Since we’re into the hardening off phase, let’s dive into it! Hardening off is a critical process for indoor grown plants, regardless of whether you’re bringing them outdoors or into a greenhouse.
The basic gist is that the sun is powerful, many magnitudes more powerful than your grow lights. Just like people, plants can get a sunburn if they’re exposed to too much sun, too quickly. It takes some acclimation to be ready for it!
The very fastest you can make this happen is a period of about five days. We start with one day at one hour, two days at two hours, one day at four hours and finally one day of 8 hours of exposure. After this, your plants can be outdoors or in a greenhouse.
The key to this are those very short exposures, early on. In our experience, you don’t need to find shade or use cloudy days, so long as those first exposures are very short. We regularly harden off in the bright mid-day sun.
Oh, and when hardening off, make sure your plants are well watered first. The outdoors causes plants to transpire much more, due to wind and that sun!
We’ll tell you, this is a pretty aggressive schedule. If you were to “improve” it, we’d suggest more days with less time up front. (Maybe another day or two at 1 or 2 hours.) That said, this “aggressive” schedule works just fine across all plants, with only very minor and occasional sunburn.
As for the timing, that depends. Some growers do this just before planting out. Others want the ability to bring their plants outdoors to enjoy nice days, on a whim. Or, there’s people like us that have a semi-heated greenhouse and want their plants in there.
It is important to pick reasonably “warm” days, we prefer over 45F for most plants. We do wait for 50F+ days for plants like peppers, tomatoes, basil, squash and other warm loving plants. Cool temperatures can damage these plants, for life.
And in case you want to know more, we have a full length article that goes deeply into hardening off and will answer any other questions you have! Linky down in the comments.
