to take the time to do this, but if you do, you will have healthier plants, with reduced transplant shock and wilting, and will get vegetables or blooms faster than if you skimp on the hardening off process.

How to Harden Off Seedlings

So, two full weeks before you want to plant seedlings outdoors, and when seedlings have at least two sets of true leaves, water your flat of seedlings fairly generously and set the flat of seedlings outside in a partially shaded location when the temperatures are above 50 degrees (I will fudge and put things out at 45 if they are cool season crops like onions & brassicas). If there is heavy wind, consider waiting for another day.

Yes, it is possible that you won’t be able to put your seedlings out every single day. And yes, this process is more difficult if you work outside the home, but an hour in the late afternoon after work is better than nothing at all, so do what you can. The first day, put the seedlings out for an hour and then bring them back in. If they seem dry at all, water them again.

photo of sunburn on a cauliflower seedling
Sun burn on a Cauliflower seedling
Leave them out a little longer on the second day, two hours or more if it’s cloudy, but still be sure to water them before and after their outdoor adventure. If you start to see white or light tan spots on the leaves, that is sun/wind burn. Give the plants an extra drink and a little bit more shade, but continue to put them out.

Over the course of the next two weeks, slowly increase the number of hours the plants are outside until they are out for the entire day. Do not leave them outside if temperatures are going to dip into the 30s, especially at night. As you increase the amount of outdoor exposure they’re getting, decrease the water. They likely won’t be getting watered every day in their permanent home, so the idea is to slowly get them used to being watered every other day, then every third day.

photo of tomato leaf with sunburn
Sun injury on a tomato leaf during the hardening off process
Once they’ve been hardened off for two weeks, they can be planted outside in the garden. You can leave them out in a sunny location until you’re ready to plant unless a late frost is expected. Keep in mind that sunburn happens (even on my beloved tomato seedlings!), especially in the sunny southwest. And try not to panic if a couple of starts don’t make it through the transplanting process–that’s why you should always start a few extras to begin with, right?