That, I am unsure of. I have a small fan-type space heater in my cold frame, set to 60 for just that reason.

I put 11 seedlings/small tomatoes out last night as a test (same day as transplant from starter to solo cup) and came back this morning/afternoon to find them smiling up at me, happy as can be. it went down to 28 here but the inside didn't drop much below 50.

-Rathos

I have a ridged harbor freight greenhouse but not using yet, here in MN. It will be end of March or so - kind of depends on how carried away I get in the house with pepper and tomato plants and room under lights and windows. The greenhouse is pretty air tight, so maybe your temperature situation has something to do with air getting in??

I have a heater on a timer that kicks on only to keep it at a minimum of 49 degrees (in April we still dip into the 30s off and on at night). As it warms up during the day the vents will open up at upper 70's. Once the plants go out there, they really thrive; the plants are easy to acclimate to the gardens and containers for planting too.

You might want to try an old trick I read about...

Put several 1 gallon milk jugs full of water in the cold frame during the day. They should warm up with the rest of the interior, then as the temps start to drop they will give their stored heat back. Should work as long as you don't get days without sun or really bitter cold nights.

You could then put a heater in as a "fail safe" backup set to keep the temps out of the danger zone only.

Agreed on the jugs of water idea. It won't work miracles, but water certainly does help stabilize temperatures, and I've started doing that in my frame as well.

As far as not extending the season - you have to understand that almost all greenhouses have some sort of heating apparatus. they work well during the day when trapping solar radiation, but at night, it's just a badly insulated structure, and it will cool off, albeit a little slower once there is some sort of mass inside of it. it's like running a freezer with nothing inside of it - if there's nothing to stay cold and help stabilize the temps, the compressor will run until it dies.

i know it seems like a pain in the ass to have to run an extension cord, but it's well worth the effort if you want to safely take advantage of the greenhouse. those little heaters aren't too expensive and if you've already invested something in getting a jump on your season, you may as well put in a safeguard.

or you can always roll the dice and see how it goes ;)

but in the end, i think running a little heater for a few hours a night is less than running lights for half a day, at least imho =)

-Rathos

I just moved a couple of black tuff-tainer (or whatever) 20 gallon tubs out there. They're full of last year's 5-1-1, so they should hold a decent amount of heat.

On an interesting note, weather.com says this about yesterday/last night:

OBSERVED
Hi 44�F
Lo 28�F

My thermometer says this for the lows:
Inside: 38.4
Outside: 41.0

So it looks like I've got quite the little microclimate going on here. I'm on a small bay and my backyard is on the south side with full sun exposure, so maybe thats keeping me quite a bit warmer than a couple miles inbound where the weather station is?

Still not sure why its getting colder inside the greenhouse than outside. Seems strange.

A couple of things;

mbellot and Rathos are bang on with respect to their jugs of water suggestion. The water stores heat energy during the day and passively conducts it's stored energy at a slower rate than the ambient air within the greenhouse when temps drop. It won't get you through the night but it can help delay the time any supplemental heat source kicks in. A lot of industrial greenhouses use concrete floors and side walls to take advantage of the same effect.

If I measured a 5 degree lower minimum temp in the greenhouse my first reaction would be to suspect the thermometers I'm using, especially if they're the cheaper varieties. Have you tried reversing the thermometers? If the same one that measured the inside lower measures the outside lower when reversed, one of your probes is off. Just a thought.

You may want to familarize yourself with some basic greenhouse and heat concepts. You may find the link link below useful.

Here is a link that might be useful: Greenhouse Temperature

Yeah, tried reversing the thermometers. Same results, colder inside.

As far as the jugs of water goes, I might suggest putting something in the water to darken it up. It will obsorb a whole lot more heat that way. Or use dark colored containers and make sure they are placed to receive a lot of direct sunlight.
Bruce

That does seem odd. I wonder if it has anything to do with different humidity in the tent. That probably makes zero sense, but, I can't imagine why inside the tent would be cooler at night than outside the tent.

JoppaRich, you asked about heaters... I have a 26' x 8' greenhouse, we had a mild winter, it got into the teens a couple of nights. Those nights I had 6 -- YES 6! heaters going, it got into the low 50's high 40's in the greenhouse. Last year when my greenhouse was only 8' x 10' I usually had 3 heaters going on the cold nights. it's very frustrating.

I have absolutely no idea why it would be colder inside than out, even with the heaters off it's still warmer inside than out.

OK, so the thermometers aren't to problem. The puzzle of temp difference continues.

In the link I referenced previously, they say:
Radiation Cooling: On clear, cold nights, plants and other objects within a greenhouse will lose heat to the outside by radiation cooling. Because objects within the greenhouse are much warmer than those outside, they lose heat by emitting infrared radiation through the glazing and into the clear sky. Under such conditions, the foliage temperature may be 5 F cooler than the surrounding air. Radiation cooling occurs very little when skies are cloudy.

If the same applies to the humid air trapped in the greenhouse, it could explain the difference.

Since surfing the net trying to figure this out was more fun than doing my income tax return, off I went.

This USA Today article explains why humid air (counter intuitively) is lighter than dry air. Ding! maybe lighter air radiates heat at a greater rate than dry air. Off I went on that tangent.

I then stumbled across this Texas Gardener article on frosts/freezes that references the temperature differences between solid objects and the air around them... back to the point made in the original link. It further explains how wrapping / covering plants (as inside a greenhouse) will protect plants from frost.

I'm more confused that when I started. I knew one day I'd regret reading comics during my Grade 2 Thermodynamics lessons.

I'm going back to doing my income tax.

My poor greenhouse suffered 100 degree heat & 38 degree cold in the couse of just one day.