I put an unheated greenhouse in my urban driveway in zone 8b, here's what happened

This post is to document my adventures of having an unheated greenhouse in my driveway.

Acquisition and construction of greenhouse

The greenhouse is an 8x12 Sunshine Gardenhouse wood-framed double-wall-polycarbonate greenhouse. A friend bought a house that came with this greenhouse, and didn't want it, so let me have it in exchange for my disassembling it and hauling it away. Disassembly was straightforward and took me about 3 hours, and I moved the pieces the 3 miles to my house on my bike trailer in several trips.

Then the pieces sat in my driveway and under my back porch for a year until I got up the motivation and time to work on them. The reason that this took so long to start working on was that 1) I had a new tiny son, and, consequently, no time for working on anything, 2) many of the pieces were rotted on the ends, and needed to be replaced. Eventually I got some replacement wood, and took each of the panels apart, and replaced the longest wood pieces with new wood, then cut off the decayed portions and used them to replace shorter pieces. I used a hand plane to remove some of the weathered wood from the surfaces. I applied toxic gick from a can from the hardware store to the wood surfaces in hopes of avoiding having to take the whole thing apart and replace a bunch of rotted wood again in a few years. I also blasted out the insides of all of the polycarbonate panels with the hose sprayer to get the moss out from inside of them, and scrubbed their surfaces.

I assembled my greenhouse in the most underutilized and sunniest spot in my urban quarter-acre lot, which happened to be the middle of the concrete driveway in front of my house. This site was appealing because the driveway is otherwise not used for anything, and I've been having good success growing some things in larger containers last year and starting and overwintering warm season plants in my unheated cold frame, so it seems plausible that I do some interesting container-growing projects inside this greenhouse. My plan is to try to overwinter a bunch of peppers, ginger, turmeric in 5-10 gallon containers, and start some other sorts of plants. During the summer when the greenhouse becomes an oven, I might try a shade cloth to dial back the heat a bit, and try growing some heat-loving things like melons or something in even larger containers.

The driveway is sloped in both dimensions, so the foundation of the greenhouse needed to be set on concrete blocks of differing heights. In order to get the base level, one corner sat directly on the concrete driveway, one corner sat on a brick, one corner sat on a chunk of 4" concrete slab from when I had busted up the "driveway extension", and the last corner sat on a concrete pier block. I mortared each of these blocks to the concrete driveway. To fill the gap around the bottom of the greenhouse, I used metal flashing which I lined with a couple of cans of expanding polyurethane foam for insulation and sealing.

Assembly was otherwise straightforward and took a couple of hours. Greenhouse was finished and caulked to reduce air leaks by mid-September.

The placement of the greenhouse in the driveway is such that there's still 15 feet of driveway in front of it where I can receive loads of woodchips from chipdrop.

Next up, I will document what I have tried to keep alive or grow during the winter in the greenhouse, how that is going so far, and what the temperature has been looking like inside of it.

Chip drop has been a great resource for me too,  have had good luck with it.   What an excellent opportunity to get a greenhouse,  good deal.    Looking forward to reading your updates.

Things I am growing or trying to overwinter

I had been growing some peppers in 5-10 gallon containers of aged wood chips with good success. I brought these into the greenhouse in early October. These have continued to ripen peppers through the end of December.
Some of the plants got some cold damage when I had them in my unheated cold frame before I brought them into the greenhouse. Some of these look like they're going to die, while other look like they have just dropped most of their leaves and gone dormant.

I extended my basil harvest by about a month, as compared to plants growing in the ground or in other pots elsewhere. Basil seems extra sensitive to cool temperatures, and it died in early December.

I have a few Meyer Lemon and grapefruit trees that I started from seeds a few years ago. I also have a couple of pineapples I grew from pineapple tops. I have kept these alive in past years in my smaller unheated cold frame. I'm treating this greenhouse as a really big unheated cold frame for keeping things marginally alive during the winter.

I have some Butterfly ginger (Hedychium coronarium) thing that can live outside here (but dies back to the ground in winter), but I brought a pot of it in and it hasn't died back yet. I brought in a pot of turmeric which has died back to the ground, but hopefully it will get a head-start in the spring.

I found a couple of massive felt grow-bags free by the side of the road. They are approximately 3 feet in diameter and 16 inches deep. I have planted a cool-season polyculture in these pots, with fava beans, peas, radish, various brassicas and walking onion. These containers are doing well, and putting on good growth since late October. They're getting to the point where I can start cutting some arugala and mustard, and I expect to get some radishes later in winter (if the greenhouse doesn't get too hot during the day and make them bolt). I'll stake the peas (and they'll grow up the fava beans) once they get a bit taller (currently around 4-6 inches tall). I cut the fava beans back in November because they were getting disproportionately tall, and they've bushed out and sent up side-shoots since then.

Temperature observations

I've have a min/max thermometer in the greenhouse and have been recording the daily min/max for the past 3 weeks. My goal is to get enough data that I can reliably know how much warmer the greenhouse will stay at night and know when I need to put additional protection over tender plants on particularly cold nights.

The greenhouse seems to consistently stay about 2-3 degrees (F) warmer than outside at night. We haven't had many freezes since I have been observing the temperature, but we had one where it got down to 30 outside and stayed 34 inside. There were several freezes of this magnitude or slightly colder before I started recording the temperatures, which seemed to damage some of the plants in my cold frame but not in the greenhouse.

During the day, the greenhouse is about 10-30 degrees warmer than outside, depending on the sunniness levels. When the interior temperature hits about 80F, the roof vents open and let some heat out. One of the wall vent openers doesn't work, so I'll probably need to replace it before we start getting summer heat.

I would like to put some thermal mass into the greenhouse to suck up some of the heat. I put a 5-gallon black bucket of water in there next to some of the more tender pepper plants, but that's not very big. I'd like to put some 55-gallon barrels of water with a simple thermosiphon heat collector made from a bunch of 3/4" tubing running in parallel up from the base of the barrel to the top, attached to a black panel under some glass. I'm not sure how much heat this would allow me to capture, but seems like it would be cheap (I already have most of the materials to build something like that) and probably better than nothing. And it might be able to smooth out some of the summer temperatures, too, to have large barrels.

hau Timothy, have you thought about painting the driveway covered by the green house black so it becomes a better heat sink for wintering trees and plants?

You could also use black 55 gal drums or the smaller 30 gal drums to store solar heat for winter temperature inside the greenhouse, just stack them or make a single row around the bottom outside the greenhouse to get some extra heat. (use a thermal blanket to cover them and force more of their stored heat into the interior of the greenhouse)

My wife suggested that since I referenced my unheated cold frame, that I put a picture of it here, too.

Predecessor to greenhouse, the driveway cold frame:

The cold frame is made from 1x6 cedar fence boards, with double-paned windows sitting on top. It faces south and sits against the southern foundation of my home. Unfortunately, it receives no sun at all November - January, due to the positioning of my neighbor's roof. In previous years, it also had a 1-inch gap between the back of the cold frame and the foundation wall that probably made it not work as well as it could have. I have somewhat mitigated this gap with some garage-door weatherstripping around the edge. During the spring and fall, I manually prop the windows open in the morning.

In previous years, it has provided enough protection to keep a couple of serrano pepper plants, some from-seed citrus and a couple of pineapples alive. It also worked well for starting tomatoes and peppers, which I set out in it in mid-march. I intend to keep using it for things like that.

This past summer, I used the cold frame to aggressively warm some peppers that I potted in large pots of aged wood chips, weeds, and garden debris. I kept the windows on (but open during the day) until the hottest part of summer arrived. This boosted temperature and water retention from containers of wood chips resulted in some of the largest pepper plants I have been able to grow in this climate. I will likely write up my experience growing in large containers of aged wood chips in a different thread in the future.

Bryant RedHawk wrote:hau Timothy, have you thought about painting the driveway covered by the green house black so it becomes a better heat sink for wintering trees and plants?

You could also use black 55 gal drums or the smaller 30 gal drums to store solar heat for winter temperature inside the greenhouse, just stack them or make a single row around the bottom outside the greenhouse to get some extra heat. (use a thermal blanket to cover them and force more of their stored heat into the interior of the greenhouse)

Hi Redhawk, thanks for the ideas. I hadn't thought of painting the driveway black, but that definitely seems like it would make sense and probably could provide a pretty significant boost for fairly low effort. And now that I have the min/max thermometer, I can quantify that improvement.

I have been thinking about black drums, probably stacked on the north inside, under the potting bench. I have a couple of 55-gallon ones, but they're white. I may try painting them to turn them into black drums, or I think there are a few places nearby where I could pick some up pretty cheap. It's tricky to stack things on the outside, as I don't have very much space on the south side of the greenhouse, and the north side abuts my (narrow) path to get my bicycles into my bicycle garage, so when I eventually put some thermal mass, it'll likely be on the inside.

I probably should put some gutters on it eventually, a) to protect the lower edges of the wood wall frames from perpetual wet and rot, and b) to collect irrigation water to use inside, and I could probably position the water collection barrels on the west exterior wall (where the wood chip pile currently is).

I was also thinking of putting black insulated panels on the north wall, since the glazing doesn't really do any good on that side. I'm holding off on this until I collect some more data about the temperature performance and add thermal mass barrels.

I like RedHawk's idea of painting the cement around the greenhouse black or another dark color to absorb some warmth. Then, in the summer, maybe cover it with wood chips to minimize heat absorption?

We got our 14 x 60ish greenhouse the same way- just having to dismantle and bring it home. It was a full day's work, but definitely worth it (to me).

Do you ever get strong winds in your area? I learned that securely anchoring them to the ground is necessary in Central Texas. My first greenhouse was a 12 x 36 hoop style house, and parts of it are still in the big pecan tree above my parents' house 2 years after a strong, early spring wind picked it up (including the t-posts it was secured to), and tossed it in the tree. When we got the current one to replace it, we bought some 6x6's and cemented them in the ground every 12' along the sides. That's worked pretty well as an anchor, but the poly panels sometimes get blown loose (but not totally off), mainly because they're old and tear easily.

I'm excited to see how your project goes! Right now, ours is heated by a gas heater for the tropicals my mom & I grow for our market plant booth (my dad works for the propane place so they get a discount). Eventually I want to put a greenhouse in at my house at the back of the property, and would likely not heat it (since I don't need another expense right now). I was thinking about thermal mass via water barrels, but I worry about them taking up space, since my greenhouse will probably be smaller than the other one. Please keep us posted on your progress!

Or just put down black landscaper fabric or black garbage bags. Whatever you have available. I like reusing the garbage bags I get leaves in, if they're in good shape, but for this "good shape" isn't going to matter. You just want a heat-sink.

My greenhouse is built into a hill and has a waterwall inside. The waterwall wouldn't take up as much room, if you want to go that direction. I just used gallon milk jugs, which sit on 2x6s (scrap wood from rebuilding the garden boxes).

Unfortunately, I haven't updated this as frequently as I intended to. I'll try to catch up and retroactively add some pictures, lessons learned, successes and shortcomings.

How much protection did the greenhouse provide during the winter?
I recorded the daily min/max temperatures from last December through June. The greenhouse seemed to provide approximately 2-6F degrees higher minimum temperature, as compared to the minimum temperature as recorded at a weather station a quarter mile away. A thermometer that records temperatures every few minutes, and a similar one outside the greenhouse, would give a lot more useful than the simple min/max thermometer that I have, and wouldn't have been much more expensive. Last winter was exceptionally mild, with only 4 days with temperatures below 30F at the nearby weather station I used for my exterior temperature data. On the coldest night, it was 27.2F outside and the inside of the greenhouse saw 30.7F.

Outcome of overwintering efforts
Many chili plants succumbed to cold damage after the night when the greenhouse got down to 30.7F. Several appeared to be limping along, before dying back fully weeks later. Three Chiltepins survived just fine, as did a couple of capsicum annuums. My largest "habanada" (heatless habanero variety) died, which I suspect was due to my leaving fruits on it to ripen throughout the winter.
Several from-seed citrus trees between 1 and 2 years old, and a 2-year-old avocado survived these temperatures.

Seed Starting
This was the first year that I had the greenhouse space for seedling starting. For some plants, this worked very well, for others not so much.

I pre-germinated tomatoes and peppers on a heat mat in early February, and by mid-February, I had potted them in small pots and set out on their own in the unheated greenhouse. In retrospect, this was probably too early to put them out in an environment where it was getting down close to freezing most nights, because they just sat there and didn't grow at all for a month or two. But they survived and did very well once the days got a little bit longer. Peppers got off to an even slower start, probably because the soil mix I was using for them  (approximately 3 worm castings : 3 coir : 1 perlite)  was too heavy and moisture retentive for such cool temperatures and low light levels, and never was able to dry out to the levels that they wanted. When I repotted them in a sandier mix, they did much better.

Ironically, some of the best tomatoes and peppers were unintentional volunteer plants that sprouted from worm castings from my worm bin. Peppers and tomatoes were the most prolific "weed" coming up from everything that I used the worm castings in. I saved some of the most vigorous pepper volunteers, and they eventually became my first ever successful bell peppers. One of them went on to live its life in a large "rootmaker air pruning pot" in the greenhouse throughout the summer, and is now 3 feet tall, very bushy, and has produced at least 6 grocery-store-sized red bell peppers and is in the process of ripening two more as of today, November 12th.

Starting cool-season crops mostly did not work very well. I started brassica, lettuce, and similar around March, planting them out in the ground through April and early May. For the most part this resulted in two outcomes: 1) plant gets immediately devoured by slugs (or it doesn't start growing quickly enough, and gets slowly devoured by slugs), or 2) plant immediately bolts (I think this may have had to do with the seedlings being exposed to too high of temperatures in the greenhouse, and then being planted out into cooler weather, thus triggering the "this is the second year, better make seeds right now" response). Regarding the second problem, the greenhouse was overheating drastically, hitting 90F+ degrees any sunny days from February on, even with the door open. In April, I had enough of 105F+ days, and got a 40% shade cloth that I attached over the outside, and which I left on through approximately October. I think that putting the shade cloth on earlier (and even rigging it up to be able to retract it on cloudy days, maybe) could help with the poor-quality cool-season starts, as would figuring out the right timing for starting them and putting them in the ground.

Early harvests
I had a lot of large grow bags and containers in which I planted warm-season crops very early. Many of these had good success, at least initially, and fruited vastly earlier than conventional in-ground outdoor growing. For instance, I germinated zucchini at the beginning of March, transplanted it Mid-March into a 7-ish gallon nursery pot filled with rotted wood chips, worm castings, weeds, and potting material from last year, manually pollinated the first flowers in early-mid-May, and harvested the first (10-inch) zucchini on May 20th. This is an earlier harvest than I would otherwise have even been planting a zucchini. These early zucchini plantings produced a few early fruits, before being rapidly obliterated by powdery mildew. A similar progression happened with cucumbers, with a smallish early harvest (which I had to hand-pollinate), and then obliteration by disease.

Attack of the spider mites
I made a mistake and plated scarlet runner beans in the greenhouse. This resulted in a massive wall and canopy of dense foliage, and a total of two bean pods, due to the pollinator requirements and pollinators not finding their way in through the door and the vents. The canopy was nice once the sun got more intense, and really kept the temperatures down during the hotter months. Before long, though, these were also obliterated, this time by spider mites that had completely defoliated the entirety of the beans, cucumbers, tomatoes, and the second round of zucchinis I had planted to replace the ones that died from powdery mildew. I had exacerbated the spider mite problem by trying to water things less, thinking that I was preventing fungal diseases. After an unsuccessful few attempts at using neem oil to halt the spider mites, I solved the problem by watering everything heavily each day to reduce drought-stress, and by blasting every plant daily with a hose sprayer nozzle.

Cucumbers that you don't have to hand-pollinate
In the late summer, I shelled out for the parthenocarpic gynoecious greenhouse cucumber seeds (variety called Socrates, costing an exorbitant nearly $1 per seed). I believe these were also supposed to be resistant to the diseases that destroyed my previous two rounds of cucumbers I had tried. These were truly a morale-booster on the cucumber front. They delivered massive yields without any hand-pollination, and continued production until last week when temperatures in the greenhouse dipped to 31F. The downside is that since these set fruit without pollination, they really make no seeds, so I am stuck paying $1 per seed if I want to keep growing them.

Greenhouse is a lot of work
I guess I expected this, but since it's a mostly closed environment, diseases and pests can make quick work of destroying things. Monitoring the temperatures and watering things all the time are a pain, too. Doing a greenhouse this way is not really a good way for a lazy person to garden. There are probably ways one could do some of this smarter to not have to work as hard at it, and set up better systems that require less intervention all the time. But maybe that's also just inherent to growing in a greenhouse.

I will try to write soon about what I have going through the winter, and my plans for how I'm going to more successfully keep tropical stuff limping along through the cold weather.

Thanks for the update, a huge success overall!

Regarding cucumbers:
I have less tomatoes in the greenhouse due to lower air flow (I know I should shake the stems regularly) but no problem with cucumbers, kiwano or other cucurbits. When the door is open or even a small ventilation window there are always enough bees and hover flies. I even have small dragonflies in the greenhouse from time to time that feast on the little flies.
I usually also have some flowering cilantro that help to bring in pollinators.

The cucumber type I grow was originally called Red Hmong but I suspect it crossed with a Russian White cucumber. Anyway, it is adapted very well to my climate and keeps producing like mad (we had cucumbers coming out of our ears as we say here).
So you could probably try with a cheaper variety of cucumbers similar to mine, they are not that long but a bit stumpy and very tasty. One dollar a seed is crazy, I get my cucumbers mostly from volunteers that sprout in the greenhouse after some cucumbers fell down to rot.
The cucumbers in the first picture are turning red, but you can eat them in all stages. When green they taste like delicious little cucumbers. I also use the young one for fermenting.
The second picture shows this year's mix, from three or more plants: Some probably white russian, some Red Hmong crossed with Lemon cucumber (they are pale and then very soon turn yellowish but they are still tender and tasty at that stage)

Keeping track of temperatures is going to be important. I have kept track of temperatures in my greenhouse since I built it, so I know I have an approximate March 1 last frost date inside, or at least it has never gotten below freezing after that date. First frost is less predictable. The first two years I had tomatoes in December, then last year we got a mid-September freeze that killed everything (got down to 8 degrees outside, 24 in the greenhouse) and this year we got the mid-September freeze but it didn't completely kill everything. One tomato came back from the root, one cucumber is still alive but only producing male blossoms. I have not had success with curcubits in the greenhouse, but I keep trying. I grow everything in-ground, so that makes a difference.

If you have something that really thrives in the greenhouse environment, keep seeds. Volunteers will save you, particularly multi-generational volunteers. Keep flowers or some kind of flowering perennial in the greenhouse at all times to draw in the pollinators. Make sure they have water in there as well. I have several herbs in-ground that flower every year, but I also have two doors which helps with the temperature problems as well as the pollinators.

Another thing you may want to consider is predators. A few years ago I was having an aphid problem in the greenhouse. I found ladybugs that had just emerged in the spring and soon had no aphids and pupating ladybugs on every surface. Now I'm seeing dragonflies and other predators in the greenhouse as well. Make it an environment they appreciate, and they will come. Many adult predator bugs also need pollen, so flowers for them as well.

Make each year a test. Try new things, and if something works well build on that.

In response to the previous two replies, yeah, it seems like attracting the pollinators and the predatory insects should be a focus. I haven't really devoted much space to ensuring that there's attractant plants blooming for them, and that would probably help entice them in more. Since this greenhouse is on a concrete driveway, everything is in containers, and I've been adding more containers outside and around the greenhouse on the concrete driveway. I should probably try intentionally growing some beneficial-insect-attractant plants in the containers right outside the door, too, since maybe they'll find that easier and then venture inside if I keep a good amount of flowering plants inside, too.

Update on things currently growing
The previous update was about how last winter's overwintering efforts, and the warm season that just came to a close. In this post, I will document what I am growing currently, and what I hope to accomplish during this winter.

Warm-season stragglers, and trying to keep them alive through the winter
I still have tomato plants that have substantial stems growing up the wire trellises on the walls and up into the rafters, but the leaves have all gotten diseased and I have chopped them all off at this point. A few fruits remain on the plants, and have been slowly ripening. My hypothesis is that if I can keep the temperature above freezing, even if I chop all/most of the leaves off, that the tomato plants will put on new growth early next year, which can be rooted as cuttings or grow as a continuation of the original plants. I'm not sure if prolonged exposure to near-freezing overnight temperatures for the next 5 months will take a toll on the plants enough to kill them, or if they are so stricken with disease that all new growth is going to be heavily diseased.

There are a lot of large pepper plants that still have a lot of pods ripening on them. I suspect that last year, leaving the pods on to ripen took all of the plants' energy that could have helped them overwinter, resulting in many losses after some slightly-below freezing temperatures (which some pepper plants without fruit were able to survive). This year I am trying to provide an additional layer of protection for the large pepper plants on particularly cold nights. The guy who rents my basement gets his groceries delivered each week in a box, inside of which comes a large metallic-bubble-wrap bag to keep his groceries cold, and I have been collecting his metallic-bubble-wrap bags. These bags are the right size to slip over a plant in a pot up to 18" diameter, and with a height above the rim of the pot of also about 18", and I suspect will give a little more protection. So, for the peppers plants that are still large with fruit, I will cover with these bags and then take them off in the morning.

Many other pepper plants that were finished fruiting or had been growing in the ground outdoors I have chopped down drastically and put into tiny pots. I put these tiny pots inside of large transparent rubbermaid tubs with lids that I found by the side of the road. The bins then sit inside of the greenhouse. My plan is to leave the lids of these bins open for airflow most of the time, but when the weather is supposed to get extra cold, to close them up. The idea is that this should provide another layer of protection to the many pepper plants inside the bins, and provide a further buffer against the infrequent case where temperatures inside the greenhouse drop below freezing.

Cool season plants
I've tried more cool weather plants inside the greenhouse this year. I've got some carrots, some of which appear to be doing okay, and many lettuces, mustard green, and assorted brassicas. Last year, my cool weather plants struggled when spring came and the sunshine cooked everything in the greenhouse, but I will put up the shade cloth when the sun gets strong in the spring and hopefully keep the temperature more even. I'm also trying some shallots and walking onion, for comparison to those that I planted outside.

Much of this stuff will likely get infested with tons of aphids over the winter. My usual aphid strategy of "blast them off with the hose every day" probably won't work during the winter, because everything will stay too waterlogged in the cool temperatures and low light. And there are minimal predators even outside to attract in to gobble them up. So, it might not be ideal, and could result in many sad dead plants. I haven't figured out a good strategy for dealing with winter aphids in the high-maintenance environment of the greenhouse.

Consider going on a predator hunt. If you have soldier bugs in your area, or late season ladybugs, or dragonflies, set up a tiny habitat for them and see if you can find any to transfer into your greenhouse. Probably easier to find are various spiders, although I'm not sure they would thrive.

If you can get the aphid populations under control during the growing season you're probably better off, as their major expansion requires higher temperatures. If they have a continuing source of food the predators may never choose to leave, although in a greenhouse that would be a massive infestation. Insect predators will eat a LOT of aphids.

Adult ladybugs will overwinter under their favorite plants, then emerge in the spring with a good source of food to lay their eggs. If you have blooming plants inside (they prefer flat flowers, like dill or yarrow) they may choose to overwinter there.

I definitely like the idea of introducing natural predators.  But I would do it by creating an entry/exit hole.   Especially for dragonflies, who require a massive amount of airspace to thrive.  They fly about half a mile up and can migrate up to 11,000 miles.  Their diet is mostly mosquitos (over 100 per day.) They also use altitude to thermoregulate.  A greenhouse would be a very poor fit for a dragonfly.  But the concept of introducing predators is great.

You could hook your barrels up to something like a the DIY Pex Solar Water Heater.  Connected properly where it can pull the colder water from the bottom and dump in the heated water at the top, you shouldn't even need a pump.  Convection would handle the circulation naturally.  It might take some tinkering to get the coil of pex tuned to provide enough heat but not too much heat.  I think if you paint your driveway now, you might regret it come summer or time to sell the house.

DIY Solar Water Heater