Hardening Off Seedlings and Plants

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Windowsill-grown plants need a period of hardening off before they can face outdoors conditions. Photo: Tomwsulcer, Wikimedia Commons

You’ve started plenty of seedlings or cuttings indoors or you have houseplants to move outdoors for the summer. No problem: you can do that! Still, it’s important to always acclimatize them to outdoor conditions before placing them out for the season. Or as gardeners say, to harden them off.

Why?

Plants raised indoors live under near-ideal conditions: it’s always warm, the light level is pretty stable, plus there is no wind to break their stems or rain to flatten them. Furthermore, most of the sun’s ultraviolet rays, the ones that give us sunburn if we aren’t careful, are filtered out by window glass, nor are they given off by the fluorescent or LED lamps we use to light our plants. As a result, our tender, mollycoddled plants will be in for quite a shock if you suddenly stick them outside in the real world where the conditions are always changing and those nasty ultraviolet rays beat down.

Plants placed outdoors too quickly, especially if you place them in full sun, often suffer from damaged or dried leaves (they are said to have “burned”), their stems can flop or break. In fact, many will be killed outright.

Hardening Off

The purpose of acclimatization or hardening off is to toughen up plants that are transitioning to outdoor life. By gradually exposing them to more intense ultraviolet rays and to wind, rain and temperature changes, you can help them adapt to move. As they cope, they start producing a thicker cuticle on their leaves, stiffening their stems with more lignin, adapting their growing system to changing temperatures, etc.

You can acclimatize most plants fairly readily, in about 7 to 10 days.

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Any spot where you can find shade is a good place to start, even if that’s only under garden furniture.

If you intend to plant out at about the date of the last frost in your region, that means you’ll have to start hardening them off before the date of the last frost, when there is still a risk of cold. In years when it’s colder than normal at planting-out time, it may be wise to delay acclimatization … and also planting out.

Under ideal conditions, it will be at least 60°F (15°C) the day you start to harden your plants off. Place them in the shade in a spot somewhat sheltered from wind. If temperatures are expected to dip below 50°F (10°C) at night, it would be wise to bring them back indoors in the evening. If there is to be a beating rain, move them to a shelter. Otherwise, they can stay outside. Repeat that for 2 or 3 days.

Now find a location in part shade in a windier spot, one that receives some direct sunlight, especially in the morning, but shade for the rest of the day. Or sunlight filtered through tree leaves. Normally, the temperature will have warmed a little bit since the beginning of the acclimatization period (that’s just way spring goes: we tend to gain a little heat almost every day). Again, place the plants there and, if possible, leave them out at night too. But by now they’re already tougher and can handle 45°F (8°C) nights. And again, this step can last 2 or 3 days.

The last step is to place the plants out in full sun and wind for yet another 2 or 3 days, leaving them it outside day and night if possible. (Obviously, with plants that will be spending their summer in shade or partial shade, exposure to full sun won’t be necessary.)

When all goes well, your plants will then be ready to face outdoor conditions, no matter where you place or plant them.

Greenhouse Acclimatization

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One of my temporary greenhouses: they partially harden off young plants.

If you’re like me, you start a lot of plants indoors where it’s evenly warm, then move them to an unheated greenhouse or cold frame when they start to grow. Such structures are heated by the sun during the day, while cooling off at night yet remaining above freezing.

This is a major step towards acclimatizing your plants to outdoor conditions, since they start to undergo cooler nights than indoors, plus a bit of wind and more ultraviolet rays: that’s because you’ll be opening the door or panels on hotter days. They’ll have begun to experience temperatures closer to those of outdoors and more ultraviolet rays will have reached them than would have been the case inside your house. As a result, they’ll be receiving near-to-outdoor conditions and will be naturally sturdier than windowsil-grownl plants. Even so, it’s still wise to harden them off just a bit more, especially to those nasty ultraviolet rays, by placing them outside in a spot protected from full sunlight for 2 or 3 days before you plant them out.

When Weather Throws You a Loop

Obviously, everything doesn’t always go as planned. If you hear frost is expected, or even near-freezing temperatures, during the hardening-off period, temporarily cancel the experiment and move the plants back indoors, even if that’s only overnight in a garage or tool shed. Even temperatures of 45°F (5°C) early in the hardening-off process can send the plants into a state of shock that will considerably delay their growth, not something you want.

If conditions are such that you have to keep them under cover for more than 48 hours, consider the acclimatization efforts you’ve made so far to have been in vain, as plants “soften up” pretty much as quickly as they harden off. Simply start all over. Sorry, that does sometimes happen.

Generally, however, acclimatization does carry on pretty much as you planned and you’ll soon have some very tough plants perfectly ready to face a summer outdoors!20170513B

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