We built a passive solar GREENHOUSE | Here’s what happened

18 months ago we built a new passive solar greenhouse to try and retain more heat in our indoor growing area and extend the growing season. The greenhouse is 35 feet long by 12 feet wide and is sited on a slightly sloping hill not far from the banks of the River Severn. In this video we'll show the whole story of how we made the switch from a 23 year old polytunnel into a brand new growing space for fruit and veggies in our organic garden in mid Wales in the UK. Where we live we enjoy a temperate climate. Our latitude is the same as parts of Canada but due to the influence of the Gulf Stream crossing the North Atlantic we experience much milder weather. That said our position in the upper Severn Valley means we live in something of a microclimate. Our garden is close to the bottom of the steep hill rising above the river and on cold nights frost rolls down the hill and straight into our patch. This has proved very challenging over the past two decades of growing in a polytunnel as even late in spring, say early May, we can experience sudden hard overnight frosts. Temperatures can drop suddenly even undercover so growing tomatoes and cucumbers was often very difficult as the cold overnight temperatures stopped them in their tracks. So when the old polytunnel came to the end of its life we decided to experiment with a new approach. Our aim was to try and hold more heat inside the greenhouse overnight than had ever been possible with the polytunnel with its thin layer of plastic. Before starting work on the new greenhouse we did some research. We spoke to friends who'd converted barns into greenhouses. We looked at innovative polytunnel designs with walls built into the back of the structures and watched many helpful YouTube videos. We eventually decided to adapt a traditional wood friend greenhouse design by adding a solid back wall to soak up the heat of the day and then radiate this heat out through the colder nights to stop the temperature dropping so low it impacted on the plants. Our polytunnel site was orientated west to east which meant that the new greenhouse's long glazed side would be facing directly south which really helped. This meant the block wall at the back on the north side would catch a lot of sunlight during the day and especially during winter, spring and autumn we felt this would be a big benefit. Before starting work on the new greenhouse we had to tear down the old polytunnel. Amazingly it had survived 23 growing seasons so turned out to be incredible value for money but it was time for a change. This was a big project for us so we hired some local builders to help us out during the building stage of the greenhouse. They started by digging some new foundations. They had to dig a lot deeper than we expected as the hillside was of stony shale and rather unstable. The first stage of the build was adding the foundations and the concrete block wall at the back. Over half of this wall was built into the hillside giving an even greater degree of insulation to the greenhouse. Then a curving concrete path was added and a small concrete pad at the far end of the greenhouse where the workbench and storage area would later be constructed. A wooden frame quickly went up next followed by the roof and by now we were starting to get a feel of what the new greenhouse space would be like. Next the doors went in at each end plus some windows along the front and ends of the greenhouse. We needed to create as many options for good ventilation as we could. We had decided to glaze the greenhouse in polycarbonate for the added insulation value and this was the final piece of work done for us by our local builders. And here's what the new greenhouse looked like so far. So we said goodbye to our builders and all the work was now over to us. Our first big job was to give the entire greenhouse a couple of protective coats of an oil-based timber treatment. We then spent the rest of the autumn and winter setting up the greenhouse ready for use. In the process we had to dig out barrel loads of stones from the ground which we used to make a pathway up to the greenhouse from the rest of the garden. We built new garden beds and lined them with river rocks and brought in a mixture of topsoil and compost to fill the beds as the thin soil in this location is very fertile. Amazingly we had been able to save a mature fig tree and small bay tree from the polytunnel days. We decided to add a small peach tree to be trained along one of the bays in the back wall. On another bay we planted a grapevine to replace the one we had been unable to save from our old polytunnel. Once the beds were finished we installed our irrigation system using drip pipes and a battery-operated timer system. The water comes from a natural spring in our garden and is gravity fed to the greenhouse. Outside we built a curved dry stone wall to retain the earth bank at the back of the greenhouse. And then it was time for the really exciting part of the project, our first growing season in the new greenhouse. Here briefly is what happened month by month. March, most of our tender plants were propagated indoors in their house and transferred to the greenhouse once they had been potted on as small plants. First courgettes and squash plants were transplanted into the greenhouse on 24th of March. So the first big test came less than a week later as our new plants were acclimatising to their new environment. On the 31st of March there was light snow overnight. The minimum temperature outside was minus 0.5 degrees centigrade. The minimum inside was 6.2 degrees centigrade. Our first seed was sown directly into the greenhouse beds, including spinach, dwarf french beans, spring onions, lettuce and nasturtium. April, the workbench came into its own at the start of April. Using seed trays we sowed cabbage, kale and early sprouting broccoli, which whilst started in the greenhouse would eventually be hardened off as young plants and planted outdoors. We also continued to sow seed directly into the greenhouse soil, including pak choy, rocket, basil, radish and climbing french beans. By this stage we had erected a wigwam of hazel pods in two trellis frames of builders reinforced mesh. Young parsley plants were transplanted into the beds, both flat-leaved and curly parsley. We used two pallets to create a makeshift table outside the back door of the greenhouse for hardening off the young plants that were destined for the outdoor garden. The coldest night so far was in early April, minus 3.4 degrees centigrade, but the greenhouse temperature stayed at 4.2 degrees centigrade. We harvested radish, parsley, flat and curled, lettuce, spinach, coriander, nasturtium leaves, fennel and bay leaves. May, the first task on our early Maybank holiday was to put up strings and plant out our young tapato plants. The plants looked very small and not the best at this point. Little did we know at this stage exactly how much they would grow over the coming season. Additional strings were added to grow cucumbers up to the greenhouse beams. By the middle of May temperatures were increasing in the greenhouse and windows were routinely opened along with doors on hot days. On a 10 degree centigrade night the temperature in the greenhouse did not dip below 17 degrees centigrade. We began leaving the windows ajar overnight. The lowest temperature for the month was six degrees centigrade and the highest 28 degrees centigrade. We harvested pak choy, courgettes, spinach, rocket, parsley, lettuce, radish and nasturtium flowers and leaves. June, seed sowing continued on the bench. Chard, chives and runner beans, all plants destined for outdoors once established and hardened off. The tomatoes, squashes, cucumbers and french beans were growing like crazy in the greenhouse by now, flowering and producing the first small fruit. We spent time tying them in and nipping out unwanted shoots. Wasps had found the greenhouse and began building their nest in a very sheltered spot above the front door. It was intriguing watching them work with chewed up wood and saliva to make wood pulp. They never bothered us at all throughout the season, even though we were regularly working just beneath the nest. The lowest temperature for the month was nine degrees centigrade and the highest was 36 degrees centigrade. We harvested courgettes, dwarf french beans, pak choy, radish, parsley, kale, nasturtium, fennel and bay leaves. Growth continued at quite a pace throughout July when the weather was exceptionally hot. The bush tomatoes started to spill out over the path and we didn't have the heart to cut them back. The squash runners were extending right up into the greenhouse roof and the cucumber plants all the way up the walls. The wasp's nest was by now enormous and there was much activity as worker wasps returned with food for the growing larvae. We were fascinated and watched with great admiration. Wasps are great for eating garden pests like aphids and also pollinate flowers and crops, so we are quite happy to welcome them into the greenhouse. The lowest temperature for the month was 11 degrees centigrade and the highest was 38 degrees. We harvested spring onions, courgettes, basil, climbing french beans, cucumbers, nasturtium, parsley and the first figs. August. In August we were away for a while. The irrigation system coped perfectly well on its own so the plants continued to romp away and the tomatoes went what can only be described as a bit wild. By the time we were back they had taken over part of the greenhouse by the front door. We decided to let them be and see how the tomato harvest progressed. The lowest temperature for the month was 10 degrees centigrade and the highest was once again 38 degrees centigrade. At the beginning of the month we harvested squash, turk's turban, cucumber, lettuce, basil, climbing french beans, kale, nasturtium and figs. By the end we added our first tomatoes and a heritage seed library variety of squash, Georgia candy roaster. September. Some of the climbing squashes had started to die back by now allowing life to others still roaming and fruiting along the ground. Tomatoes dominated the scene still along with the fig tree which had put on masses of new growth with figs sprouting all along the branches. The lowest temperature for the month was eight degrees centigrade and the highest was 32 degrees centigrade. At the beginning of the month we harvested tomato, Spanish big globe, a heritage seed variety, squash, climbing french beans, cucumbers, parsley, figs and by the end of the month endless varieties of tomatoes and more squash. October. Autumn was definitely on the way outside the greenhouse but inside there was still plenty of action. Tomatoes, figs and some squash were still growing furiously. The lowest temperature for the month was eight degrees centigrade and the highest was 34 degrees, actually slightly higher than September. We harvested figs, tomatoes, our first butternut squash and climbing french beans along with herbs. November. Many of the summer crops had come to the end of their season by now and we finally cut back and pulled out most of the tomato plants, french beans and squashes. The lowest temperature for the month was four degrees centigrade and the highest was 28 degrees. We mainly harvested herbs and figs plus the last of the tomatoes. December. The lowest temperature for the month was minus three degrees centigrade and the highest was 20 degrees centigrade. We harvested herbs, several butternut squash, the last of what had been an absolutely amazing fig harvest over the season, well over a hundred fruit in total and then just before Christmas the last crop of the year was a surprise find of new potatoes from self-seeded potato left over from the polytunnel soil. All in all we were pretty impressed with the way things had gone for our first greenhouse season. Looking back on performance we focused on a few key areas. The greenhouse design had a big impact on the temperature we were able to maintain in the greenhouse. It was quite impressive. Throughout most of the season we monitored the minimum and maximum temperatures. The highest temperature recorded was 38 degrees centigrade in July. 2022 turned out to be one of the hottest summers on record in the UK. In July there was a heat wave across the country and even in normally cool mid-Wales temperature shot up to 34 degrees centigrade outdoors on the 18th of July. Until December the lowest temperature recorded was four degrees centigrade. Then after a relatively mild autumn on 10th of December the country was plunged into an icy blast of weather nicknamed the Troll of Trondheim for a whole week. For the first time since it was glazed the greenhouse internal temperature dipped to minus three degrees centigrade. It was minus 10 degrees outside. These were the coldest winter temperatures we had experienced here in over 10 years. So for our first greenhouse season we experienced some of the highest and some of the lowest UK temperatures for over a decade. To be honest we couldn't have asked for a better test. During an average winter it seems very unlikely that the temperature would ever drop below freezing in the greenhouse and during early to mid-spring when temperature is most critical to young seedlings and plants they never drop below four degrees centigrade. Good ventilation is really important in the greenhouse environment to help prevent mould forming on plants. Whilst during most of the summer the ventilation provided by our two doors and six windows was adequate the 2022 heat wave caused some greenhouse plants to droop a little. We decided to cut up some of the metal hoops left from the old polytunnel. These small sections of pipe could be slotted onto the metal window latches which allowed the windows to be opened much wider for increased air circulation. At the top of the block wall we had added ventilation openings with removable covers just below the roof. These had already been removed for the growing season. We're not entirely sure how much they help but hopefully they improve air circulation higher up in the greenhouse. During periods of high humidity and also on cold nights after sunny winter days we do see quite a bit of condensation forming on the roof panels in the greenhouse. This did result in some mould on some of the more densely planted tomato plants during the summer but overall it wasn't a serious problem. The irrigation system we installed before the start of the new season worked perfectly for us. We set it to come on for two hours each morning initially but as the ground grew wetter we reduced this to one hour each day. The only additional watering we had to do was on some odd corners or stretches where the pipe didn't quite reach and we watered all the seed trays and young plants in pots on our workbench by hand using a By December we had dismantled and drained the irrigation system and were very pleased we had as by then the exceptionally cold weather could have caused problems with burst pipes. As it was our first season we had very little trouble from slugs and snails as they had yet to find their way inside the new greenhouse. However we did have some issues with green fly early on in the season. We did our best to control these by spraying soapy water onto the affected plants and rubbing off the green fly. The issue was at its worst in May so quite early in the season. Later on the plants grew so quickly they seemed to outgrow the green fly particularly as we helped to keep them under control in the early stages. Our resident wasps may also have helped to keep the agar population under control. So with the end of our first season in the new greenhouse it's been good to reflect on how things have gone and what we can learn for future growing seasons. Perhaps the main lesson is plants really like this environment so we don't need to grow so many as they can go a bit wild if left untended even for a short period. Fewer plants means better spacing and improved ventilation and less time is required to keep on top of the pruning. We're very pleased to have added the back wall in the greenhouse construction and believe it makes a massive difference to maintaining a decent temperature. It's also a great space up which to train trees, vines and climbing crops. We're looking forward to experimenting with different crops over the coming seasons in our greenhouse. It's a wonderful place to work especially at the beginning and end of the season when the sun warms up the space beautifully and gives us a cozy place to garden even when it's cold outdoors. Thanks for watching and happy greenhouse gardening all!