Hardening-Off Seedlings: How & Why
Excerpt from The Ever Curious Gardener by Lee Reich


Imagine that you had not set foot outside all spring…better yet, that you had spent all spring in a warmed cave…then tomorrow you went out and stayed there. At the very least, you would have to put your hands to your eyes for a while to shield them from the sun. And if the night was very cool—not unusual in spring—well, you would shiver. Fresh air and sunlight are great for the constitution, but you would have to first acclimate yourself to them.

The same goes, even more so, for vegetable and flower transplants. Indoors, where they get their start, they are, after all, coddled. They know nothing of wind, which can shake them up and dry out their leaves by too quickly drawing water from their stomata. Their tender cells know nothing about dealing with cool temperatures, or temperatures that swing 30 degrees Fahrenheit within 24 hours. Their leaves have yet to experience blazing sunlight.

Vegetable and flower transplants bought from a nursery would not necessarily fare any better. They’ve spent the better part of their youths in greenhouses, exposed to more sunlight than home-grown seedlings and, perhaps, the breeze from a fan, but still nothing compared to the great outdoors. The high humidity of a greenhouse does nothing to prepare them for drier air on the other side of the glass or plastic.

What’s needed before setting transplants out in the ground is to have them undergo a process called “hardening off,” which gets these plants acclimated to increasing intensity of sunlight, gusts of wind, fluctuating and cooler temperatures, and soil moisture levels that might border on drought one day and a week later turn boggy.

The kinds of changes that the lower temperatures of the hardening off period induce in coddled seedlings depend on the nature of the seedlings themselves. Seedlings of cabbage, lettuce, snapdragon, pansy, and other plants that can eventually stand up to temperatures below freezing develop a tolerance for cold by building up sugars in their cells. Cold also changes the composition of their cell membranes.

Tomato, marigold, zinnia, and seedlings of other plants that hardly tolerate temperatures below freezing begin to suffer from so-called chilling injury at temperatures below 50°F. Resulting changes in their membranes interfere with photosynthesis, causing a buildup, instead, of damaging toxins in their leaves. As a tomato or other warmth-loving plant becomes hardened off through gradual exposure to cooler temperatures, it becomes better able to repair and prevent such damage.

Once seedlings move outdoors, direct sunlight—whose intensity is as much as ten times that of light streaming through a sunny, south-facing window—can also do damage. Gradual exposure to more intense light, beginning in dappled shade or with just a few hours each day in full sun, thickens cell walls, fibers, and cuticles on both existing and new leaves. With increasing light exposure, chloroplasts, the green, light trapping energy factories in leaves, also move around and align themselves in such a way that the leaves turn darker green. And the stomatal pores of the leaves, through which water is lost and carbon dioxide and oxygen are exchanged, become more quickly able to open and close in response to changing conditions.

Hardening off needs to be gradual. Trying to toughen up plants too severely too quickly could send them into shock. Annual  flowers and vegetables might respond by  flowering prematurely. Flowering ruins a vegetable like celery, putting the brakes on stalk production and making those that remain too coarse too eat (fresh, at least; they’re fine for soup). And you don’t want flowers on a marigold plant before it has become big and bushy, or its growth will be stunted. The same goes for broccoli buds.

So what’s needed is to find some cozy spot outdoors, a spot that’s sheltered from wind and receives sun for only part of the day, or else dappled sun all day. I have a south-facing, brick wall along one side of my terrace that makes an L with a west-facing, white stucco wall; overhead is an arbor. The extra heat in that corner, the shelter from wind and all-day sunlight, and the part shade a orded overhead by the arbor make this corner ideal for beginning the hardening off of seedlings. About a week at that location starts the process.

Watering needs special attention because, barring rain or overcast conditions, plants are going to dry out much faster outdoors than they did indoors. Keeping plants slightly on the dry side makes for tougher growth, and gets them used to a condition they may have to experience once they are on their own in the ground. Too much water stress, though, could cause shock and its attendant effects.

I also pay attention to the weather. If a night threatens to be cold, I might, depending on the plants and the degree of cold, have to move the plants to a more protected spot outdoors—to a bench right up against the north wall of my house, for example— or indoors. (Fortunately, my garage/barn is on the other side of the brick wall of the L.) Despite the extra work, I move seedlings if there is any doubt about the weather. One cold snap could snuff out weeks of care, especially tragic if I’m growing ‘Italian Sweet’ pepper, ‘Carmello’ tomato, ‘Lemon Gem’ marigold, or other unique varieties that couldn’t be replaced with ones from a local nursery.

After about a week, plants get moved to a more exposed location, one that takes just the edge of gusty winds and broiling sun. I continue to keep a close eye on watering and nighttime temperatures. A week at this second location and plants will be ready to be planted out at their permanent homes.

During the couple of weeks of hardening off , plant growth becomes slower and stockier. This is good; it shows that the plants are getting ready to face the world. Mother Nature can be fickle, though, so I stand ready to protect even these hardened off plants, once they are out in the garden, with overturned flowerpots or sheets if a late frost threatens. Ideally, this gradual hardening off, along with further protection, if needed, eases seedlings’ transition to the garden so they hardly know they’ve been moved. Which is as it should be.



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