Hardening off of plants

weebus(Z8 Sunset 5 WA)November 14, 2005

JB (Junior Balloon) started a the great subject of hardening off of plants. It is so big and intricate, yet so simple in many ways, that I thought it deserved its own thread.

Let's try and keep this to just hardening off of plants in the spring.

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weebus, you are just wanting to discuss hardening off INSIDE some kind of greenhouse/coldframe structure, correct?

Yes, I think that there are LOTS of folks who grow their own plants indoors with lights who are interested in learning how it is done in a greenhouse. The whole issue is confusing to me, when, I spend about three weeks acclimating my plants to the outdoor elements (after having grown them inside my home), yet, if I purchase a plant from the nursery, it has never left the greenhouse until I bring it home. How is it that greenhouse operaters effecively harden off?

    Bookmark   November 14, 2005 at 1:14PM
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My sister has a 35x65 greenhouse in Iowa, Z4. She grows and sells mostly bedding plants. She told me she does not harden off anything. She feels that because she has fans running all the time and opens her double doors on many sunny days that the plants are sufficiently hardened. I can attest to that from the many plants she has given me that I put directly into the ground with absolutely no sun scald or drooping at all.

I read somewhere that the wind is as important to hardening as is full sun. They need to be exposed to some breezes.


    Bookmark   November 16, 2005 at 7:05AM
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The breeze causes the plants to move and stretch. The result is it helps the cell structure on the outside of the stocks to hard or more stiff. Some folks tell of their grandparents would take their hand and gently brush their seedlings everyday. Same process.

HAF fans in your greenhouse will serve multiple purposes.
1: Constantly circulate the air to eliminate hot/cold spots.
2: Create a fresh supply of CO2 around the leaves so they don't starve from still air.
3: Move and stretch the plant to make it tougher.

After all Mother Nature isn't exactly nice to a seedling.

As for hardening off in a large g/h can be done by turning down the thermostats to help acclimate them to the real world. That's what we do and it seems to work fine.

    Bookmark   November 16, 2005 at 9:59AM
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I would like to hear a little bit about the sunlight part of hardening off. As a grow light grower inside of my home, my hardening off process is acclimating:

1st to outdoor air and temps in the shade in daytime. Then nighttime temps.

2nd gradually exposing to sunlight. This is done in conjunction with the temperature changes as well.

3rd gradually exposing to total full sunlight.

It takes me about three weeks before I feel safe enough to plant out. All the while, my plants are also acclimating to wind (though, I do use a lot of fans in my grow rooms). For years, I have had to do this process without any kind of structure for protection. The wind is my worst enemy, next to rainy stormy weather, which requires me to move my plants very hastily. There were many times that I had to strap the twins into their car seats and leave the van running in order to get all the flats moved quickly, so that I didnt leave my babies unattended. There were many days that I had to do this twice or more per day, depending on the nighttime temps. I had to do this with over 200 flats, sometimes every single day! Yes, I know some of you will find me insane, but those of us who are obsessed with growing will go to these extremes.

Now that I am planning for a cold frame, this will save me so much time and wear and tear on my back!

Now, since I am still going to be growing indoors for 2006, and then be hardening off in a coldframe structure, my HO process will still be different than for those who have grown the entire crop in a greenhouse. I still have to acclimate to full sun. From what I have learned, using grow lights is the equivalent to growing in "really bright shade", which is why I have to keep my plants almost touching the lights at all times, in order to ensure stocky, bushy plants that receive light from the top all the way to the base of the plant. I even space my plants in cell packs in order to ensure this for certain plants.

However, "full sun" inside of my cold frame, will that still mean a shade covered exposure? Do you ever have your plants in a non-shade covered exposure?

I would like to learn more about how you all do this. I throughly understand the hardening off process, I have been doing it for many, many years. But it is a bit different than growing from day one in a greenhouse.

    Bookmark   November 16, 2005 at 12:22PM
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Cold Frames have white plastic not clear. If you put clear on it your going to heat it up way too much and stress your plants even more.

You wil not have full sun if you use white plastic.
"non-shade covered exposure" If it's covered then there is shade. At least to some degree.

    Bookmark   November 16, 2005 at 1:21PM
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weebus(Z8 Sunset 5 WA)

There are cold frames that have clear plastic on them. Just throw shade cloth over it and expose the plants to sunlight in increasing increments of time. Mylu's right, you will definitely need a fan, and I would move them out of the house into the cold frame during a spell of overcast days. That way they can acclimate at a slower, less stressful pace. Also, with cloud coverage, the nights won't get quite so cold.

Should only take about a week to a week and a half to harden off.


3 things to consider when hardening.

You can harden things off in several ways. Fans will help with wind.
Shade cloth will help with the sun and temps getting too high. some type of heat will take care of the lower temps. A 12 X 20 GH should get by with just an electric heater for this year.

One question. What is your last frost day for your area? Why do you want to move them out in March to harden them off? OK two questions...

Here is a link that might be useful: cold frames primer

    Bookmark   November 16, 2005 at 1:49PM
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Damn I knew I should have said "generally speaking" on the white cover.

Weebus I'm refereing to a cold frame that is used to overwinter hardier perennials not a coldframe to heated house evelution thing.... (like the big boys when we get bigger :) Yes i saw that post!

Her last frost date should be either April 15 or May 30 depending on if she is 6a Like me or 5b like most of the state.

    Bookmark   November 16, 2005 at 3:06PM
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I start hardening off in March,usually, because I am OUT OF ROOM in my house by mid April, you would not believe it! I have to make room for more plants under the grow lights, sometimes I have a rotation going on for the more established plants, they will only get about 12 hours of light, as compared to when they are seedlings and just beyond a seedling, where they get 16 hours of light. Last year, I started hardening off begonias, impatiens and petunias, and other plants by late mid March, and the weather was great. Then, come mid April, it all went to hell and we ended up getting frost until exactly May 1st. In 20004, me and everyone else around here had plants in the ground by Mid April.

I know it was too early to start hardening, but I had to make room for everything else. And it worked just fine, except for having to move the plants around a jillion times. Once I get them acclimated outside, all I have to do is keep them from getting frost at night. There were many days that the highs did not get above the 50's, and I am sure the growth rate was incredibly slow, if at all, but I did not lose a single one of them, and they all were great, healthy plants. I had hundreds of wave petunias, about 3000 wax begonias and about 1500 impatiens, all hardened off before the second week in April.

I know it shouldnt take so long to harden, and my plants are probably ready well before I set them out. However, I know that other grow light users do struggle with acclimation to full sun. Every year a lot of my plants will lose some of their older leaves, that never seen true sunlight until I began exposing to them. However, the new basal growth does not get sunburned one single bit. Almost every year I will lose practically every single leaf on Salvias when I harden, but they regrow very quickly and I have great Salvias (all species).

I guess maybe it takes longer for grow light users to harden? It does for me anyway. Or maybe I am being too careful. I have never lost a single plant that I grew due to sun exposure problems.

I can choose what kind of plastic I use, right, white versus clear?? I need to learn this, research it more. If white is used, are you still acclimating to full sun. So much to learn, so quickly.

My zone is 6a, I am in that small area that is not 5b, but close enough. Normally, I do not put out any plants until after May 1st. However, in 2004, we had an early spring and everything worked out for everybody to begin early.

    Bookmark   November 16, 2005 at 3:34PM
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Also, I meant to add, that many plants need to get acclimated so that I can finish growing them on in the full sunlight. Because I seriously run out of room in my home. I will harden them off a bit younger than I would have if I hadnt run out of room and then let them continue growing on outside, with full sun as my light. There are many many times that I will also transplant to larger cells or pots and do this at the exact same time. Again, with no loss at all.

I havent gotten too many scientific reasonings for my success with this, I am sure people will argue and say "Oh you should not do this or that" for this and that reason, I gladly welcome these arguments, becuase I always learn from them. I have posted tons of questions that go unanswered.........presumably because others just dont know the answers, or, because they just dont like me! :)

There are a lot of techniques that I do that I dont fully know why it works, I just know that it does. However, I dont get intimidated by "not knowing everything" because oftentimes I will go to nurseries and strike conversation with the owner/grower and they dont seem to know everything either.

    Bookmark   November 16, 2005 at 3:41PM
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We do the same as you,Ellen, except with vegetable transplants. Every year in mid March our house is crammed with flats of plants. The outside weather is the critical factor since I won't subject plants to the harsh weather in moving to the GH that often occurs in early March. Plus, I'm never sure I can maintain a suitable GH night temperature when outside temps are in the teens. And besides the plants usually appear so content to enjoy the house temps that we enjoy(We don't pay extra to heat the rooms where the plants are because we are there too).

I must give credit to my wife once the plants are in the GH because we have never used shade cloth. What she uses is the shade of the benches(placing flats on ground) and misting with water to gently harden off the delicate house grown plants. Even a half hour of direct sun on some plants can be fatal so the flats are moved to the floor for shade and the back to the bench tops when the sun isn't so strong. The process requires constant attention to the weather and an hourly check of the plants for signs of over-exposure. I only take credit of keeping the wood stove going through the nights. We usually move tomato plants into the GH first since soon after they are hardened off they are immediately planted into the ground of one of our high tunnels. The process of shuffleing plants into and out of the GH seems ongoing until all plants are transplanted to the tunnels or the fields. But even then the small (15x40') GH is still loaded with ornamental peppers, potted herbs and a few flats of secondary planting plants, all of which had to go through the same hardening off routine.

I won't say we never lost a plant because there have been occasions where I had to transplant peppers or eggplants and immediately move them to the GH because there was no more room in the house( When I transplant one flat of seedlings to cell packs I usually end up with 6-8 flats). I don't recommend this practice of submitting plants to double shock . For melons and cucumbers (started in peat pots) I like to move the to the GH as soon as they are emerging so they never have to adapt but I start them only 4 weeks before transplant time of mid-May.

To sum up: House grown plants need graduadual adjustment to direct sunlight of a greenhouse but it can be accomplished in many ways. Wind and temperature are controlled in the GH with much less effort (but more cost). And you never want to move plants from a mild environment to harsh one and forget about them.

    Bookmark   November 17, 2005 at 4:52PM
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