As we get closer and closer to our last frost, some of you might be getting pretty desperate to get your plants outside. So, let’s talk about “how low can you go?”
For the sake of science and sometimes necessity, we’ve done some experimentation with leaving our plants outside when its still quite cold overnight. We’ve learned those limits and we’re here to share them.
With most cold hardy and frost tolerant plants, we generally recommend no colder that 36F. This is a good, safe and frost-free temperature. But, if safety isn’t your primary goal? You can go well into freezing temperatures if you have to. Cold hardy seedlings can withstand low overnight temperatures down to 30F. Even a tiny bit lower than that is usually OK, but they’re definitely going to start showing cold stress at that point.
Now, we’ll tell you, this isn’t a perfect science. Even though a particular plant might be classified as cold hardy or frost tolerant, there’s a lot of variability between genetics and maybe even some luck involved. For example, if you compare a flat leaf vs. curled leaf parsley? The curled leaf will outperform the flat leaf by a wide margin in colder temperatures.
Some strategies you can use to reduce risk from cold over night temperatures? Keep your plants close to your home. The thermal mass will definitely help and can be several degrees warmer than your ambient air temperatures. Also, get those plants off the ground. If you can prevent thermal transfer, especially into the soil, your plants will weather colder temperatures better. Thermal transfer occurs any time there’s a temperature differential.
Of course, when it comes to those frost sensitive plants, we can’t even mess around with these. A frost is going to wipe them out and it’s game over. Frosts can technically occur anytime temperatures get below about 36F or so. Even though that’s not quite freezing, small microclimates, wind chill and other factors can definitely drop those temperatures below freezing. But, if an evening is predicted to be above 40F? Maybe 38F? 36F if you have no choice? Frost sensitive plants will be fine outside!
But, if you’re growing those warm loving plants like squash, peppers and cucumbers? Protect them at all costs! The cutoff for these plants is 50F, day or night. We never really consider getting these plants outside until we’re much closer to our last frost. Sometimes even after last frost, depending on the season.
At this point in the season for you northerners, chances are good that a lot of your plants can simply live outside. But, as we get closer to our last frost, those evening temperatures will continue to flirt with these borderline freezing temperatures. They’ll also rise, the closer we get to last frost.
If you want to know more, we’ve taken these concepts to some fairly deep levels. You’ll find one of those handy links down in the comments!
