Heating the Greenhouse (continued)
We’ve got a small solar system on one of our chicken tractors that can be switched over to control the thermostat on the heater and the exhaust fan if we lose electricity long-term in the greenhouse. If we need to, we can move the woodstove back in, but for now this system works well. No matter what kind of heating system you choose, having a backup plan makes you sleep easier at night. At minimum have a kerosene heater and a few cans of fuel on hand that you can move into the greenhouse for a night or two if your main heat or electricity goes out. It will save a greenhouse full of plants and a season’s worth of food.
Temporary Greenhouses and Tunnels
Instead of a permanent wooden or metal-framed greenhouse, you can use a less expensive, temporary greenhouse design made from PVC. However, I wouldn’t recommend it, because they will only last a year or two and have no snow load capacity. The PVC will get brittle in the sun, and a good wind or a small snow will break them down. This will most likely happen when they are full of plants and/or there is a blizzard. Trust me, I’ve constructed several over the years, and the end result is always the same. Unless you absolutely can’t afford anything else, just save yourself the trouble and put up a more permanent structure. If you don’t want to frame up a wooden greenhouse, I would suggest getting a greenhouse bow bender from Johnny Seeds . These are less than one hundred dollars and you can bend the bows to construct several sizes and styles of greenhouses from readily available, and affordable, chain link fence piping.
They also sell low tunnel benders, which make short tunnels about waist high from ½-inch or ¾-inch electrical conduit. These are pretty much extended cold frames, which serve the same purpose. Low tunnels have become a mainstay in our production, as they are affordable and very quick and easy to put up and take down. We cover them with plastic early to get a jump on the season in the spring, and then use a woven floating row cover when it warms up. The clear plastic can heat up fast and kill whatever is inside without proper venting. So, if you aren’t going to be around to constantly monitor your low tunnels, then just stick to the woven covers, which will give you less heating but at the same time won’t overheat. The added benefit to these covers is the protection from insects.
Most of our early mustard and greens crops go under these woven covers to exclude flea beetles, which if left uncovered will be decimated without a lot of pesticide sprays. Use of these covers allows us to grow these greens perfectly without chemical sprays, which is worth their cost alone.
You can also use the lighter weight fabric without a frame, just letting it float over crop, thus it is sometimes called a “floating row cover”. In both uses the edges have to be weighed down to keep them in place and to prevent insects from getting underneath. We’ve found that using sandbags to weigh down the edges is the best option, as they don’t tear holes in the fabric. Just making use of this material alone will increase the quality and yield of your produce exponentially. This is one of the items that you should buy and lay up for a rainy day while it is still readily available. If protected from the sun it will keep indefinitely and is one of our most valuable resources on the farm.
A high-tunnel is essentially an unheated greenhouse in which you grow everything in the ground. Even a single layer of plastic allows you to plant about three weeks earlier in the spring and lets you extend the season at the end a month or more. Those extra weeks make a big difference in the availability and amount of food you can produce in a year. Late summer plantings of cold-season crops like collards, kale, chard, spinach, lettuce, and carrots can be made in the high-tunnel and harvested throughout the winter. You can put low-tunnels within high tunnels for the coldest climates. The plants don’t really grow in the winter, but rather if they are fully mature in the fall they are held in a living dormant state in what is essentially a giant walk-in refrigerator.
Most high tunnels have roll up sides or end-doors that you open up manually for ventilation. If you aren’t around during the day, you’ll need an automatic venting system, as a high tunnel can overheat and kill your plants, although not as fast as a low-tunnel. All of these structures need to be covered with special greenhouse film that is UV treated, or it won’t last. Treated 6-mil film is guaranteed for about four years, although I’ve left it on a lot longer. It just becomes opaque over time and lets less light pass through.
I’ll give an example of how your greenhouse and your high-tunnel allow you to exponentially lengthen your growing season and your potential food production. As I mentioned before, we are in USDA zone 6 and east of the Mississippi. If you are located in colder Zone 5 or warmer Zone 7, you can tweak the dates I use a bit, but there won’t be a huge difference, and most of the country’s population will find themselves in one of these three zones.
Let’s take tomatoes for an example, as everyone likes to grow them. If you want to direct seed tomatoes directly in the ground, you can do that around the end of April or the first of May when the soil is warm enough so that the seed germinates directly in the ground. From that direct seeding you will have a ripe tomato from most common 70-day varieties around four months later, or in late August/early September. The first frost comes in October, so you’ve got about six weeks of ripe tomatoes. You’d better grow a bunch at one time and have the capacity to preserve them if you want tomatoes to use year-round.
Now here is an alternative that we use to extend our season. Our first tomatoes get seeded on February 1st in the greenhouse. That gives us large, healthy, eight-week old transplants the first of April that go into the high tunnel. We get some frosts in April, but the unheated high tunnel is enough protection in most years; if it gets super cold, we can cover the plants inside with a woven frost blanket, or even move in a portable kerosene heater for a night or two, which has always been enough, since we’ve never lost our early crop to frost. We use a super-early tomato for our first crop. (We like Stupice or Polbig). By doing this we can have ripe tomatoes by the middle of May.
We continue to seed and plant a succession of field and high-tunnel tomatoes throughout the season with the last being seeded when the first tomatoes are ripening. The very latest varieties are transplanted into 15-gallon pots and moved into the now empty greenhouse. They’ll grow just fine there with a minimum and very efficient use of supplemental heat. If the weather gets really cold and it doesn’t make financial sense to keep burning propane, then we can harvest the rest of the tomatoes and move them into the basement. This is normally right around Christmas.
By using storage varieties (Burpee’s Longkeeper, “Reverand Morrow’s Longkeeper”, Giraffe, et cetera) that have a long shelf-life, you can continue to have edible fresh tomatoes for another six weeks or so, which gets you through January and into February if you are lucky. So then, you’ve got three months of canned/dehydrated tomatoes that will get you to your first ripe tomatoes in May. Having fresh tomatoes nine months of the year in our area is just one example of the worth of the investment in season extension.
For most, these structures will end up being multi-use. We often use our high tunnels in the fall for temporary animal housing. They clean up the weeds and fertilize the soil with their manure, which is tilled in and decomposed before we replant. We also use ours for drying crops in mid-summer when it’s too hot to grow anything inside. We use the better-insulated greenhouse for overwintering plants that need a bit of help, like our potted figs and semi-hardy herbs, like rosemary. Outside they might be winter killed, but in the depths of winter our greenhouse is mild even without heat. A greenhouse kept heated above freezing gives you access to those favorite tropical plants of ours, like tea, olive, citrus, or banana that may not be possible in your part of the country outside a heated environment.