Unheated greenhouse in zone 4; what to expect

Tomorrow I’ll stake the layout for my greenhouse, a 20’ by 9’ lean-to on the south facing wall of the house. Lots of sun to be had there. With a bit of luck ill even start the holes for the sonotubes that will be the foundation. It is meant to be an unheated greenhouse so I can get an early start on plan propagation, host my bottom heat/mist bed, potting workstation, and shelving to hold hundreds of plants. When I retire I plan on running a backyard propagation nursery and the greenhouse will be an integral part of those efforts as my growing season in Alaska is short with zone 4 winters.

I haven’t had a greenhouse before so I’m not sure what to expect. Here March and April is still middle of the winter. Early May the days may warm up but it still freezes at night for a good chunk of the month. I’m curious as to the nighttime temperatures will be when it is in the 40’s during the day and into the low 20’s at night. I figure sunlight can heat it up petty good during the day and being next to the house ought to help, but I habe no idea how much warmer it will be at night compared to the outside.

Is there anybody here running an unheated greenhouse under similar conditions? If so, how is that working out for you?

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I use propagation heat mats under trays filled with compost and old potting mix, placing my starts in their own containers on top (I let roots grow into those trays but prune them every few days by lifting pots. This reduces the need to water and helps get bigger plants in smaller pots). On cold nights I add an additional blanket of floating row cover over the starts and everything else down to the greenhouse floor to hold heat close to the plants. They can survive down to about 25 degrees outside. Actually, I’m not sure how low they can survive because I’ve not kept them in there in freezes below that. My greenhouse is completely uninsulated.

My greenhouse is up against the south side of my house. It is 4 layers of glass thick 16n feet wide by 4 feet deep and 10 feet tall. My guess if I have a good sunny day I am probably safe over night down to the teens-F. If it is cloudy all day with a high of 40 I will need to ad heat. This is done with a small fan in a widow to push house air into the greenhouse.

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A greenhouse that’s unheated will be close to outside air temperature at night. On the south side of your house it might remain 1-2F warmer than outside but that’s all I’d expect unless it’s highly insulated or your house leaks lots of heat.

Think about cooling. That’s just as important as heating. In May when it’s in the 40s and sunny the GH will get close to 100F unless it’s well ventilated. You’ll either need large vents, roll up sides, or an exhaust fan capable of at least one air exchange per minute.


The plan is to install several of the heat activated vent openers. We can get 19 hours of intense sunlight, I’m sure heat will be an issue.

While in the summer I may hijack the greenhouse to grow tomatoes and such, for the most part I’m interested in getting an early start on propagation of cold hardy stuff. Here plants starts waking up around mid May, and by mid September the growing season can be over. There is very little propagation being done locally because it is extremely hard to fatten plants to market size from one season to the next.

Take haskaps: starting cuttings in mid March in a cold room with bottom heat gives me a huge jump on getting them ready to soak up the coming season. Heck for that I don’t even need to warm up the entire greenhouse, I just need to insulate the bottom heat bed and in fact make sure the top side stays cold. With the propagation schedule I’m figuring out I don’t need to worry about taking the edge off cold nights until the end of April/early May.

Heck most of what I want to start early doesn’t mind overnight frosts at all. The goal is to gain a month+ of growth. It may sound like not that much but that would be a 25%+ increase to my season, which may be enough to start perennials early one year and have them market ready by the next summer.

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Have you followed these Alaskan Youtubers? They have a greenhouse for a few seasons now. Last year they weren’t able to ripen tomatoes if i remember right, but i think it was very rainy/cloudy. --they are somewhere around Willow, Ak.

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I think this is a great idea. I have something similar in mind but not quite as big.
Currently, I have two small unheated hoop houses with a double layer of plastic and infiltration fans. If managed correctly, the night time temperature is 10° warmer than outside. Depending on what plants are in the greenhouse, I will use a small electric space heater with a fan during wretched cold spells. A 750 watt fan/heater combo helps ME sleep on a 17° night. And I use the floating row cover too. Number of layers and thickness depends on the temperature. I’m in zone 4 and have successfully overwintered large Rosemary plants(pots in ground) with no winter heat. We also plant cold hardy salad greens, carrots and green onions in September/October for harvest in late winter/early spring.
I’m always on the lookout for tough varieties and new tricks to use. I have read some interesting hypothesis from intelligent and credentialed people that believe our sun is entering a “minimum” cycle around 2025… maybe sooner. That scenario would be rough. I’m currently in zone 4 but we experience severe weather events often. Two years ago, in October, we had a storm blow in that took us from 60° to -3° in 12 hours and brought snow and 25mph winds.
Looking forward to you sharing your project!!!

fieldsofgreen, have you seen these guys?

They are great for chicken coops and other applications where you don’t want a cold spell to sneak up on you. With good placement (the sensor is the cube itself) the on at 35f / off at 45f would kick in a small heater as needed. Heck for my application the on at 20f / off at 30f ought to do it; April/May timeframe all I care to ward off is a deep freeze not your garden-variety Alaska morning.

There are all sorts of tricks to coax things to grow. I have grapes that are about to get evicted because they sit right next to that south facing wall where the greenhouse is going. The house heat keeps the soil right next to it warmer than even 3 feet away so I take advantage of a highly localized nano climate. Vines are hung in such a way that they get taken down for the winter and buried in wood chips and snow cover. You could even do a cold frame to warm it up earlier (grapes are the very last thing to wake up) in order to give them enough time to ripen fruit.

Heck I keep a pile of dark compost near that same wall so it thaws early. Once spring gets going good enough I spread it with a shovel over the snow cover; the dark compost warms up with the sun melting the snow coverage faster. Once the soil underneath is exposed the darker color does the same number, helping to remove the thermal snow blanket a week or two sooner.

But honestly, while I like to push things to see if they will grow there is great satisfaction in looking at the window, haskaps flowering and frosting over, knowing that it won’t bother the flowers one bit. There are wonderful things that will grow on zones 2 and 3.

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In the spring, if it’s cold enough outside to kill a plant, it will also die inside the greenhouse at night (like tomato). You can put plants out there in the morning, and bring them back inside at night. It gets old fast.

Mail order plants- put them inside your greenhouse on arrival, and they spring to life and put out a lot of growth compared to planting directly outside. Sometimes if a plant is weak or struggling, you can put it inside and it might come back. It’s like a plant hospital. You could also have more options with propagation and seed starting.

I’m trying to grow some watermelons and pumpkins. You have to be constantly watering and opening the doors as it gets hot inside.

As others have noted, a greenhouse does NOT stay warm on its own. It will be at ambient outside air temp unless some form of active or passive heat is present. I’ve seen greenhouses full of black 55 gallon drums of water that absorb heat in the day and release it at night.

Double or triple layers of glazing (or plastic) that covers the greenhouse will slow heat transfer significantly. Build it to take the weather it will be exposed to.

I recommend having at least one heat source that gets turned on somewhere above 35 F. Most tender plants can survive down to 35 though there may be problems with growth rate. You might want to put in a wood heater as a backup.

Last but not least, put in at least a year learning how to use your greenhouse. Allow for the fact that you will likely lose some plants. Learn how NOT to lose them.

The last caution I will give is about a hard blowing wind with below freezing temps. Under these conditions, temps in the greenhouse will drop drastically and may actually be a few degrees colder than the ambient temperature outside.

I’d give a thought to how a high humidity environment may affect whatever the exterior of your house is made of. Might not be so big a deal if you’re just using it for starts for a month or two. But mold problems are bad problems to get into.

It will depend a lot on what kind of glazing you use for the greenhouse. I have twin-wall polycarbonate (8mm) and even though there are some air leaks, it usually bottoms out around 5°F above ambient just before sunrise, with no meaningful heat source (a couple 32w LED grow lights and a couple 40w fluorescent tube lights).

If it gets real hot in there one day, often it’ll bottom out closer to 10° above ambient the next night, because the heat retained in the gravel and rain barrels and soil is slower to dissipate with more insulated glazing. In the dead of winter with short, cloudy days, it might be closer to +4°F overnight, since there was less heat trapped in the day.

Here are the ∆°F charts for yesterday morning and today (both prior days were mostly cloudy), as an example for this time of year:


I’m not growing anything during the winter but a huge benefit is that even if the inside of the green house hits -20f, the outside can be -20f with 45mph winds. On days like that I look out of my window and marvel at how my trees and bushes can put up with that.

Well the Lapins cherry didn’t, but I knew that was at best a borderline choice.

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My greenhouse with no heat is very near outside temperature by one hour after sunset and stays there all night. The only exception is on a very overcast night it can stay 4F warmer than outside. And while it’s not the best insulated the roof is double layer inflated woven poly that heavily IR treated. That means the poly slows both heat lose at night and heat gain during the day. It’s noticeably more effective than double layer inflated 6 mil greenhouse poly.

Greenhouses don’t stay meaningfully warmer than outside at night without some active heat source. I hear reports of 20-30F warmer than outside without heat. That’s hogwash. The R value of my greenhouse is probably 1.5, my house at least 20. My house won’t hold 30F warmer than outside without heat at night. It’s ridiculous to think that a greenhouse would with 10-20 times faster heat lose.

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Definitely hogwash. I’d say mine ranges from +3°F (rarely that low) to +10°F (rarely that high), with most nights in the +5-6° range. It’s possible the lights add a degree or so to what my greenhouse would be on its own, but that seems unlikely since they are relatively low power LED and fluorescent tubes, not HID. Out of curiosity, though, I’ll turn them off tonight and see if it makes much of a difference. I have two rain barrels, too, but I’m skeptical of claims about those making much of a difference. I assume the twin-wall rigid 8mm polycarbonate is the main reason mine tends to stay that warm by dawn each night. I couldn’t find an R value for this particular thickness, but the manufacturer says their panels range from 1.5 to 3.8, so I’m guessing it’s somewhere in the middle of that range based on it being near the middle of this chart showing the range of thicknesses they offer:

You have lots of cloudy nights in winter. That seems to add 4-6F to mine. I don’t know how big yours is but it’s not unheated. Add that all up and 3-10F is reasonable.

Also it’s not easy to get perfect temperatures both inside and out. My greenhouse is sometimes 10-20F colder than the airport reading two miles away. Sometimes they are way warmer than my place.

I won’t be surprised if the greenhouse provides some protection above what would happen with open sky/air exposure. So 28F might cause more damage on a clear night than 28F with two layers of poly overhead. I can’t really say because I operate in the 34-45F range in winter with outside as low as zero. But if someone said their plants survive freezes better in an unheated GH than outdoors I couldn’t argue the point.

For my particular neck of the woods that would be -20f but protected from 45mph winds. I’m sure that alone would have a positive impact on first year seedlings going into their first winter. Heck I do want them to get cold so I can transplant only the hardy stuff.

I figure for propagation purposes I could inject heat in May to keep it above freezing. Just about everything I plan on growing probably thinks 33f is balmy weather suitable for growing.

3-10 is with heaters off, it’s 16-20° with the heaters on. I use the same sensor chips on an outside probe and one at 8’ in the greenhouse, which is about 300 sq ft with the peak of the roof around 12’.

Here’s a representative week with a mix of cloudy and clear nights about a month ago, heaters off the entire time (but exhaust fans on most days):

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Lights turned on are heat. If there’s a 40 watt bulb on it’s heated. My heater will hold 50F above outside. Normally I’ve got nothing else in there that’s even 2 watts in a 1700 sq ft GH.

I mean yes, but those are LED and fluorescent tubes that feel cool to the touch. I’ll turn them off for a few nights and see if it makes a difference, though, that would be good to know.