Building a greenhouse in the high desert

I am so glad I found this forum!! I live in SE Arizona at 4800 feet, so we get fairly cold in the winter and hot in the summer and I have so many questions!!!

My DH just finished building a storage shed for our yard, he is very pleased with himself and now wants to build me a greenhouse/potting shed. His idea is just build a small shed with large window facing SE and the SE portion of the roof being covered with clear poly panels. But I have so many questions and I need to get information quickly before he starts, as we have already been having disagreements about the design and I need to make sure I know what I want before he gets too far. He is fairly handy, but the shed is the first big project he has built.

First, let me point out that I am not looking to grow large plants or vegetables. I just want someplace sheltered where I can start seeds in the late winter and hopefully overwinter some plants that don't like the freezing cold. I have been doing a lot of reading about greenhouses and it is just making me more confused. Anyone with any experience who can help, would be greatly appreciated.

Here are my questions/issues:
1. I have been reading both, that you need good ventilation, and that you need to keep it sealed? I understand that you don't want drafts in the winter, but with the heat here, I was going to put in windows that opened so I could open them on really hot days to get the heat out. I was also planning to vent the roof peak so it would always let out the heated air and just put plastic over it in the winter. Do you think that will work?

2. The location my DH wants to put the shed faces roughly SE and will get good morning to mid-afternoon sun most of the year. I see most recommendations are for south facing, but as it is being placed next to the shed, that isn't really feasible.

3. We have no power near the shed, so I was looking into passive winter heating. Has anyone had any luck with water as heat storage? Does it need direct sunlight to work? (I was thinking about putting water storage under the plant shelf against the wall with the morning sun, but I don't know if that will work.)

4. The foundation is going to be on wood on top of cinder blocks, but open under the plant shelf area with some weed cloth and wire mesh underneath to keep out the weeds and critters. Will this be a problem?

5. We are building it out of wood, with windows on the top half of the east and south facing walls, and siding for the bottom half and a few small windows on the north and west sides for cross ventilation. Is that sufficient? And should we insulate the west wall? I am so confused.... ugh!

Ok, I think that is enough for now. I am posting a picture of what we are basing the design on, but with bigger windows and a picture of our new shed, which the garden shed will hopefully match in color. If anyone has built something similar, I would love to see it. I know some of you are very creative in your yards and I could use some inspiration. :)

Thanks for any advice!

My uncle has a quonset hut type greenhouse in Nevada. The winds blow right over it, it is fiberglass - clear, with window and one end and door at the other. (not sure what direction they face - in Texas I would (and did) put those east/west for the main ones. I would think a metal shed would be very hot in summer, cold in winter, since no winter sun could come inside.

The shed is going to be wood, not metal. Although the roof might be metal with insulation.

Wood is a bit less chilly. My greenhouse is plywood on the bottom 3 feet or so, windows and fiberglass upper walls and roof, with enough wood to be able to mount the large windows. In Texas, ventilation is at a premium. North side of mine is the back wall of my garage to block the north wind. A privacy fence blocks the west wind.

Hi Dionosaur, we haven't built a greenhouse from scratch, but I've had a 10x12 Harbor Freight greenhouse for about six years now, and my high/low temps are very similar to yours (almost identical.)

I use my greenhouse to store cacti and succulents year round, and to overwinter plants that can’t live on my patios during the winter months. The light in the greenhouse is so much better than the winter light in my dark house...they love it.

My greenhouse has the long side facing south. I would think your orientation would work fine (you and I don’t live in cloudy climates!) If you find that any area of your greenhouse is getting too much light (your plants will tell you) you can experiment with shadecloth, perhaps during the hottest parts of the year. You can always decrease the light with shadecloth, but it’s hard to add light after you build.

If you want to use the greenhouse to overwinter plants, I think you will likely need to get power to the greenhouse. From my reading, it’s really very hard to store enough water in a smallish greenhouse to help raise the temperature enough for most plants. Usually what happens is that the heat gain from the water does help warm the space in the early evening hours, but by the time the critical low temps are reached (in the wee hours before dawn) all of the heat has already been lost from the water, and it's just not enough. I don’t want to discourage you from exploring this, but I think folks who make this method work well are probably the exception, not the rule. I think it requires very careful design and great insulation to work. In other words, it’s tough, if at all possible in your (and my) climate...so just a heads up.

To heat my greenhouse, I use two 1500w cheap electric heaters. I used this website to help determine how much power I’d need to keep my size greenhouse at the temp I wanted, so you can play with it:
http://www.littlegreenhouse.com/heat-calc.shtml

The same website also has a calculator that lets you play with what size exhaust fan you might need, to exchange the air in the summer.

You are so right that ventilation is critical. I finally made screens for the south wall, and remove most of my solid panels on that side in the summer. I also keep the doors open, with a shade cloth curtain in place (to keep birds out) and I installed an exhaust fan on the opposite end, to help pull the hot air out. I keep two cheapo oscillating fans in there, running 24/7, to keep the air moving. Roof vents are a great idea, but if you plan to pay for winter heat, you’ll want to be able to close down the gaps, so you don’t lose money through the roof.

Cooling is harder for me than heating. If it’s 100F outside, it can easily reach 115F inside my greenhouse, and that’s with one wall made of screens, the double doors open, and an exhaust fan running! It gets very hot in there during the summer, and without power for fans, your greenhouse may well be too hot for summer plant storage (which you may not need, anyway.) Desert greenhouses are challenging in the summer.

Many people insulate their north wall, and some insulate part of the north side of the roof. We insulated the north wall, even though most of the cold north winds are blocked by our house.

I know all of this is really confusing when you start. This is my first greenhouse, and it’s one long learning experience, but I’m so happy we took the plunge. Keep reading all you can, there’s so much great info on the internet!

I maintain a little blog about building the Harbor Freight greenhouses. Most of it will not apply to you, but I’ll add a link here, in case any of my info about heaters, fans, insulation, and other things is helpful. This is a link to my last section, about Greenhouse Enhancements, since you don't need to know all the Harbor Freight assembly stuff.
http://hfgh10x12.blogspot.com/2017/08/part-seven-greenhouse-enhancements.html

Sorry this got so long! :-)

I bought some white corrugated plastic sign material and siliconed it to the roof of my greenhouse last summer. Lowered the temp, plants didn't sunburn, with windows open, a nice breezy 85 to 90 was achievable on a 100 degree day. Wind took it off over the winter, but I intended it to be removable, will be putting back on soon.

oldmudhouse - Thank you so much for all the info. I will post a picture when mine is done. I am going to reconsider insulating, as my fiance will have a fit if I start talking about putting in heating and electricity. The shed is not very close to the house and it would mean trenching and hiring an electrician which, considering I am currently unemployed, ain't gonna happen. I had already planned on figuring out a way to close off the roof vent in the winter, and we are planning on having roof runoff water storage up against the west wall of the shed, so I am hoping that will provide some insulation as well. I guess it will be a trial and error thing, hopefully I will be back to work soon and then I can afford to add heat if I need it.

Gypsi - I was thinking about building a frame structure that I could stretch over with shade cloth and then use hooks to anchor it on the roof during the summer and then just take it down and store it in the winter.

Yeah, adding electrical service really does add to the cost, I know. I think it's trial and error for all of us, and doing things in phases is a good plan!

Regarding shade cloth on the roof, your plan is what I did; I have a piece of 60% Aluminet shadecloth on my roof. I hand-hemmed it and put grommets in the edges, about a foot apart. Then we added a strip of wood to both edges of the roof, so the grommets can catch on screws (I added a bit of wire to each screw so the grommets wouldn't flip off the screws in bad winds.) It worked great. I found I can leave my shadecloth in place year round (there's that trial and error thing, again) so I never have to take it down.

I hope you do post pictures of your progress!

If you have got a source for the sign material it is about $10 for a 4'x8' sheet, tube of aquarium silicone (so it is removable) about $5. Much faster too. I did it by myself.

With our winds, shadecloth gets frayed out too fast.

I just wanted to post a picture of the shed. I took this over a week ago, it is further along now and I finished painting it a few days ago. He is putting up paneling inside and insulating all the walls up to the ceiling. I will post a better picture in the next few days.

I have much respect for both of you, working on an outdoor project in the heat, this time of year. Wouldn't it be great to watch a nice monsoon rain, through those new windows?

I saw your post in the SW gardening forum also, with photos of the dutch door you refinished. You did a great job on the door (wish you would come here, and tackle some of my own refinishing projects.)

I'm thinking about putting four cattle panels together in a protected place in the yard and building a rocket mass heater with cobb and exhaust tubes in the soil. The pipe would go out the opposite end of the heater, but I could put a little door in the pipe so that smoke could go outside until it turns to steam and CO2 (5 to 7 minutes for the smoke to clear in the pipe) then open the valve/door and let the plants have the CO2 at mid-morning/time of feeding. I'd build a frame of 2x12s to set the hoops into, insulate the ground with styrofoam, then make a gigantic raised bed under the hoops with the RMH pipe down the center. It gets -40 here in the winter, so not expecting to grow all winter, just gain four months on beginning and end of season. Rocket Mass Heaters use very little wood, and re-burn the smoke.

I'm not sure how you are, temperature-wise in Las Cruces, but in Alamogordo when I lived there, it got pretty cold in the winter. This might be helpful for places such as that, also. Lots of wind, here, so will have to build it in a protected place in my yard...I learned my lesson last year with a big one that got lots of wind. It tore the plastic off and the wind mashed down the hoops. That was scary watching it. Not doing that again, lol. I may build a frame GH there next year, and dig down so it won't have to be so high. We get such terrible wind off the mountains, here.

Solace we get into the low teens a few times each winter, but rarely get lower than that, so your climate is much more challenging for heating than mine. I complain about our winds here (especially in the spring) with gusts some days in the 45-55 mph range, but I know other folks have it worse. Only good thing about losing a structure like you did is you sure learn (in a hurry) what you want to do differently next time, lol. Good thing gardeners are usually stubborn optimists. :-)

Very nice shed, nice shiny new windows! (mine are all recycled and the frames don't all match)