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The weather is warming, your garden is tilled, and you can’t wait to plant out all the little seedlings on your light shelf. Stop! Put down your trowel. Your seedlings need a little time and help to get used to the Great Outdoors before you put them into the ground.
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!
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
About Jill M. Nicolaus
Better known as "Critter" on DG, Jill gardens in Frederick, MD. The weather is heating up, and I'm trying to catch up on planting and transplanting. Between the plants enjoying the protection of the deck canopy and those on the patio waiting to be planted out, it looks a little like a nursery right in our own back yard! I love it, and DH is patient with it as long as he can get to the grill... (Images in my articles are from my own photos, unless otherwise credited.)
Posted by CompostR (from south central, PA) on April 15, 2008 at 8:48 AM:
Yep, this was one of my first big lessons - have a protected area outside for seedlings! Good suggestions! We forget the wind, rain, and blazing sun can ravage a tender seedling.
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Posted by critterologist (from Frederick, MD) on April 15, 2008 at 9:45 AM:
You're right! Even rigging temporary protection will work... I've turned a plastic table upside down, using the legs to support a row cover "tent," and put flats of seedlings under the row cover.
I'm giggling a little at the visual image I got from "ravage" -- picturing a sweet young seedling tossing its leaves dramatically over her "face" -- oh, save me! (probably I need coffee)
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Subject: Very useful information! Thank You.
Posted by plantgirl1 (from Raleigh, NC) on April 14, 2008 at 1:37 PM:
It's been a crazy Spring in Raleigh, NC. Need al the tips possible to make sure our hard work isn't all for naught!
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Posted by critterologist (from Frederick, MD) on April 14, 2008 at 2:30 PM:
Good luck, and happy planting!
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Subject: Hardening
Posted by CaptMicha (from Brookeville, MD) on April 10, 2008 at 8:11 PM:
I've been putting all my plants out on the front porch (sheltered) when I start bringing them out for the spring and summer. It works great... But the porch gets taken over!
Great advice!
This message was edited Apr 11, 2008 6:12 PM
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Posted by critterologist (from Frederick, MD) on April 10, 2008 at 9:25 PM:
A sheltered/shaded porch is ideal... I use my front porch, too, especially when hardening off "shade" plants.
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Subject: Great article!
Posted by Bookerc1 (from Mackinaw, IL) on April 10, 2008 at 10:41 AM:
I've lost whole flats of seedlings in the past by putting them in too much sun, or exposing them to too much, too fast. This is the article I needed 15 years ago when I bought my first seed flat!
Booker
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Posted by critterologist (from Frederick, MD) on April 10, 2008 at 10:42 AM:
It sounds like you've got the hang of it now... I think a lot of us had to learn this the hard way!
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Posted by Sharran (from Calvert City, KY) on April 10, 2008 at 11:11 AM:
Thanks, Jill..excellent article.
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Posted by critterologist (from Frederick, MD) on April 10, 2008 at 11:19 AM:
Thanks!
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Subject: Greenhouse-grown plants. . .
Posted by LariAnn (from Miami, FL) on April 10, 2008 at 9:10 AM:
. . .respond well to this procedure as well! They've been babied in the greenhouse all winter long but now you want them to get all the advantages of the summer warmth, so out they go. They need a little hardening off time as well.
Very good and detailed information that will surely save many baby plants this Spring. Thanks!
LariAnn
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Posted by critterologist (from Frederick, MD) on April 10, 2008 at 10:44 AM:
You're right! When I buy plants at the nursery, if they're on the "outside shelves" and look like they've been fending for themselves for a while there, I don't worry too much... but if I pick them up from inside the protected area, I know they need to be gradually acclimated to outside conditions rather than just thrust out into my garden.
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Subject: So Helpful
Posted by jojoringer (from Conroe, TX) on April 10, 2008 at 8:55 AM:
Your series of articles has been so helpful. This is the first year I have used heat and lights to start my seeds. LOL I will probably do better next year since I have been a week or two ahead of you season wise and find your article and have slapped my forehead a couple of times going "ohh wish I had done that!"
I have a question about fertilizing. Maybe. How do you get your seedlings so big. Mine are healthy, but just aren't getting the fullnes that yours have. My peppers are just little guys and I'm at the point where they have to go out. So I'm wondering are you using fertilizer to get them that large and full? And what are you using if so.
Oh BTW I started some of the Yellow Flying saucers, Brazilian Rainbows and the White even though it was late, I just had to try them! : )
Thanks!
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Posted by critterologist (from Frederick, MD) on April 10, 2008 at 10:49 AM:
Sorry about the timing, Jojo! But I'm glad you've found the articles useful anyway. I actually put this one on the schedule a month before I'll start hardening off here, because I realized that folks further south needed the info asap. Next week's article will have tips about planting out seedlings...
I start peppers 8 to 10 weeks ahead (10 to 12 weeks for some hot varieties because they just seem to be slower), and I germinate them with 80 degree bottom heat to speed things up. I do fertilize once in a while, with water soluble fertilizer at quarter to half strength, but I think heat and time make the difference. There's a thread on the propagation forum with more detailed information on what I do with peppers (maybe it'll be an article for next year!). See [HYPERLINK@davesgarden.com]
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Posted by jojoringer (from Conroe, TX) on April 10, 2008 at 10:52 AM:
Oh no need to apologize! I'm glad for the info LOL I started a little late this year so maybe that's my problem. It gets so hot so quick here! Next year I'll be ready though. Thanks again for all the great information. Your articles have been some of my favorites!
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Posted by critterologist (from Frederick, MD) on April 10, 2008 at 10:54 AM:
:-)
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Subject: No sunburnt seedlings ...ease them out.
Posted by chrissy100 (from Sydney
(Australia)) on April 9, 2008 at 6:07 PM:
Thankyou for that great read ...I am always surprised when folk get a shock at the poor little leaves burning off after growing in these sheltered conditions. The same applies to plants you buy in sheltered situations in the Nursery. Home they go and are planted into full sun and the poor gardener wonders why they sometimes even drop dead from shock.Any protected and sheltered growth growth that is suddenly shoved into full sun will suffer. your article is well timed ...and very important.
chrissy
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Posted by critterologist (from Frederick, MD) on April 9, 2008 at 10:14 PM:
Chrissy, you're absolutely right!
I guess the thing to remember is that plants can be remarkably adaptable, as long as change happens s-l-o-w-l-y. If you change their environment suddenly, they may not be able to cope!
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.
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.
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! 
