Minimum nighttime temperature for seedlings

californianFebruary 25, 2009

I have a lot of tomato seedlings that I just transplanted from my seed starting trays into individual 4 inch pots. As a result I don't have room for them anymore under my indoor fluorescent light. So I have been taking them outside during the day and bringing them back indoors at night, but not putting them under a light. This is getting old in a hurry. What is the minimum outside nighttime temperature at which I can just leave them outside all the time. Right now in my part of southern California the minimum nighttime temperatures are in the low to high 40s, but I am worried about what if we drop into the high to mid 30s.

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Your Question has a few answers. Basically as long as you keep your plants from frost or freezing they will live. But they grow at a slowed pace when subjected to lower temperatures,even if just during nights. I would strive to keep them out if above 50F and if temps dip into the 40s you still have a safety net. You need to be real careful where you record your temps when you get in the lower 40s. Any eye level thermometer reading below 40F means that you can have frost at ground level so at the least the plants should be covered.

There are many options available to you. You could simplify your task by putting the plants on a cart that you can wheel into your garage,etc. You could use a row cover fabric and even leave it on during days- often these give you protection for a few degrees below 32F. Other materials like plastic or even a bedspread will buy some protection overnight but you need to remove them after danger of frost and before the sun is up long.

    Bookmark   February 25, 2009 at 9:59PM
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digdirt2(6b-7a No.Cent. AR HZ8 Sun-35)

Agree. Our greenhouse thermostats are set at 45 and their location is vital as there can easily be a 10 degree difference between waist level and the floor/ground.

Outside as yours are, you need to factor in wind as well.


    Bookmark   February 25, 2009 at 10:38PM
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timmy1(6a ri)

Although I've never read of this anywhere, I believe a healthy tomato transplant of good quality is a result of a slow steady growth with optimal balanced nutrition.

A plant stunted by cool or cold temps will take time to recover and many times a little plant seeded 3 weeks later will pass it hands down. Also, a stunted plant is more likely to develop disease due to deficiencies and general age of the plant tissue.

If you grow your seedlings on at say 65* nights, then have to subject them to the low 40's for a week or more then you may have jumped the gun on your seeding date.

I always select a seed date by deciding on the transplant date, then back up 6 weeks.

The weather does not always cooperate but one can only make plans based on past weather history.

    Bookmark   February 26, 2009 at 11:06AM
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I agree nwith moser's 50°.

IF they have been growing well and are already hardened, I will leave them out covered with remay down to 45°. That won't necessarily stunt them more than that one night. Definitely block the wind or take them in if windy and below 50°.

Below 45° for extended periods and you will likely see some foliage damage in about 2-3 days.

    Bookmark   February 26, 2009 at 1:18PM
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anney(Georgia 8)

Two months later, here I am with the same problem as the OP of this thread. Daytime temps in the 60s and 70s, nighttime temps in the low 40s to low 50s for at least the next ten days according to the forecast.

Plots are ready, supports are up, most of the tomatoes are transplant size (a few dinky ones still), they're all hardened off with even some balmy outside overnights, but the weather is a problem.

I guess I'll just keep bringing them in when the forecast is for 45 and below. What a pain after doing it for so long anyway!

My son advises me that all this reluctant weather warming is a "sunspot problem" and explains in great detail why that's true. Who knew?

I think I'm going to aim for a cold frame for next year.

    Bookmark   April 19, 2009 at 2:55PM
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A cheap cold frame has gotten me by for years, and I am in zone 6. Unless an overnight frost is predicted, the plants are out in the frame. They look fabulous by their May 15 planting date.

    Bookmark   April 19, 2009 at 5:46PM
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I would add you want to make sure your plants have been acclimated some before allowing them to sit in lower temps.

Some plants have shown damage at 50F (depends on variety, how acclimated, wind etc).

    Bookmark   April 19, 2009 at 6:28PM
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anney(Georgia 8)


Yes, they're already hardened off to wind, sun, dryer soil, and temps above 50 degrees, so all I can do is wait for nighttime temps to stabilize at 50 or more. I am VERY careful about hardening plants off. Soon out in the garden to stay, I hope.

    Bookmark   April 19, 2009 at 6:33PM
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I have a hoop house I left 2 plants in last night after a 65f day it was only supposed to get to low 30's but they still froze and died. So not sure when as we will be in the 70-80's in the day and mid 30's to 40 at night.

    Bookmark   April 19, 2009 at 7:05PM
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jrslick (North Central Kansas, Zone 5B)

I have been having the same problems here in Kansas. I have 102 plants in one hoop building. They have been there since March 28th. They are growing great, minus the ones that died when it was 18 degrees. It is 50-80's in the day and 32-50 at night. I have been covering them up with row cover every night it is suppose to be below 45. Then take it off each morning. It is alot of work, but I can tell you that I am seeing more growth this year than last because of it. I also have 400 seedlings in another hoop house. They are under one layer of row cover also. They have been out there for a week. They went straight from the basement lights to the hoop house during a cloudy 3 day warm spell. They were not stressed.

I don't get too worried about 40's, with the hoops. I do start to worry when they say 38 or below. I just double up the row cover. After the 18 degrees and had a majority of everything come through ok, 32 doesn't scare me like last year. Do I worry, a little, but I still sleep good.


    Bookmark   April 20, 2009 at 3:25PM
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Jay, I envy you because I've had to set my alarm to get up to check greenhouse stoves at least once nightly during most of the last month. But a bit of advise for you and Mtinaz that was reported by a reseacher that kept records of hoophouse inside and external temperatures. He showed that the inside temperature can actually dip below the low of the outside temp.

Row covers can make the difference between a damaged plants and a saved ones even when the temperature is at freezing in the vicinity of both covered and uncovered plants. Apparently tomato plants can sense the proximity of covering and adjust heat loss from the plant.

the problem is that when you are walking the fine line of temperature regulation near freezing points you better be prepared to take a loss.

    Bookmark   April 20, 2009 at 9:04PM
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timmy1(6a ri)


Get a sensaphone and get some sleep! He He.

The other thing that works is a 12volt car battery wired through a honeywell to a car horn. Just as long as you can hear it from your house.

I also wired in web cams to look at the furnace's

Here is a link that might be useful: sensaphone 1104

    Bookmark   April 20, 2009 at 10:11PM
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sprouts_honor(5, southern shore of Erie)

Wow, timmy1. You're wired. A car horn?! I'm sure the neighbors would love that! LOL

    Bookmark   April 21, 2009 at 7:29AM
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We live on the northcoast (5 miles) in Humboldt. Each area here has it's own microclimate. After three years trying tomatos I gave up... until two years covering with 2x4 corner stakes, then with plastic. This kept the mist and rain off and got good results with cherry and currents.. except towards october, with higher humidity, lower temps. Three yeas ago invested with a 20'x14' green house. The first year was fine until october with 95-100% humidity, with temp below 50's, which hit late bloom, killing else. the second year, early blight started in april... so I used a cheap heater keeping night temp 50-60, which also lowered humidity to 80-85. This year I am having terrific success (so far) with a base heater 50-60, which keeps humidity lower than 75-80%. I also purchased a de-humifier last year with the cheapy heater which dumped 3-4 quarts each night. So far, this year have not used the dehumidifier.

    Bookmark   June 18, 2011 at 9:45PM
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