Let’s talk hardening off strategy! We have started our hardening off process and so we’re going to talk about why we’re doing it so early in the season.
Some growers are simply going to raise their plants from seed indoors and start hardening off all their plants around their last frost, typically about a week (or maybe 2-3 weeks) before that last frost date. This is definitely the easiest and lowest risk way to go. If you’re in a similar climate to us, we’re still 7 weeks to last frost. So, if this is your strategy, you still have quite a bit of time to go.
For us, we want to start shifting our cold hardy and frost tolerant plants out to our greenhouse as soon as we can. It’s important to know that our greenhouse can be minimally heated (to 40F) and is guaranteed to not get frosts or cold temps, should the low overnight temperatures drive back down. So, most seasons, we can start to look at hardening off fairly early in the season. Typically, 3 to 6 weeks prior to last frost, depending on the seasonal weather patterns.
The biggest reason we try to make this push out to our greenhouse as early as we can is to optimize our indoor growing space. Growing over a thousand garden starts each year takes up a lot of space. So, if we can get our plants outside sooner, we can run less grow lights and use less of our living space. Also, the sun is better than our grow lights!
We are generally looking for day time temps in the 40’s and overnight temps in the 20’s as “the point” where we’ll start doing this. This helps us ensure we’re heating our greenhouse against relatively reasonable outdoor temperatures. Heating always has a cost, and it’s often not small. You technically can heat against any temperatures you have the budget and equipment for, but for us, we like to minimize that cost. These are our cutoffs and we’ve generally found it to be highly competitive against using indoor growing and grow lights instead.
For our push to the outdoors, we use a general strategy where cold hardy and frost tolerant plants go first. These plants are going to handle those cooler temps just fine. As outdoor temps continue to rise, we also start putting our frost sensitive plants out. We usually do wait for our warm loving plants (peppers, squash, cukes, etc) until low temps are a minimum of 50F. These plants really don’t handle cool temps well, so it’s important to keep them well protected.
Even if you don’t have a greenhouse, you can usually use a very similar strategy to get your plants outdoors. As long you watch day and evening temperatures like a hawk, you can often get your plants outdoors many weeks before last frost in most seasons. We have a guide, found in the comments, that talks about temperature tolerance of plants and provides guidelines for low “spring season” temperatures and the various types of plants this affects.
If you don’t have a greenhouse that can be heated, it’s important to know that the biggest risk is overnight low temperatures. You can still get frost in a greenhouse! Greenhouses are great, but they aren’t this perfect tool that protects your plants from all potential risk. You can still use it, but we generally recommend following the guidance we offered above for outdoors before you simply throw them out there.
We’ll discuss our actual hardening off process tomorrow. But, we figured some of you “aggressive northern growers” might want to know where our head is at. This specific season is allowing us to do this quite early, but we always look at it based on the weather conditions we get each season.
Demystifying Cold Hardy, Frost Tolerant, Frost Sensitive and Warm Loving Garden Plants
