Even thick greenhouse glass would not keep the air inside an unheated greenhouse warm at night, and certainly not on a zero degree night. I think it is more likely that his plants died due to cold exposure than from the heat.

If a person does not have a heated greenhouse, the air inside the greenhouse can drop just as low as the outside air. For some reason, this always shocks people. I do not know why. Even a wood-framed house with insulation in the walls gets awfully cold if the house is not heated, and glass offers very poor insulating value compared to a wood-framed wall filled with insulation.

To some degree, you can mitigate the cold air if you have a double walled greenhouse with two plastic layers inflated by a constantly running air blower. The layer of air maintained between the two layers serves as insulation and helps keep the greenhouse warmer, but you still would have to have a heater inside the greenhouse. Otherwise, the layer of air between the two layers of plastic is just cold air.

You also can help the air inside a greenhouse stay warmer if you built it as a lean-to on the south side of a house so the house shields the greenhouse from north winds, or if you build it as a sunshed with the heavily insulated shed wall on the north side.

You also can help the air inside a greenhouse stay slightly warmer without a greenhouse heater if you fill the greenhouse with water-filled containers that serve as solar collectors---with the water heating up all day and releasing heat at night. However it takes a huge amount of space to put enough solar collectors inside the greenhouse to make a difference in the winter time. My water-filled solar collectors run entirely along each wall of the greenhouse beneath the tables and I use large containers----molasses feed tubs---- along the south greenhouse wall, with each tub being roughly the size of a whiskey half-barrel planter. On the north wall, I use some plastic trash cans and the rest of the area is Tidy Cat cat litter buckets stacked 2 or 3 buckets high and covered with black plastic to help the water inside of them heat up more during the day. Even with all those water-filled containers in the greenhouse, it barely stays above freezing at night on the colder nights and sometimes it drops below freezing.

In fact, this year, instead of filling up the containers with water, I left them empty (because I want to take everything out of the greenhouse in January before I move seedlings out there and put in a new plastic ground cover before Spring arrives) and only put the most cold-hardy potted plants in the greenhouse. My orange and lemon trees are indoors in the unheated mudroom which never drops below 50 degrees since the sunroom is right beside it, and there's nothing in the greenhouse right now that cannot handle pretty cold weather.

Heat buildup is a problem, but the heat likely didn't kill the plants because the cold already had killed them. Even if y'all had dropped to only 15-18 degrees, it is likely the air inside his greenhouse dropped just as cold as the outdoor air and his plants would have died at temperatures below twenty degrees. Certainly his plants had no chance of surviving a 0-degree night without a heater on all night to keep the air in the greenhouse above 20 degrees.

Without shadecloth, a greenhouse gets insanely hot. My greenhouse is substantially larger than his, and it can hit 145 degrees by 10 a.m. in the winter on a sunny day if I fail to open the doors and 4 vents early in the day. Keep in mind, my greenhouse has 50% shadecloth covering it and still gets that hot.

The smaller a greenhouse, the harder it is to keep it both warm enough at night and cool enough during the day. A person can't just buy a greenhouse and put the plants in it and be done with it. I cannot emphasize that enough. A small greenhouse without a heater is virtually worthless.

The minimum and maximum temps must be monitored daily (preferably multiple times a day) and doors/vents opened in the morning to vent out hot air, and then closed in the afternoon to help build up enough heat to help the greenhouse stay as warm as possible at night. A heater must be used if someone is trying to maintain a temperature that is substantially warmer than the outside air. This can come as quite a shock to someone who thought that the greenhouse effect which warms up a greenhouse so much during the day would make much difference at night. It just doesn't. In the absence of sunlight or a heating system, there's nothing to provide warm air at night. You know, a 7' x 7' greenhouse is smaller than a car, and cars get very cold at night.

The best way to understand how much a greenhouse heats up during the day and how much it cools off at night is to have two Min-Max thermometers and to record those temperatures daily. Put one inside the greenhouse. Put one outside the greenhouse. Track the temperatures of both for a couple of months in the winter time, compare the two to one another and you'll see just how much hotter the greenhouse is during the day and how cold it gets at night. This always comes as a huge shock to people who automatically assumed the daytime heat would hold over at night.

Thank you Dawn that answer socks both of us :0

I really thought a greenhouse would be a little above outside Temps. When I get my greenhouse built I will definitely be heating and venting. There is so much I need to do in a greenhouse. Soon.

Well I wonder why my peas and kale didn't die and they were outside. Of course he's in Lubbock and I am here. I would have thought the cold wind would do more damage than just cold.

It also is important to water before a freeze. It helps plants survive. That's how I broke my wrist I was out watering before all this ugly weather. too bad I only got two beds done. Any Greenhouse serves as a Wind Block which can make a big difference too. But I have watched my maximum minimum thermometer and my experience is much like Dawn's.

He said it was very wet and lots of condensation. He is having a hard time figuring out fall/winter is so different. He floods everything every week just like he did in July. He will learn just like we all do. My stuff is all moist so I haven't watered in about 2 weeks maybe.

Plants that were outside likely were hardened off to the cold. Plants in a greenhouse, especially following a warm autumn, might not have been as hardened off to the cold as much as the plants outside were. Wind can cause windburn, of course, but plants that have been outside already have been exposed to all kinds of wind too, unless they were newly moved from indoors to outdoors or newly sprouted, so they've developed a hardening to the wind as well. Hardening off plants to wind is just as important as hardening them off to ever-increasing amount of sunlight and is one reason a lot of us run a fan in the room where we raise seedlings indoors under lights and why we run a fan in the greenhouse.

If his plants were flooding wet, that didn't help them either. Overwatered plants can be as vulnerable as underwatered plants. Like Amy, I always water in advance of a big cold front, but I don't flood them. I always aim for only moist soil, never wet. If he is continually keeping the plants too wet, that also leaves them less healthy so more prone to handle extreme heat or extreme cold badly. And, if he constantly underwaters and then leaves the greenhouse completely closed up without good air flow, he's just creating ideal conditions for fungal root rot and other diseases to develop.

Yesterday, I watered the tomato seedlings for the first time since the day I planted the seeds, which was December 1st. I check the soil moisture daily, and that was the first day the growing medium felt dry and the plants needed water. Three weeks indoors without watering is pretty good. Once the seedlings are older and bigger, I'll likely have to water them about once every 7-10 days. I'm not one to water overly much as it encourages diseases. I know people who mist and water seedlings constantly and then are dismayed when the poor little plants develop damping off and die. I'm the exact opposite. I water as little as possible and dare them to die. They don't die, but they do develop good root systems as they stretch and seek moisture. Every one of us learns by doing, and what works for one person in one situation will not work for another. That's why learning by doing is so important---everyone has to figure out what works for them in their soil, their growing conditions and, especially, in their microclimate. I have high spots and low spots in my garden and some plants can survive cold nights in the high spots but not in the low spots (because warm air rises, leaving the low spots colder than the high spots). That's something I just had to learn from experience over time.

My unheated greenhouse gets just as cold as the outdoors after a cloudy day. After a sunny day, when the greenhouse can gain as much as 40 degrees over the outside temp, the greenhouse temp can remain as much as 10 degrees above the outdoor temp if there is no wind. It is a large greenhouse--16x24--so the ground temperature takes a long time to cool off. There is also an area of pavers inside the door that heats during the day and radiates at night. I also have some water storage, although not as much as Dawn. On very cold nights--below 20--I cover my inground plants with sheets in midafternoon on a sunny day. Last week when it was 2 degrees outside, it was 9 inside and 18 under the sheets. So most of my cold hardy stuff is still alive although a few plants closest to the wall froze out. So far kale, mustard, bok choy, tatsoi, Swiss chard, spinach and winter onions are still alive. A few lettuce plants survived, but some of those froze and some damped off way back before it even got cold. I have had kale and spinach survive 0 temps outside in years past, mulched with oak leaves.

Unfortunately, there is some kind of critter in there that is eating the growing points and surrounding leaves out of some of the plants. It started on the spinach, working at night, and then moved on to the Swiss Chard during the two full days that I kept the plants covered last weekend. When I pulled the sheets off, there was very little of the chard left. I have sticky traps, regular mouse traps and a mouse bait station, but so far the only casualties are crickets and a couple spiders. This also happened a few years ago in an outside cold frame. After several days of leaving the cold frame covered in a heavy quilt, I found everything inside eaten off and a nest of oak leaves and grass tucked into one corner. I'm pretty sure that was a mouse. This time in the greenhouse I found a small hole in the ground. I am wondering if that means a vole. I killed two voles in the garden this past summer. One was under an eaten out hill of sweet potatoes and came out and ran away as the hill was dug into.

I still haven't caught the varmint that is eating down my greenhouse greens. Any ideas on what is doing it? Or what to do about it?

Vole? Field mouse? Rat? Cottontails, especially babies? That is, if the greens are low enough for rabbits to reach them. Those are my best guesses. Maybe a baby possum if it has a way of getting into the greenhouse, like if you leave the doors open for a good portion of the day and it gets in there and hides until night.

As far as I know, all I've ever had in the greenhouse (and none so far this winter) are voles, but I saw a field mouse in the garage the other day so there's always the chance that a mouse will show up in the greenhouse or garden shed.

I've had wild birds eat young, recently sprouted and still smallish greens back to the ground out in the garden in winter or early Spring, but not in the greenhouse where they grow in a flat tub up on a table.

I wonder if you put a flat type glue mouse trap near the greens if it might catch whatever varmint is eating your greens?

I did consider the possibility that you have either a big grasshopper, big crickets or big, fat caterpillars hiding in the greenhouse, overwintering and sneaking out at night to eat, but if you did have any of those, I am sure you'd recognize the signs of them, including their frass. Nowadays, with such wild weather swings, perhaps anything is possible. Yesterday I found ladybugs inside the house but they weren't alone. I found one brown stinkbug, a moth and mosquitoes. I guess they are flying in when someone opens the front or back door in the evenings. I never see them come in, but I sure find them crawling around or flying around.

I set a regular trap, two sticky traps in box shapes and set out a Dcon bait station, So far only crickets in the sticky traps. Something tripped the regular trap without getting caught. Whatever it is has now started in on the kale. Darn it. In retrospect it is clear that we should have dug down and put a tin barriar under the walls of the greenhouse for a couple feet instead of just setting it on the ground.

How about a hot pepper and maybe garlic spray? I know chickens don't mind heat, but I'm thinking mammals do. You could put a cat in there, but that might cause more damage.

Maybe just sprinkle dry cayenne pepper around the targeted veggies. If the creature is a vole, mouse, etc., it will get the pepper on its feet and....if small rodents wash their faces like cats do, the pepper might chase them away. Or, to try to capture small paw prints in order to get an ID, sprinkle flour on the ground around the targeted plants and see if they leave any discernable pawprints, a trail, etc.

Great ideas here. In the morning I will reset traps, sprinkle flour on cardboard and sprinkle ground cayenne pepper.

Well, the traps and bait station are still untouched, but the newspaper with the flour sprinkled on it shows tiny little feet, although very indistinctly, and a tail trail. So now I know that whatever is in there is small.