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amideutch January 18, 2010 12:11 PM

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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Marko January 18, 2010 01:41 PM

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After reading [URL=""][/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:

amideutch January 18, 2010 02:52 PM

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

Tania January 18, 2010 03:29 PM


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...


chalstonsc January 18, 2010 03:32 PM

Ami, thanks for you use the method?

salix January 18, 2010 04:39 PM

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.

clara January 18, 2010 05:22 PM

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

dice January 18, 2010 08:04 PM

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:


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


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.)

amideutch January 19, 2010 12:46 AM

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

Granite26 January 19, 2010 11:53 AM

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

b54red January 20, 2010 12:35 AM

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.

amideutch February 22, 2010 09:09 AM

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

yotetrapper February 22, 2010 11:47 AM

Interesting. Hmmm.

mtbigfish February 22, 2010 01:43 PM

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


roper2008 February 22, 2010 09:52 PM

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.

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