I love greenhouses and their potential. I've always found, though, that smaller "hobby" greenhouses don't live up to their hoop-la. The small size makes it difficult to control temperatures -- too hot in the summer (even with "venting") and too cold in the winter (little heaters heat uneveningly in such small houses). And, maybe more importantly, the small size doesn't allow for good placement of plants, enough space to actually grow anything in worthwhile quantity, and good traffic flow. They almost always quickly get stuffed with miscellaneous items just for storage. Along with spiderwebs and dust. With that said, get the biggest one that will fit your site. I've always told my gardening and design students that the minimum for even a "hobby" greenhouse is 14 by 21. That allows for side benches and a middle "work" table/bench. Use the greenhouse for seed germination (flowers, veggies, specialty plants, etc.). Move transplanted seedlings out to a tall coldframe to actually grow them on and harden them off. Grow tomato plants in the ground. There are hundreds of cool-season/short-season tomato cultivars that will do well in Sonoma. In a greenhouse, you'd have to grow them in small pots and tomatoes don't really like small pots. Grow lettuce outside in the winter and hose off those aphids. Lettuce grown in a greenhouse suffers tremendously from the occasional overly-hot winter day (compounded in the greenhouse). Grow some just-too-tropical plants that need a little winter protection and a little more heat than your summers provide: various orchids, bromeliads, epiphyllum, Eucharis, begonias, plumerias, and so much more. Joe

Where I live, the outside temps can get down to 10F at times. I have an unheated greenhouse, so at least the plants are protected from wind. But the inside temperature at night is the same as outside. So I cover my tender plants with plastic film, and put a mist nozzle on top and let it run all night. This keeps the plants very close to freezing temperature, which seems to be good enough for them to survive. This works with outside temps down to the low 20's F. If you wanted to do it right, I guess you could turn your greenhouse benches into little hoophouses and rig the mist nozzles above them. If you don't have a greenhouse, you could cover the plants with plastic and rig a mist nozzle. But you have to make sure that the wind doesn't just blow the mist away. If wind is a problem, you could use a nozzle that makes a larger droplet size. But it will take more water. Or you could move all your plants into the house or crawl space a couple of times a year.

Nick, the average tomato plant won't set fruit below fifty or so degrees F, so to get much production you'd have to keep the temps around them higher than that (not sure if that can be done without a greenhouse, or a rather extensive structure of some type). There are some types that are more likely to produce in cooler temps ~ Siberian types, one called "Oregon Spring", etc. ~ so I'd grow those for sure. If you put them against the south side of a large building (preferably your house), that will help as well. And I don't know if daylength has anything to do with it, but I surely wouldn't be surprised to find out it did. You may want to try some supplemental lighting on a few to test it out if you can't get a straight answer anywhere else. Since tomato blooms are self-fertile you won't have to uncover them to let insects pollinate, but you will need to move the blooms to do it. The natural action of the wind or an insect landing on the bloom is what triggers pollen to drop inside each flower onto the pistil (female part) and pollinate it. So either brush them with your hand repeatedly or install a small fan to move them around. Overall, and you probably already know this, but don't expect the same production as we get in summer. In cooler temps, growth of everything slows down, so it'll take longer for those tomatoes to form and ripen.

An electric space heater works great as long as it is one of those with a thermostat that will shut off when it reaches a certain temp. It should be sitting on something like a paver so it's stable and won't topple over. But if it should topple over it needs to be the type that automatically shuts off. Also, if you're using a heater, you need a fan to move the warm air around. Otherwise your hot air builds up in a hot channel, pooling at the highest point directly above the heater, and plants farther away from it will still freeze. I use three fans, the largest one is mounted near the ceiling peak opposite the heater and blows directly at the heater. This forces the warm air to circulate all around the GH. Four feet below it is a small oscillating fan that keeps air moving in constant random patterns. On the floor I have another small oscillating fan that keeps the coldest air in the GH from settling down low. My GH is only 10' x 12' so keeping it at a steady temp and moisture setting isn't very hard. If it was larger I'd probably use more fans. Generally though, it's not necessary to keep the temp any higher than just above freezing for most plants to thrive through the winter. The only ones you'd want to keep warmer than that would be seedlings and very tender tropicals. I have astigmatism, that's why my pics are all tilted, but you can see the swamp cooler and the heater mounted directly above it in this one. Cheryl