Temporary Raised Bed Greenhouse

The first fresh picked garden tomato is an annual treat. Unfortunately with our higher elevation, late spring and early fall freezes and snows, the growing season can be short and the ability to harvest vegetables even shorter! Getting fresh tomatoes from early August to mid September is about the norm. So my solution was to build a temporary raised bed greenhouse that covers the bed in spring to ward off the freezes and the snows. This allows my tomatoes to grow and make fruit earlier in the season. Please note that this is where things get a little over the top for my gardening. As I have spent extra time and money to create the concept. But it works. And, by planting tomatoes in early to mid April (4-6 weeks BEFORE the last frost), you can harvest the first ones in late June until mid September. That’s 11 Weeks of fresh tomatoes instead of the more normal 6 weeks!

The concept starts with a good solid raised bed base. I made my south raised beds from 4×4 Douglas Fir 13 years ago. Back then, the treated wood available still had arsenic in it and was not ideal for use in vegetable raised beds. The Douglas Fir has held up pretty well, but after 13 years there is some rotting and deterioration. (My new raised beds are built with 4×4 garden safe treated lumber.) The raised beds are 5 foot by 10 foot, outside dimensions. They are level and have PVC sleeves spaced and “built-in” around the inside perimeter. The PVC sleeves allow me to easily install and remove my temporary greenhouse with ease. The bed walls also have “caps” placed on top of the 4×4 sides that enhance the appearance and help provide a seal between the greenhouse and the raised bed.

The raised bed greenhouse is a PVC framed structure with solid end walls. The PVC structure and the end walls are designed to set into the PVC built-in sleeves for stability. The covering for the greenhouse is a twin layer of plastic sheeting. A blower inflates the space between the layers, providing insulation. The solid end walls are cedar insulated walls on the lower section and clear Plexiglas on the upper section. One end wall has a vent fan and the opposite end wall has a motor operated louver and two smaller dryer vent louvers. By using separate thermostats and sequencing the operation of the vent fan, and the motor operated louver I can control the high temps that occur in the greenhouse on sunny days. For the cold nights, I have two small thermostatically controlled 1500 watt electric heaters. One primary that runs every night and one secondary for extra cold nights. Once erected, I use a series of hold down straps and bungee cords that connect to the raised bed foundation. These stabilize and hold the greenhouse down in high winds. This is very important, as we usually get a spring blizzard that tests the temporary greenhouse strength. The raised bed greenhouse has withstood the high winds, snow and severe cold without any problem. All while the tomatoes are growing safe and snug inside!

I sow my tomato seeds in my sun room the first week of February. Transplanting them into a 5 inch and then a 9 inch pot so that I eventually get a 8-9 week old tomato plant. I then plant the transplant into the raised bed sometime during the first two weeks of April. I am careful to make sure that I pick a planting weather window that has some consecutive warm days. There is always a chance of a significant snow that can delay the planting but so far I have had good luck with the calendar. (See my post on Transplant Shock and Blossom End Rot to learn more about how I transplant.) But, one key for planting early is warm soil, and I use hot water to make that work. The whole process of planting 8 plants, adding the tomato cages, wall-of-water and installing the temporary greenhouse takes about 4 hours. The raised bed greenhouse is designed to allow the plastic cover to be rolled up to let the plants get some direct sun and air on warm days. I remove the temporary greenhouse in late May and if all goes well, am ready to harvest in late June!