Giving Seedlings the Cold Treatment
 

Ever wondered how commercial seedling providers get these beautiful, stalky, plants with thick stems to market? It's called the cold treatment and has been going on at commercial greenhouses for over 30 years. Here's how they do it.

This was taken out of the book "Greenhouse Tomatoes, Lettuce & Cucumbers" by S.H. Wittwer & S. Honma where they recommend 'Cold Treatment' for hardening off tomato seedlings.
The cold treatment should be started just as the first true leaves emerge, whether the seedlings are still in seed rows or pricked-off. Air and soil temperatures should be lowered to 52 to 56 deg F for ten days to three weeks. A ten to twelve day cold treatment is adequate during periods of good sunlight. Three weeks are usually necessary in the fall and early winter when most of the days are cloudy and plant growth is slow. The amount of cold during the ten-day to three week period is more important than the time of day in which it is given. Cold exposure during either the day or night, or both, is effective. Night temperatures of 52 to 56 deg F are recommended when the days are sunny and partly cloudy.
Following the cold treatment, night temperatures should be raised to 58 to 62 deg F. Cool daytime temperatures (60 to 62 deg F) should be maintained in cloudy dull weather. On bright sunny or partly cloudy days, temperatures of 65 to 75 deg F accompanied by good ventilation are suggested.
Tomato plants properly exposed to a cold treatment develop large cotyledons and thick stems, with fewer leaves formed before the first flower cluster, up to double the number of flowers in the first, and often the second clusters, and higher early and total yields.

Basically this cold treatment is used to give healthier, more stalky seedlings that will give increased yields and earlier harvests. In regards to light intensity and duration they had this to say.
The tomato is a facultative short day plant which flowers and fruits earliest if the day is not extended beyond 12 hours by artificial light. Young tomato plants do not need the light intensities of full sunlight. Where there is no overlapping of leaves, light saturation is reached at intensities from 2000 to 3000 foot candles, or about one-fifth to one-third the intensity of direct sunlight at high noon. If artificial lights are used, an intensity of at least 500 foot candles should be provided at the leaf surface. Tests with fluorescent fixtures reveal that Wide Spectrum Gro Lux is slightly superior to cool white.
Hope this helps. Ami

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After reading [URL="http://www.kdcomm.net/%7Etomato/Tomato/start.html"]http://www.kdcomm.net/~tomato/Tomato/start.html[/URL] I also had some questions regarding cold treatment, especially since last year I planted just one tomato plant in the ground on 7 april, three weeks too early. Just to see what will happen. Last april was frost free, but temps were often in 40's. Plant survived, flowered and I've never seen so much flowers on one truss before.
Sadly it was a spitter, Moneymaker, which I got free from Thompson&Morgan and planted of curiosity, but experience was very helpful.
I have one bright SE facing unheated room where temps are just right for cold treatment and this is the way I'll go this year.
Here's a picture, there was 16-18 fruits on one truss, and the plant was pruned to single stem:

Good stuff Marco, keep us posted on how it goes. Ami

Ami,

This is awesome info - thank you so much for sharing.

I am a believer of the 'cold treatment' - as my laundry room where I start tomatoes and peppers is naturally cold (not heated), the night time temps are ideal for the cold treatment, so whether I wanted it or not, I have been always going through the process :)

For folks who tend to over water the seedlings - beware that cold and wet is not an ideal combination for the root system of the young plants. Been there in the past, and lost/stunted quite a few transplants myself...

Tania

Ami, thanks for posting...do you use the method?

Have used the cold treatment for many other seedlings, but never tomatoes, eggplant or peppers because I thought they required only warm temps. Will definitely do half that way this year as a trial and hopefully will be able to post positive results.

Ami, thank you very, very much for posting this!! I will try this method this year, can't wait for starting! clara

I always get a lot of seedlings that have been through that,
simply because I run out of room indoors under the fluorescent
lights after potting up from seed-starting cells to newspaper
pots. Most of them end up outside in these things for several
weeks before transplant:

[url]http://www.greenfingers.com/images/superstore/extraProductImages/LS6257D/20_3_Tier_Steel_Mini_Greenhouse.jpg[/url]

(With an incandescent light bulb in the bottom in a shop light
fixture like this:

[url]http://www.google.com/products/catalog?hl=en&as_epq=&as_oq=&as_eq=&num=10&lr=lang_en&as_filetype=&as_sitesearch=&as_qdr=all&as_rights=&as_occt=any&cr=&as_nlo=&as_nhi=&safe=images&q=%22trouble+light%22&um=1&ie=UTF-8&cid=8461157546131076455&ei=awRVS7egH4bosQO_q6j1Bw&sa=X&oi=product_catalog_result&ct=result&resnum=2&ved=0CBEQ8wIwAQ#ps-sellers[/url]

to keep it above freezing at night. My seedlings actually get
quite a bit more hours of cold temperatures than what Ami's
post described, with many nights in the 40s F.)

What I try to do but not always possible is to kill the heat to my upstairs room when my seedlings get their first true leaves. But do to the large glass area that I later put them in front of can cause the room to heat up when the sun prevails. So for that 2 week period it depends on the weather. Ami

Well this year, cold treatment it is! Thanks for the info!

Marko that is one loaded plant. Too bad it was a spitter.
Tomatoes are kinda like weeds in that way aren't they. If you really want a particular variety, you can't keep it alive and if you can't stand the fruit off of one, you can't slow it down short of pulling it up.

Now that we are getting more questions about light periods and hardening off seedlings I think I'll bump this thread for those that missed it. Ami

Interesting. Hmmm.

yeah I don't even start my seeds inside but under a patio cover on seed starting mats - and when I pot up they are outside on a seedling cart with a clear cover and fluoro lights go on in the day and are off for 8 hours at night (it was 40 degrees a couple weeks ago and in the low 50's the last week)- got 30% germ in 5 days and more poping up every day
many cells already have 100% germination today

Dennis

The first seeds I planted are Bhut Jolokia pepper. They went into my sunroom
after they were about 2 inches high. I have a small heater in there which keeps
it in the 50's. I don't know if it's because of the variety of pepper, or because it's
cool in there. They are nice and stocky. I haven't started my tomatoes yet, but
they will go into the sunroom also.

Giving seedlings the cold treatment.
 

Following the cold treatment advice I thought I would make a start last night, but to gradually reduce the temperature. I am using a propagator and have some additional greenhouse heating, but trying not to set the temperature for this higher than necessary. I set the propagator temperature(for soil temp) at 14C(57F). I put a max/min thermometer in as well as seedlings and the air temp got down to 8C(46F). In the other propagator containing the younger seedlings set at 17C(63F) the min air temp was 14C(57F).
It is obviously difficult to maintain a soil temp close the air temp without considerable extra heat in the surroundings. Of course if I choose not to heat the greenhouse any more the dilemma is whether I should set the higher soil temp for the 'cold' seedlings or for the younger ones! Any ideas?
Gill

Gill, keep us updated how your cold treatment works out and if you can take any pictures, please do and post them. Ami

I finished cold treatment today. Plants look healthy and have a purple tinge due to lack of phosphorus. Temperature was constantly 11C/52F. Now I have to wait till fruit set to see the benefits of cold treatment.
Here's the photo, it's the six pack in front, that was in cold room:

Marko, very nice plants with thick stems. I think you will notice a difference in the first fruit cluster.;) Ami

Interesting. I've heard hardening off produces better plants, but it is interesting that it actually increase the blooms and produces earlier harvests, since all I've ever read was about stem thickness and hardiness. I guess it makes sense!

I have been doing this for years.
My room where the plants are grown has the damper closed to the heater and the garage is not heated, any day that it is 40 or above the plants are outside.
The only time they come inside is if there is a chance of freeze or frost.

They get this till it warms up and then they get planted.

The reason I do this is out of necessity. The plants come out looking great.

Worth

My plants have gotten the cold treatment out of necessity, and they look fantastic. It's only my first year growing but they look much much better than the plants I bought from the nursery last year.

I can't comment on the fruiting, but the plants are definitely robust and sturdy.

Interesting topic...

I've always babied my plants that I have on a rack with 16 hours of artificial lighting, until about the last 2 weeks where I acclimate them.

This year I'm using heat mats to help germinate my peppers better, so if I were to try this technique I would use the heat mats until they have true leaves then stop the supplemental heat?

Basically we're throwing them outside once they get their true leaves as long as there's no threat of frost right?

interesting thread, I start my seeds in the basement on heat mats, once germinated they go under lights for 12 hours a day, I have a thermostat in the basement and keep it cool at about 50 degrees, and normally the seedlings do fine until they are potted up then a few weeks later taken outside to acclimate them to the sun and different temps. This year, I'm not too sure what is going on, the seedlings don't look all that great, the room is cool but even though they don't have true leaves yet, I just might pot them up into different containers, 16oz. cups. Am I rambling? LOL, I think I am, so let me end now, :)
Alberta

My seedlings did not do as well this year. Everything was done the same way with the same products. Oh well, we can't control Mother Nature.

Neva

This cold treatment sounds interresting. This year I started 400 plants from seed. I started them all indoors under lights and when I transplanted them to larger pots all the plants went on the back porch because there was no more room inside. I had a space heater on the back porch just to keep the plants from freezing. I was not aware that there was a cold treatment method but my plants grew great this year. Best year I ever had. I was telling my babies sorry that it is so cold because I thought tomatoes love it hot. I am building a greenhouse to start more plants but I cant afford much heat. I am beginning to like this cold treatment method, I will study this more and I am going to ask around, how do the big boys do it.
Joe

Study carefully before you do this. It is dangerous if you don't watch carefully and know what to do.

The "big boys" do tomato plants by growing them in huge greenhouses that never get below about 70 degrees. A tomato plant can be forced to salable size in 5 weeks at this temp and that is exactly what the "big boys" want. Unfortunately, these plants have never been stressed and when you set them out in the garden, they sulk for 2 or 3 weeks before deciding to grow.

Folks like me who grow up to 50,000 plants a year usually are a bit more in touch with their plants. I grow in a greenhouse and I definitely use the cold treatment methods when feasible. There are a few gotchas that you must know how to avoid. The first is that low temps significantly slow down plant growth. Allow an extra week or two for cold treated seedlings to grow. The second is that tomato seedlings are VERY sensitive to excess water. What's excess? You must let tomato plants dry out until they just start to wilt if you want them to be healthy. Give them plenty of water when they are dry, just be sure they are thoroughly dry before you water. The last caution is that you need air circulation even when it is cold. Be sure you have a fan blowing across the seedlings at least part of the day. It encourages the plant to grow a sturdy stem.

Does it work?

I can assure you that cold treatment of tomatoes is VERY effective. It shortens time to first flower truss and boosts overall production.

DarJones

I came across this when looking for information on when a seedling is considered leggy and how to prevent it.

The information on internodal spacing talks about cool temp treatment of seedlings

[URL]http://www.enjoygardening.com/?s=internode+&x=6&y=8[/URL]

Hill60

@ Fusion power:

I'm glad that you stressed holding back watering. Water-stressing has been a part of my hardening off process for years. If you allow the foliage to begin wilting at this early stage, the plant's self defense system begins growing more roots to seek out the needed water. (Bottom watering @ this stage forces a deeper root structure which increases the plant's drought tolerance.)

Last season I had somebody thinking my seedlings needed watering, so they watered the ones they saw. Throughout the season, those plants seemed to need frequent watering, while the ones that they missed seldom needed watering. At seasons end, those that had been "voluntarily" watered early, had root structures less than half the size of those that had been water stressed during hardening off.

DarJones, Very interresting reply. This is good information on "There are a few gotchas that you must know how to avoid". Perfect description. I have printed your post and saved a hard copy when I start my seeds for next year. Thank-You
Joe