Hardening Off Plants

Spring is an exciting time because I can start planting cool-season crops. It’s also a frustrating time as the weather changes from one extreme to another. Within one week we’ve had lows of 28°F and highs of 81°F. That’s why you need to know the best time for planting different kinds of seeds and transplants in your garden. Soon it will be time to start transplanting warm-season annuals here in zone 5, so now we should start getting our seedlings ready to go outside. We do this with a process called hardening off: the process of helping plants adapt to being outside by gradually exposing them more and more to outdoor conditions for about two weeks.

That’s right: it takes about two weeks. If you wait to buy the plants until you’re ready to plant them, you’ll be tempted to skip this process. Don’t do it! You’ve got to baby those baby plants before you put them out in your garden. After being sheltered inside a nice cozy greenhouse or under your grow lights, those seedlings run the risk of being damaged by the brighter sunlight, stronger breezes, and fluctuating temperatures of outdoor life. Even if they do not exhibit any visible damage, they will take longer to adjust out there and resume normal growth. Thus you won’t save any time, and you’ll end up with stressed plants.

I don’t trust big box stores to harden off their plants before they put them outside, so I recommend getting seedlings from a garden center greenhouse and doing it yourself. I buy from the big box stores only when the price is good enough to compensate for getting stressed out plants. Plus, I don’t buy anything that is especially susceptible to cold temperatures, such as impatiens, from them.

You’ve got to baby those baby plants.

the gardener wife

Here are two methods for hardening off seedlings. The first one requires more work, and the second method requires a cold frame or–what I prefer to use–a mini greenhouse or two. Both of these methods require ten to fourteen days, so you should begin the process about two weeks before your planting date.

Method 1: Moving Plants In and Out

1. On a mild afternoon, move the seedlings outside to a shady area that is sheltered from the breezes. After two to three hours bring them back inside.
2. Repeat the process each day unless the weather is too harsh. Gradually increase the amount of time outdoors and the amount of sunlight the seedlings receive. 
3. Keep watering the seedlings as needed so they do not wilt, but water them less often while they are hardening off.
4. After they have been outside for several ten- to twelve-hour periods and the nighttime temperatures have risen high enough for the hardiness of the seedlings (in the forties for hardy seedlings and in the sixties for tender seedlings), leave them outside overnight for a couple of days. Then they will be ready to transplant into the garden or outdoor containers at the appropriate planting time.

This method gets wearisome if you have lots of seedlings to harden off like I do. It also requires that you have a space to put the plants inside each afternoon or evening. The space requirement, by the way, is one of the reasons you should not buy tender annuals too early in the spring. They must be kept where they will be warm enough and get enough light until it is time to start hardening them off.

Method 2: Putting Plants in a Cold Frame or Mini Greenhouse

I do have the space for keeping seedlings indoors because I have seed starting shelves with grow lights in my basement. But once I start hardening off seedlings, I do not want to carry all of them up and down the stairs twice a day for two weeks. That is why I prefer to use mini greenhouses. First I harden off the hardiest seedlings in them and then the half-hardy seedlings. By the time those are ready to transplant outside, I can start hardening off the tender seedlings.

Mini greenhouses like mine are now available at most big box hardware stores. But if you cannot find one near you, you could order them here.

1. Set the mini greenhouse up in a mostly shady area. Be sure to brace it by attaching it to something or weighing it down so that it would not topple over in a big gust of wind. I lay heavy bricks or pavers on the bottom shelf of mine.
2. Move the seedlings inside the greenhouse. Now instead of bringing them inside everyday, you can just zip the door shut. Start on a mild afternoon and leave the door of the mini greenhouse open for just a few hours. 
3. Protect the plants on windy days by unzipping only one side or part of the door. To open the door fully, you can roll it up and tie it with the ties provided. However, I prefer to flip the door up and over the back, and I use a clamp to hold it in place. Water the plants only as needed, and keep increasing the length of time that the door is left fully open. When nighttime lows are mild enough for the hardiness of the seedlings, leave the door open overnight.
4. After one week, move the mini greenhouse and then the seedlings to a sunny location. If you have two mini greenhouses, like I do, then you can set up the second greenhouse in the sunny spot. Then all you have to do is move the seedlings from the shady greenhouse to the sunny greenhouse.
5. Continue to keep the door open during the days, and close it when the nighttime lows may get too cool for the seedlings’ hardiness level. You may rotate the seedlings around the shelves to adjust the amount of light they get. By the end of the second week, the seedlings are ready to transplant at the appropriate planting time.

Whichever way you do the hardening off, your plants will be stronger and grow better. If you have any questions or comments, write them in the reply section below. And if you found this post helpful, please share it so I can help others to grow something, something beautiful—even better, something to eat!

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