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Bracing Up: Hardening Off Transplants

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Bracing Up: Hardening Off Transplants

by Thomas T. Watson

There are those who say that you are not a real gardener until you learn to start your own transplants from seed. I can't say I completely agree with such a notion, but I do maintain that very few aspects of gardening are as deeply rewarding as watching those seedlings come up under the lights, and then (if everything goes well, of course) grow into plants as beautiful as anything you would ever see at the nursery. Gardeners in regions where the winters can be long, cold, and dark will understand when I say that those little bits of glowing green under the fluorescent lamps can go a long way toward keeping you sane as you wait for spring.

For the sake of those seedlings, whatever crops or flowers they will become, you will no doubt go to considerable effort to get them up and growing. You'll probably spend hours fussing over them, making sure they are well-watered, warm (or cool) enough, and under an adequate light source. And although I believe you should avoid being too nice to your future transplants (as you will see as you read on) this is as it should be. Those small plants are vital; they are the future of your garden.



That having been said, remember to beat up on them a bit before you take them out to your carefully prepared plots of soil.

By beating them up I am, of course, referring to the process known as hardening-off. If you've gardened for more than a season or two you have almost certainly run into this concept, and learned that it is a straightforward process that gradually acclimates the seedling to life in the great outdoors. Experienced gardeners take it all for granted, part of the seasonal chore cycle you accept as a gardener. If you are relatively new to gardening, you've probably wondered what the fuss was all about, why, if you've done everything correctly from the start, the plants should not be sturdy enough to go out and immediately face the world. The truth is they cannot simply because you have taken such pains to be sure they had the right amount of water, good artificial lighting, and the right temperatures.

You've spoiled them rotten.

Seedlings grown under such gentle conditions have a variety of characteristics that make the big move out into the garden a challenge. The regular waterings and sheltered (windless) environment mean the seedlings have never been drought-stressed since their emergence. (A good thing, too, since drought stress on a brand-new seedling is a death sentence.) However, a plant developing under such conditions will grow somewhat broader leaves than one out in the open, and will have a thinner cuticle. The cuticle, by the way, is part of the 'skin' of the plant, a layer of waxy materials deposited by the outer - epidermal - cells that cover every bit of a plant, making a barrier between the plant and its environment. The waxy cuticle of this plant epidermis is there to help keep the plant from drying out. A plant produces the waxes in the cuticle to a degree that is partly dependant on its environment: if a plant develops when things are on the dry side the cuticle tends to be thicker, while in a relatively moist environment (the one you are providing, for example) the cuticle is thinner and less able to protect the seedling from dehydration.

The indoor nature of the average seed-starting environment also influences the seedling's hardiness with regard to mechanical stress. Unless you arrange otherwise, the space under the lights is a calm, windless environment. Since moving air causes a plant to lose moisture faster, a windless environment contributes to the circumstances that encourage a thin cuticle. It also lends itself to the development of more slender stems, not adapted to the buffeting provided by the wind out-of-doors. Plants respond to mechanical stimulation (wind, raindrops, the gardener's dog taking a shortcut) by growing a bit slower and changing the way they build their cells. The cells end up shorter, stockier, and have thicker walls, making for shorter, sturdier plants. Without such mechanical stimuli, which are usually absent under the lights in the house or shed, the plant tends to make longer cells, with thinner walls. Nothing wrong with this; the plants are simply responding to the environment in which they are grown, and as long as that environment does not change, they're just fine. They are well adapted to that environment. But of course, their environment will change, and change dramatically, the day they are transplanted to the garden proper.

As for temperature, here the regulated environment under the lights has an even deeper effect. Those seedlings, like all living things, are the products of thousands of intricate biochemical reactions which, ultimately, control the rate of the seedling's growth. Warm things up a bit, the reactions speed up, and the plants grow faster; cool things off (or heat them up too much), and growth slows down, or stops altogether. No big deal; you've seen this every spring since you started gardening, when that inevitable cold spell sends you scrambling for hot caps and row covers. In gardening terms, we refer to the effects of inconvenient temperatures as 'setting the plants back..' The reason for the set-back, in most cases, is the suddenness of the change; the plants simply did not have a chance to adapt their inner workings to the new conditions. So guess what happens when you pull them out from under the lights and put them outside all in one motion? Even if the temperatures are acceptable for that species of plant, will they be the same as what the plants are used to? Almost certainly not. And so the seedling is set back, perhaps turning into a less productive plant (which may be an advantage if you habitually over-plant zucchini) and possibly weakened enough by the stress to succumb in the long run to a pest or disease.

And then there is the light. If you've done your job well, the light provided to those seedlings has been bright, even, cool, and of considerable duration. No matter how bright it has been, of course, no fluorescent light can hold the proverbial candle to the sun. To make the most of the light you've given them, the plants have grown leaves that are a bit wider and thinner than those of outdoor plants of the same type. Inside the leaves the chloroplasts, those little bundles of chlorophyll (that stuff that makes the plants green), are arranged in thin, even layers throughout the leaves, to better capture and use every bit of light striking those leaves. In other words the leaves are adapted to be excellent solar collectors.

Take that plant directly out into bright sunlight, and the intensity of light is overwhelming. Plants grown in direct sunlight respond - as they grow - to this very different environment by producing slightly smaller leaves and by arranging the chloroplasts inside the cells of the leaves so that they are not as exposed to broadside blasts of sunlight; they sort of stack up, instead of spreading out. A seedling grown indoors has not made such an adjustment; the leaves are over-exposed and sometimes, quite literally, burn out.

Take that plant directly out into bright sunlight, and the intensity of light is overwhelming. Plants grown in direct sunlight respond - as they grow - to this very different environment by producing slightly smaller leaves and by arranging the chloroplasts inside the cells of the leaves so that they are not as exposed to broadside blasts of sunlight; they sort of stack up, instead of spreading out. A seedling grown indoors has not made such an adjustment; the leaves are over-exposed and sometimes, quite literally, burn out.

And so, when the time for transplanting seedlings approaches, this is what you are most likely to have under the seed starting lights: healthy, lush, green little plants with wide leaves, a thin cuticle, long cells with thin walls, and chloroplasts arranged to grab as much of that gentle fluorescent light as they can. Seedlings perfectly adapted to life indoors and under lights, but very poorly suited to the real world of your garden.

Existing leaves cannot grow smaller, but the cells of which they are made can rearrange their chloroplasts well enough - given time - to prevent serious damage while the plant is busy putting out leaves better adapted to life in the sunshine. (After the plants have established themselves in the garden take a close look. Compare the old leaves with newer mature leaves. You can often see the difference.) A plant can thicken its cuticle by depositing waxes on the outer surfaces of its epidermal cells, and can thicken existing cell walls to better brace the plant against wind and the battering of raindrops. These matters take time, and that is what hardening-off is all about, giving those seedlings the time they need to make the essential adjustments.

To best prepare seedlings for the real world, start well before the time most gardening books recommend beginning the hardening-off process. As soon as the seedlings are up provide some sort of mechanical stimulation. At first, just jiggle the trays and pots enough to make the seedlings quiver. As the plants grow and produce leaves, find a way to move the air around them. A small electric fan stirring the air will do nicely - just don't blast the seedlings to the point that they end up growing sideways. If you use a fan, pay special attention to watering, since the moving air will dry things out faster than would otherwise be the case. If an electric fan is not feasible, try to fan the seedling with a piece of stiff paper or cardboard a couple of times a day. The key to this process is gentleness; at the first true leaf stage actually touching the seedling is not a good idea - moving air is all you really need. Later, when the plants are taller and stronger, you can brush them lightly with a feather duster to continue the encouragement of sturdy stems.

The process you're stimulating has an absolutely wonderful name: thigmomorphogenesis. (Say that three times real fast!) It's the kind of word that, when most people see it, causes irregular heartbeats. Don't panic. This comes from a Latin word meaning touch, morph means shape or form, and genesis means to bring something about. So what you have is the bringing about of a change in shape or form through touch. The plant responds to the moving air or the feather duster by changing the way it builds its cells; it makes them shorter and wider, with thicker walls. It also thickens the walls of existing cells. If you give your seedlings a little touch therapy from near the beginning of their little green lives, you come to the time for hardening-off well ahead of the game. And you will see the difference, believe me. The seedlings will be shorter and bushier - trust me, this is a good thing!

If you employ a fan (and I'm talking a very gentle flow of air here, remember!), you add the benefit of increased evaporation from the leaves of the plant. Moving air causes a plant to lose moisture more quickly than would be the case in still air, for much the same reason that waving wet hands dries your skin more quickly. As a reaction to increased water loss the plant makes a thicker cuticle. So now you're two steps ahead of the game.

A couple of weeks out from your planned transplant date (which is determined by considering the average last freeze date for your region and by watching the weather reports diligently) you should reduce the amount of water the seedlings get. This is not to say you should let them become desiccated, but you should not be keeping them constantly moist, either. Let the soil become a bit dry-looking between waterings. You will know you've gone too far if the plants wilt a bit. At this stage a bit of wilt is not a kiss of death, as it would be to newly emerged seedlings. Water them right away, and try not to let things go so far in the future. The reduction in water has an effect similar to that of the moving air; the plant responds to drying by toughening the cuticle to retard water loss. There are also some changes (which are poorly understood) in the roots that make them a bit better at extracting water from the soil, a trait the plant will find to its advantage in the less controlled garden soil environment.

At least a week out from transplant time, start exposing the plants to the great outdoors. You want that first exposure to be numbered in hours, so it might be a good idea to start on a Saturday, and plan on being home that weekend. Put them out in a shady, protected place for a few hours (say, mid-morning to early afternoon). How you define shelter will depend on your local conditions. Here in Tucson, Arizona, I just set the flats and trays on the back porch, in a spot that won't receive any direct sunlight that morning. In a colder climate you may want to have a shaded cold frame available. If the day is breezy, avoid putting them in the direct path of the wind.

After a couple of days of short exposure, assuming you've engaged in thigmomorphogenesis therapy (you had to know I would bring that word back into this, right?), and assuming the weatherman has not predicted snow, you should be able to leave the seedlings out for the day, still in the shade. Each day, nudge them closer to a spot that gets full sun, or uncover more of the cold frame. Assuming the weather does not turn nasty (watch the weather reports!) by the end of the week you should be able to leave those seedlings fully exposed to the elements, day and night. While the seedlings are hardening-off the reduction in watering you began indoors could become counter-productive. Water as needed, of course.

If the plants aren't showing any signs of distress, and the weather is looking good, the seedlings are pretty much ready to go after a week and a half to two weeks of hardening-off. These times are, of course, generalizations. In regions with really cold climates, a longer and more gradual buildup to total exposure is a good idea. Be prepared to bring the plants back indoors any night the weather is likely to be too cool for the plants, and you should have hot caps and such ready to use at a moment's notice after the transplants are in the garden. I've heard it argued that a lengthy interruption of the hardening-off process (say, by a week of out-of-season snow storms that put the seedlings back under the lights indoors) reverses the plant's adaptations, but by my experience this is not true. Plants change more readily in one direction than the other; once they have laid down thicker cell walls, for example, that's the way those walls will be.

Follow through on the hardening-off by transplanting on a calm, cloudy day, or by shading the transplants if sunny weather prevails. Be prepared for sudden changes in the weather; have shade ready for unusual heat, and row covers for that late spring cold snap that always seems to be waiting for you to get the transplants in the ground. Watch the transplants for signs of stress, and don't be too quick to douse them with fertilizer; let them settle in a few days before giving them anything stronger than compost tea.

Whatever conditions prevail after transplant day, you can be sure that by encouraging thigmomorphogenesis (there, got away with it three times!) and by following the time-honored practice of hardening-off, you will have given those seedlings an advantage. That edge can make a huge difference between a productive set of plants, and a garden that just sorta-kinda bears fruit.

Copyright by Thomas T. Watson. All Rights Reserved.

About the Author Thomas Watson is graduate of the University of Arizona College of Agriculture, where he earned a BS in Agriculture, majoring in plant sciences. When he is not writing he is usually working in the garden, re-landscaping his Tucson home, studying botany and plant identification, or "out there" somewhere, studying the natural history of the Sonoran Desert. This is all, of course, based on the assumption that it isn't baseball season, in which case he'll get all of the above done before the next Diamondbacks game.

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