Hardening Off seed starting

Hardening Off Your Seedlings

The day after I posted Tomatoes – Starting Your Own is Easy, I received several emails from gardeners fairly new to seed starting asking me how to harden off seedlings.

When plants are sown into the garden under weather conditions in which they thrive, they start acclimating themselves to the outside world the minute they poke out of the soil.

In order to get the longest production possible from heat lovers like tomatoes and peppers, we have to give them a head start inside under artificial conditions. We don’t even think of putting them into the “real world” until temperatures are much warmer.

These pampered seedlings must have a transition period to become accustomed to the world of wind, sun and rain. A chance if you will — to become more “thick skinned”.  And that is exactly what happens in the hardening off process.  The plant’s cuticle (skin)  becomes thicker.

The time it takes to harden off will depend on the plant, the temperature and the fluctuations in temperature. Generally, the transition can be made in about 7 days.

The key word to remember is “gradual exposure to the elements“.

If you don’t harden off your seedlings you stand a good chance of loosing all your invested time and effort.

You can do a couple of things from day one of their emergence to help the hardening off process.

  • Air movement (a fan) where your seedlings are located inside really makes a difference.
  • If you have been brushing your seedlings with your hand once or twice a day, this too will have helped make them thicker.

If you haven’t done these two things, give your seedlings a day or two of this before proceeding to the great outdoors.

Two important things to keep in mind before you begin hardening off outside.

  • Your plants will still need protection from cold, strong wind, and driving rain.
  • When you water your seedlings, water them well and then let them almost dry out before you water again.

Guidelines for Hardening Off

The only way to learn the hardening off process is to jump in and do it. It’s not an exact science. But there are some simple guide lines you can use.

  • Place your plants in a sheltered, shady spot outdoors out of direct sun light.  (Maybe under a tree or on your porch.)   Keep an eye on them for any signs of stress like wilting or drooping.  If things appear to be ok, leave them out for about 3 or 4 hours. Day 2 through 5,  add 1 or 2 hours to the time you leave them outside.  Still keeping an eye on them just in case.
  • About the 4th or 5th day, place them where they will get direct sun for a few hours.  You might want to expose them to morning sunlight first, rather than hot afternoon sun.  Return them to the shade in the afternoon. Each day increase the exposure to sun.
  • By the 7th day they should be able to handle sun all day.

They should be able to handle nighttime temperatures without problem after 10 days. Of course, you’ll have protect against any freezing temperatures. And they’ll still need your protection again high winds and sudden downpours until they are large enough to fend for themselves.

(My tomatoes seedlings – 5 weeks old and 3 inches tall – went through their first hard downpour today.  They passed the test with flying colors!)

Encountering Mishaps

If during this process you encounter some mishaps – like some of the leaves of your plants turning white and drying up (sunscald) — there is still hope.  Just take them in and give them some time to recover and start over.  It won’t hurt to loose a few leaves.  Look for new growth.  That’s what’s important.

I’ve seen lettuce in a flat totally wilt from the stress of too much rain or sun and then have some of it make a comeback when moved to cooler more desirable conditions.  So don’t throw in the towel until you know for sure that all is lost.

There are too many variables for this to be an exact science. Just use your best judgment.  You’ll have to experience it to learn.

A Cold Frame

Using a cold frame or some type of covering (it doesn’t have to be fancy) can make the process a bit easier, quicker and you won’t have to “hold their hand” so much.

I put a rock or brick under the cold frame to make sure my plants get the outside air.

Follow the guidelines above.  But on day 3, after the seedlings have been out in the air for the allotted time, place them in a cold frame. For the remainder of the day, make sure the cold frame is open enough that air can get in and circulate. If temperatures are not forecast to dip low leave them in the cold frame all night.  Make sure it’s closed.

(Here you have to make a judgment call.  My tomato seedlings were out of the house and into the cold frame after they reached the ripe old age of 2 days old at the end of February.  Temperatures at night ranged from about 39 to 50.  If you want to be more cautious, go with 50 to 60 degrees.)

Two day old tomato seedlings.

Another Judgment Call

Some seemingly pretty knowledgeable gardeners say tomato seedlings subjected to low temperature (like mine in the example above) will never grow and produce to their potential in the garden. The only thing I can tell you is that over the years I have subjected tomato seedlings to some pretty horrible conditions.  (I’m not suggesting that you do that.) In spite of it, I have always had bumper crops of tomatoes for the past 33 years. So – it’s your call.

Transplanting to the Garden

Once your plants are hardened off, you can transplant to the garden.

Helps for Young Seedlings in the Garden

  • You might still watch for low temperatures and severe weather and be prepared to give them some protection with a cold frame.  (Even a gallon jug with the bottom cut out and the top off makes a great mini-coldframe.) Once they’ve grown some this won’t be necessary.
  • If that doesn’t work for you, pull your straw mulch up very close to the plants.  Maybe even lightly sprinkle some over the plants.  Works for me all the time when I don’t want to take a cold frame back to the garden.

I removed the top and cut the bottom out of this plastic gallon jug and fit it snugly over the seedling until the weather warms.

Final Words

Now go out there and harden off those plants!  It’s time!  8)


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  • Great article, thanks! I have a bunch of plants that are growing nicely especially my tomato plants, they are so tall it’s very exciting. Can’t wait to harden them off & get them in the ground!!

  • Sounds like you’re off to a great start.
    Yes, Alicia — it is exciting! Best of luck with all your plants!

  • How do you harden off plants if rain is in the forecast for the next 10 days and has been for the past three weeks?

  • That’s an excellent question, Mail. It’s not a lot of fun when the weather totally refuses to cooperate. But it can happen so we need to be as creative as possible.

    If I were in your position I would put them on porch that at least had an overhang and place them so they would not be pelted with wind or rain. A screened porch would be excellent.

    Or -if you have some kind of make-shift cold frame that you can cover them with but have it open to the air —- that would certainly be a step in the right direction.

    Another way is to put them in a garage that is open but protected.

    I don’t know what you have available to you, but hopefully this will give you some ideas. The main thing you’ll need to do is keep an eye on them to make sure that what you are doing is accomplishing the goal rather than setting them back. Once you get them hardened off even a little, it won’t be as bad.

    Keep working at it. I feel sure you will be successful. Let me know if you have more questions.


  • Theresa,

    Thank you for responding. I have been able to sneak them outside for an hour or two here and there between rain. Unfortunately, I do not have any garage or screened porch I can put them in. What about a window seat with the window open? Would that work? I figure it would work the same as a garage, wouldn’t it?

    Thanks again.

  • Window seat with the window open is definitely a step in right direction, Mail. Keep getting them outside for an hour or two here and there between rains too. You’ll have them hardened off before you know it. 8)

  • Great! I will do that. Now I have another question.

    I was telling a friend of mine who is a garden enthusiast that my cucumber, zucchini, squash and watermelon seedlings have very long stalks (about four to five inches) with leaves at the top. She said that that is indication that they are not getting enough sun.

    I can move them to a sunnier spot but I will not be able to get them into the ground for the next two weeks.

    So I was planning to put them into larger pots for the time being and was wondering if I can bury some of the stalk in the pot to keep them from snapping in two from the weight. I know you can do that with tomatoes but am not sure about these. Please advise. Thank you.

  • Putting you seedlings into larger pots is an excellent idea, Mail. Try not to disturb their roots when you do it, especially since they will be transplanted to the garden in two weeks.

    If they were my plants I would be careful about burying a lot of the stem on seedlings. I do put them a little deeper than how they are set in the pot when I transplant to the garden —-but not much. You’ll probably will be ok. If you have 4 of each — why not try putting one down deeper and see what happens. (And do let us know.)

    As squash, cukes and melons grow in the garden they appreciate the opportunity to grow new roots from some of the stem. As they age at bit I always like to peg down or cover stems at various places like a joint and have them grow more roots. If really helps them.

    Again, let us know how you do. Sounds like you’re going to have a nice garden this season!


  • I’m gardening for the first time…in Alaska no less. The
    Rule of thumb here is to plant no earlier than June 1st.
    I’ve started “hardening off” my plants a few days ago. Last night when I brought them in they were white tipped and wilting. Should I start the hardening off phase again or bring them in for a few days.

  • You didn’t say what the temperatures were but the white tipped and wilting (to a degree) is probably from the cold.
    When you say “start the hardening off phase again”, I’m not just sure what you plan.
    If temperatures are going to be below 50, bring them in. Take them back out when temps rise.
    If they’ve been hardened off enough to take bright sun, then give them as much sunlight as possible when they’re outside.
    You didn’t say how big your plants are, but if they’ve been healthy and robust till now, they should recover from the tip burn.
    Also, don’t over water. Make sure the grow mix is dry before you water again. Soggy soil will prevent air from reaching the roots and cause wilting.
    Good luck! And congratulations on growing your own seedlings the first year! Puts you way ahead of most beginners.

  • Hi Theresa, I wish I had found your article sooner! I am brand new to vegetable gardening, and basically have a black thumb with past houseplants. I recently decided to try my hand at growing chili peppers and got a plant from a nursery. The plant was already 8″ tall, but skinny, with only a couple branches. I transplanted it to 14″ self-watering pot with a good potting mix but I did not harden it. After being outside for 5 days now, its leaves are all drooping and the bottom leaves have been falling off. Should I bring it back inside for a couple weeks and then try to harden it? Is it too late?

    At the same time, I also transplanted some herbs and baby lettuce and they seem to be doing fine, but not this pepper plant! 🙁 Any advice or insight would be greatly appreciated. Thanks!

  • Vuthy, with plants, you never know if it’s too late until you try.
    Peppers (all veggies) have to have adequate light. Unless you have lights set up for growing plants, you probably can’t give it the light it needs.
    You didn’t say where you’re located, so I have no idea what your temperatures are.
    You might find a more protected spot for it outside.
    You also might think towards starting your own seed next year. That way you won’t be limited to just one plant from a nursery. (Search TMG for all kinds of information on how to start seed.)
    Even if you loose this pepper, that doesn’t mean you’re bad a gardening. It just means you’re learning.
    Keep trying. You’ll do fine. Ask if you have questions on various things. I’ll try to help you.

  • Thank you for the response! I’m in Los Angeles, zone 10b according to a map I looked up, and the more I think about it and read about hardening, the more I realize it’s probably what I should have done. I was a bit too ambitious with my first ever attempt at growing veggies! The plant was very green and healthy looking from the nursery, but after being outside in 80+ degree weather (it’s supposed to hit 92 today), I think the shock is getting to it. I brought it inside last night and it’s now near a sunny window. Maybe I could put it back outside in the shade? I feel like I’m just winging it! And yes, I think seeds are a much smarter and more cost effective idea. In my location, do you think I still need to pay attention to things about starting early (for seeds/transplant)? It seems articles I’ve read are concerned about avoiding frost, but we don’t get frost here. The lowest it gets is maybe upper 50s in late December/January and it never last long. Basically, I’m wondering, given my location, is it ever too late “in the season” to start seedlings?


  • Vuthy, a sunny window (unless it’s a very bright southern exposure) will not be enough for vegetables.
    After your plant recovers a bit in shady spot (if it does) then gradually move it to the sun again.

    Search my site for seed starting information.
    Try starting seed about 8 weeks before the temperatures will be in the 60s and 70s. Whatever that time frame is, should give you an excellent start.

    Cool weather crops can be started even earlier.

    Do your homework and search TMG for what and how to do. Then if you have questions, let me know.
    Good luck and have fun with this!

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