best temperature for seedlings to grow in

engineeredgarden(7, nw Alabama)February 4, 2010

I've read in the FAQ's that once germination has occured, 65 degrees is the optimal temperature for the seedlings to grow in. However, other information on the internet even talks about anything from 55 to 75 is ok. What would be your preffered temperature?

The reason I ask, is because my programmable propagation system lets me enter any range I want, and I would like the absolute best.


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Al recently posted that 65 day and 55 night is ideal.

    Bookmark   February 4, 2010 at 2:34PM
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digdirt2(6b-7a No.Cent. AR HZ8 Sun-35)

Once germinated and transplanted the first time to cell packs I shoot for a max low of 45 and a max high of 65 in the greenhouse. Warmer then 65 and you get leggy growth and increase the odds of damp-off IME.


    Bookmark   February 4, 2010 at 5:10PM
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Tomato seeds should germinate and sprouts emerge at 75 - 80*F where possible.

Emerged seedlings benefit from temperatures holding at 65 - 75*F until the first set of true leaves are nearly fully emerged or fully emerged with just the wee bit of the growing tip emerging from between the first set of true leaves.

You should then expose the tomato seedlings to a "cold treatment" when the first true leaves have emerged fully, say 55 - 60*F if possible but I wouldn't go below 50*F during the cold treatment. The cold treatment will stimulate stockier plants with better blossoming potential.

After the the cold treatment for say 10 days - 2 weeks, I like to keep the little plants at temps below 70* just to retard leggy growth. For me, room temps of 75 - 80*F at the pre-plant out holding/growing stage always has resulted in plants way too tall by the 6 - 8 weeks from germination to plant out. And when weather dictates holding the plants even longer, whew ... too much heat results in woody lower stems and 12 - 18" transplants.

But after saying all that, I think seedlings should be potted up individually to 3-inch or 4-inch transplant pots when the first true leaves are fully emerged. Like when you can easily prick them out of the starter cells (where there may be several little sprouts per cell) and bury them up to the cotyledons. Then I keep the temps up to 70 - 75*F until the little sprouts settle in and the second true leaves can be seen as a little peak of growing tip between the first true leaves. Then the cold treatment begins as outlined above.

I've found the processes as described above to result in nice, stout, stocky transplants with shorter internodes than if left to grow under higher temperatures such as "normal room temps" for living quarters or even higher daytime temps common in covered seed beds or hoophouses.

    Bookmark   February 4, 2010 at 5:19PM
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anney(Georgia 8)


Thank you. That's great detail on temps that tomatoes need to thrive before being set out. I shall keep it in mind.

    Bookmark   February 4, 2010 at 5:29PM
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Cool temps (20,000 lux) will give you slow growing but very bushy plants with a great root ball, especially if you eschew nuits high in nitrogen.



    Bookmark   February 4, 2010 at 9:28PM
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engineeredgarden(7, nw Alabama)

Great info! I'll have to adjust my program for a lower temperature range now, because I currently have it keeping the temp between 70-75. Thankfully, the seeds haven't been sowed yet. Man, i'm glad I asked this question! Thanks everybody!


    Bookmark   February 4, 2010 at 10:56PM
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How do you guys keep the temp below room temperature? Would turning off the air in a basement room be enough drop the temperature by 20 degrees or is there an easier way to do it?

    Bookmark   February 5, 2010 at 6:38PM
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digdirt2(6b-7a No.Cent. AR HZ8 Sun-35)

How do you guys keep the temp below room temperature?

In the greenhouse it's easy. You just set the thermostat to turn on the exhaust fans and pull in outside air.

When growing in the house I don't know how you'd do it unless you have a room that is shut off from the rest of the house or, as you say, use the basement, open a window, etc. Using a fan, as is usually recommended for seedlings, would also help some with the air temp but only a couple of degrees.


    Bookmark   February 5, 2010 at 8:21PM
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sunsi(z5 NY)

"Warmer then 65 and you get leggy growth and increase the odds of damp-off"


Never knew that might explain problems I've encountered in the past, thanks.

    Bookmark   February 5, 2010 at 11:06PM
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engineeredgarden(7, nw Alabama)

Tworivers1 - it's easy for me, because I have all of my equipment in an unheated shed out back. Of course all 3 propagation systems are enclosed too, and I just use the lights for a heat source - then turn on ventilation fans based on temperature for cooling.

Dave - I have mine set to maintain 60-65 as of today. This temperature range will of course be active only after the seeds have germinated, and the seedlings have their first set of true leaves.


    Bookmark   February 6, 2010 at 12:27AM
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mrs.b_in_wy(5a WY)

How do you guys keep the temp below room temperature?

This isn't an option for everyone, of course, but, since we turn our thermostat down at night, "room temperature" by morning is normally in the mid-50s. It's 54 right now. Myself, I won't bother with turning it up when I get up since I've normally left within an hour. My husband will move it up to 65-68 or so when he gets up.

I have half a dozen tomatoes going for my cousin's window garden. They seem quite happy with the arrangement. Peppers, on the other hand, I might have to take them to work, where my office runs 77-82 most days :)

    Bookmark   February 6, 2010 at 9:35AM
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sprouts_honor(5, southern shore of Erie)

I too had a hard time keeping tomato seedlings cold enough. So I borrowed an idea from the winter sowing forum. I wait until night time temperatures are in the mid 40s before I sprout the seeds indoors on a heating pad. After the tomato seedlings have a true set of leaves, I transplant them into a two liter pop bottle or a milk jug that has been cut almost completely in half with holes punched in the bottom for drainage. One to two inches is left uncut and functions as a hinge. The cap is removed to provide ventilation and they're placed outside. If the weather is fair during the day, I tip the top back. I close it back up at night (or late afternoon so it can gain some heat before nightfall). If it gets really cold at night, I pull them inside. But that usually isnÂt necessary especially if theyÂre next to the brick foundation of the house.

A small piece of duct tape helps keep the top down during windy inclement weather. They stay short, with a thick stem and develop a good root system. The plus side is that I donÂt have to harden them off because theyÂre grown outside and there is more space in the sunroom for peppers and other tropical plants.


    Bookmark   February 6, 2010 at 10:49AM
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I just bought a thermometer and shut the vents on the two unused bedrooms in the basement and I'll see if I can keep the temp down to where it needs to be. I'll also get a couple of small fans for the air circulation. I've got a while to practice before I start the seeds.

Sprouts - I think I will try your method, with just a couple of plants though.

Mrs. B - your approach won't work with my wife. Despite from originally being from Iowa, or maybe because of it, it's a constant fight about the thermostat with her.

    Bookmark   February 6, 2010 at 4:35PM
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never more than 65 for heirloom tomato transplants

Here is a link that might be useful:

    Bookmark   February 8, 2010 at 10:50AM
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heirloomtomatolife, you seem to be spamming. I see you posted that link on at least three threads so far, including this one whose topic has nothing to do with buying tomato plants online.

    Bookmark   February 8, 2010 at 11:22AM
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anney(Georgia 8)


If you want to advertise on GardenWeb, which is what you are doing, you must contact the GW folks and pay for the ad, which won't appear in the discussion threads. If you continue spamming, you are likely to be banned from everything on GardenWeb. I suggest you contact the site administrators, apologize, and ask that your spam be removed.

    Bookmark   February 8, 2010 at 12:06PM
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"How do you guys keep the temp below room temperature?"

My garage is attached but I don't heat it unless the outdoors temps drop way down below 25*F, and by March, the temps in the garage stay above 55* all the time.

Don't know about basement temps. I guess that would vary depending on the HVAC set-up.

I've also kept young plants under lights in my yard barn from mid-March until May 1st or so, and when we had a real cold snap, I just draped old blankets over the light fixtures which were set directly atop the plastic tote tubs in which the plants grew in 4-inch square starter pots. The heat from the florescent fixtures is enough to keep the temps in the tubs above 50*F in my experience.

    Bookmark   February 8, 2010 at 12:31PM
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