Hardening Off Seedlings

MOST GARDENERS WHO’VE ENTERTAINED THE IDEA OF STARTING SEEDS INDOORS have at least heard that they should harden off seedlings before transplanting them outside. Some also know that plants that were started in a nursery’s greenhouse also should be hardened off too.

So what exactly is hardening off and why is it more critical than most people know? Hardening off is the process of gradually getting your plants used to the much more difficult outdoor environment. Wind, colder / hotter temperatures and sun scalding are all potential threats that can shock or even kill the baby seedlings you’ve worked so hard to nurture from seeds. To this point, all they’ve known is the cushy life under the indoor grow lights or the hoop house and hardening off is a kind of boot camp to prepare them for the war of natural selection that is waiting for them outside of your home.

In his excellent article on Hardening Off Seedlings, Kenny at Veggie Gardening Tips suggests that you gradually build up the amount of time seedlings are outdoors over the course of a full week and that you start with the early morning and late day hours and gradually increase to the middle of the day. This will be less “tiring” on your baby plants.

In Southern New Hampshire, the two biggest dangers when hardening off seedlings are cold temperatures and wind. You can mitigate both risks by bringing plants inside when the weather changes, but when you are succession planting seedlings, moving plants in and out can be a full time job. It’s good to know when your plants are actually in danger so that you only move them when needed. Here’s a handy temperature chart from Johnny’s Seeds that lets you know when it’s too cold to leave them out:

Recommended Minimum Temperatures
All Temperatures Are In Fahrenheit

40 degrees –  broccoli, Brussels sprouts, kohlrabi, cabbage, onions, leeks, parsley
45 degrees –  celery, Chinese cabbage, lettuce, endive
50 degrees –  squash, pumpkin, sweet corn
55 degrees –  cucumber, muskmelon
60 degrees – basil, tomatoes, peppers

Can plants get sunburned?
I originally thought that wind and cold temperatures were the only reasons for hardening off, but it turns out that the sun itself is another critical threat. Amy at My Suburban Homestead wrote a great article on how too much direct sun too quickly can dehydrate plants in a way that is similar to sunburn in humans. It turns out that cutin, which is a waxy substance that protects the leaf from dehydration, is built up gradually with exposure to sun. Too much sun all at once and your seedlings could go into shock from dehydration … or worse.

I’ve even noticed that plants that are moved from inside to a cold frame or hoop house need to adapt to the climate slowly. Although the glass or plastic covering seems to reduce the impact of sunlight, especially if your plants received some sun through a window as they were growing, cold frames still present challenges for your seedlings. For example, temperatures in a cold frame fluctuate much more wildly than indoors. Again, seedlings can have real trouble with this when all they’ve ever known is a constant 70 degrees Fahrenheit.

You also need to get plants ready to exit the cold frame well before it’s time to transplant. Depending on the outdoor temperatures, you should try to gradually get seedlings used to the uncovered world by opening the cold frame more and more each day.

The bottom line is when you are putting plants outside, even in a cold frame or hoop house, it should be gradual. The more time you take to gradually get them used to the great outdoors, the better your chances of success.

What’s your process for hardening off seedlings? Let us know by commenting below.

Related posts:

1. Transplanting Seedlings Outdoors
2. Transplanting Strawberries
3. Transplant at Proper Soil Temperature



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10 Responses to “Hardening Off Seedlings”

  1. May 11, 2011 at 8:16 am #

    Bill – this is some very solid information on hardening off seedlings. Hardening off seedlings is something many gardeners take for granted.

    I was guilty of this until I began losing plants and finding out it was because of not hardening them off properly.

    Now, I harden my seedlings off very slowly and it has increased my success rate quite a bit.

    Great points about setting the seedlings outdoors in the early morning and early evening hours when sunlight and wind is less intense.

    I usually start out by setting my seedlings out for about two hours per day once the temperatures are at least 65 degrees (depending on the vegetable types).

    I then increase the time by two hours every three or four days. I continue this over a two week period until they are outside for most of the day.

    It can be a tedious task moving plants in and out, but definitely worth the work.

    • May 11, 2011 at 9:03 am #

      Take note that Tee takes a full two weeks to gradually harden off his seedlings. It’s tedious, but he has found that it is worth the time and effort to do it gradually.

  2. May 12, 2011 at 6:44 pm #

    with the terrible weather I have had to move all gardening chores to one month including hardening off…I moved the seedlings to the garage and outside during the day in a part sun porch..in when the late PM sun was upon them and back in the garage… weather has been in the 70s so we kept them in the garage…weather changing to 60s so I will plant them under a row cover to give them protection from sun, cold and wind…they will hopefully be fine…we shall see…

    • May 12, 2011 at 8:33 pm #

      Good luck Donna. Hopefully the weather will cooperate so you won’t have to battle against the cold and the wind.

  3. May 13, 2011 at 12:41 am #

    Oh I almost forgot to bring my eggplants back in! Oops!

    • May 13, 2011 at 6:28 am #

      I almost forgot to open the cold frame yesterday. There sure is a lot to remember this time of year.

  4. May 19, 2011 at 3:08 pm #

    I have definitely made some mistakes with hardening off. 4 of my tomato seedlings got sun scald issues this season, however, I only lost 2 and the other 2 bounced back.

    BTW, here are the ways I like to use horseradish! Thanks for leaving a comment on my blog.

    • May 19, 2011 at 3:47 pm #

      Meemsync — You definitely have to gradually expose plants to the outside. Otherwise, it’s really hard on your seedlings. Thanks for the info on horseradish.


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