Hardening off isn't tricky or complicated, but it can be a matter of life or death for your seedlings. Your beautiful little seedlings may keel over in shock if you take them straight from your light shelves to your garden. Hardening off simply means getting your seedlings used to outside conditions gradually. Give them a little more exposure to wind, sun, and temperature variations each day, until they are ready to be planted out. Although the process could be accomplished in as little as 3 days under ideal conditions, I like to give seedlings as much as a week to toughen up before transplanting them.
Find a sheltered spot for hardening off. Gradually move seedlings to a less sheltered location. Scout around and figure out where your seedlings will spend their first hours outside. A location that's out of the wind and partly shaded, especially from harsh afternoon sun, is ideal. I've learned the hard way not to set them directly on the patio. One year, some varmint bit the stems off several dozen pepper seedlings! Now seedlings get set out on a table, so they're a little less accessible.
Increase sun during the day. There's a big difference between fluorescent lights and the brightness of real sunshine. Their first day out, seedlings should get no more than an hour or two of sun. Ideally, they'd get another hour or two of sun each day until they're enjoying the same amount of sun they'll get in your garden. If that's not practical, putting them in a partly shaded location for several days before moving them to a sunnier location will work fine.
Decrease protection from cold at night. Always bring them inside if temperatures drop near freezing. The first night, bring them inside. The second night, bring them inside again unless the low temperature forecast is especially mild. By their third night, they'll have started to toughen up a little and can stay outside in a sheltered location, perhaps up against the wall of the house. After a couple more nights, they'll be OK in a less protected location such as a few feet away from the house. The idea is to get them used to cooler night temperatures in stages.
There are alternatives to moving seedlings around so much. I use spun polystyrene row cover to protect seedlings from wind, sun, and cold. Giving the plants more or less exposure to "real" conditions is simply a matter of putting the row cover on or off. Put something underneath to hold it up off the seedlings. Row cover provides better protection against cold when it doesn't touch the leaves. A sheet would also work, but I think the spun row cover does a better job of insulating. It will also let water through to the plants.
Those little plastic zip-front "greenhouses" are also wonderful for hardening off plants. You can drape row cover or a sheet over them to provide some shade. You'll probably have to unzip them during the day so your plants don't cook. As your seedlings progress, leave the door unzipped at night also.
If you're in a rush, even a little hardening off is better than none. Here's my 3-day quickie method for use during mild weather. Be sure to water them as needed. Day One: Put seedlings outside in a sheltered, mostly shaded location and bring them in at night. Day Two: Give seedlings a half day of sun, and cover them at night. Day Three: Put seedlings in sun for most of the day, and let them stay outside for the night, uncovered but out of the wind. This is a minimal approach; I'd suggest taking 2 or 3 days at each stage.
Even if you winter sowed seeds in closed, vented containers, you're not entirely off the hook. Your seedlings are already partly hardened off, since they're already outside. But the protective cover is sheltering them from wind and keeping them warmer. And you've probably put them in a semi-shaded location so they won't cook in the sun. So they'll need a little gradual adjustment too, before you plant them out in a sunny, exposed location. Some people gradually increase the size of the vent holes to get seedlings used to direct outdoor conditions. I take the covers off my winter sowing containers just during the day at first. If a frost is in the forecast, I cover them back up. I also give them increasing amounts of direct sun while they're uncovered. By the time I'm ready to plant them, they're ready too!
It's hard to be patient while hardening off your seedlings. After all, you've been nothing but patient since you set up your shelves and sowed your seeds. You can see your seedlings practically bursting from their pots in their eagerness to get into your garden. After several days of hardening off, however, you'll notice a difference in your seedlings. The stalks and stems of your plants will thicken, and they'll look healthier and sturdier. The wait will be worthwhile. Now, they are really ready to be planted out in your garden!
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See my other articles on growing plants from seed:
For more tips on irrigation, see Paul Rodman's article, Watering Wisely.
Photos by Jill M. Nicolaus