My first experience with season extension structures was a cold frame. I have found cold frames to be of best use as solar powered plant starters. I grow lots of seedlings in my two cold frames, beginning with brassicas plants in February and ending with brassicas plants and lettuce in September. In September, I set out the kale, collard, and maybe chard plants into beds that will have a low tunnel over them for the winter and I space the lettuce seedlings around inside the cold frame. I harvest the lettuce on a cut-and-come-again basis until about Christmas. By that time it will have become somewhat tough and weathered, having been frozen and thawed many times, even with the glass lid on. I leave it go and by about February 10 I clean out the cold frame and plant the first seeds for spring transplants. One year I planted onion seeds there on January 10. I’d do that more often if I remembered to order the onion seeds in time. Each time I harvest seedlings, some soil goes with them. Each time I plant seeds, I add compost before planting.
Over the years, I have put collards and kale in the cold frames to harvest all winter. That was okay, except for the fact that in February it is still there going strong when I need the space to start seeds. I don’t start anything in the house anymore. About the third week of March, and for sure by the beginning of April, I have set out the cool weather plants in the garden, mainly brassicas, chard, and lettuce, leaving room for the tomatoes, peppers, flowers and whatever other warm weather things I’m starting in the cold frame. My last frost date is around April 25. The soil will warm up in the cold frame with the lid on and those warm weather things will sprout. I open the lid on warm days and close it at night. You don’t want to have cool weather crops still in the cold frame when you put in the warm weather seeds. By that time the lid needs to be off for the cool crops and it needs to be on for the crops requiring warmer weather. If their occupation time in the cold frame overlaps, you would do well to have two separate cold frames. Anyone who has started tomatoes and peppers at the same time knows that peppers take longer to germinate. They also like warmer soil. Usually I treat them the same as the tomatoes, but I’ve considered covering the pepper seeds with a cloche of some sort inside the cold frame to provide extra warmth. When the tomatoes and peppers come out, the sweet potatoes go in to produce slips. By that time the lid is put away for the summer and the cold frame is just a safe place to produce transplants. There always seems to be something to put in there.
I used to start my tomatoes and peppers on March 1 in the house under lights. They would require careful attention in the house, then more attention to harden them off so that they would be prepared for the elements when I set them out in the garden. Starting the seeds in the cold frame is a whole lot easier. The plants produced in the cold frame don’t need further hardening off. If too many seedlings are crowded together, I might prick them out and put them in pots or flats to have some more space to grow before putting them in the garden.
I used Four Season Harvest by Eliot Coleman as a guide for my cold frames. A friend gave me two 3′x6′ pieces of glass, so I made the cold frames to that size. The low side is from a 2×8 and the back is from a 2×12. I took Coleman’s suggestion to add a 2×2 along the bottom so that when it starts to rot, I could replace the 2×2. My cold frame sits on solid cement blocks that serve to raise it a bit and to protect the bottom from rotting. My cold frames are made of untreated pine and painted with latex paint. The 3′x6′ wood-framed glass lid is heavy and it takes two of us to put it away each year. I notched a 2×2 to lift the lid for venting, as shown in Four Season Harvest. I never hinge a lid to the frame. If you do that, the only way you can vent it is to lift the lid. Sometimes on warm days it needs to be removed completely. Cold frames can heat up quickly when the sun is shining. Sometimes it is to your advantage to just slide the lid one way or another to vent slightly to keep from overheating. Rather than have a large lid I think it is better to have two or more lids that can be stacked on the cold frame to allow venting, but still provide some protection. When they are removed in the garden temporarily, or stored away for the season, they wouldn’t take up so much room as one large lid. When one of my glass lids broke I used 2′ wide pieces of twinwall panels, also a second-hand gift from a friend, resting them on the cold frame and shifting or stacking them as needed.
Although I would like to not have any plastic in my garden, most years I use a low tunnel made from plastic hoops covered with greenhouse plastic to protect collards, kale, and chard over the winter. At 2 1/2′ tall it allows more space for the greens than the cold frame. I can usually harvest once a week, skipping a week in the coldest times. Here in zone 7, I’ve found that I don’t have to close the ends, although I keep a piece of plastic handy to put over the ends if it really gets cold. This structure protects the plants and doesn’t need as much attention as a cold frame. I transplant the winter greens in early September, but they don’t need to be covered until late October or early November. You want the rodents in your garden to find cozy homes elsewhere before you put the cover on. You can use regular construction plastic for the cover. It’s best to make sure it’s 6 mil thick. There are clips made especially for attaching the plastic to the hoops and I recommend them. I only need them for the ends.
My low tunnels have a ridge pole along the top, attached with a screw at each hoop. The hoops are no more than 4′ apart. I use 8′ lengths of plastic pipe to cover a 4′ wide bed. The ends of these hoops fit into pieces of larger diameter pipe. This larger size pipe is cut to 2′ lengths and is put one foot into the ground with the other end sticking up for the hoops to fit into. I put screw eyes (going through both pipes) on alternate hoops along each side to slide the nylon cord through that I use over the plastic to hold it to the hoops. With this method I don’t need to use any weights, rocks, boards, etc. to hold the plastic down and it is easy to raise it up for harvesting or for venting. To the screw eye on each end, I attach a bungie cord that the nylon cord is tied to. This provides tension to hold things tight. In the summer, shade cloth can be exchanged for the plastic.
These low tunnels are nice, but in the spring the winds can be pretty strong. When the sides are raised for venting, the winds can be harsh and I thought that being able to keep the sides closed and opening the top to the rain would be nice. Of course, that sounds like the cold frame, but I wanted a taller structure. I have used 2′ tall mini-greenhouses that I made from chicken tractors that were no longer useful in the field. I installed vents in them that open at 70 degrees and close at 40 degrees. They are the same kind of vents that you can use in the crawl space of your house. In the home building stores you would probably find them near the cement blocks. For a 4′x8′ mini-greenhouse I made two 2′x8′ lids hinged together. I could just fold one over the other to vent or remove the whole thing and lean it against the mini-greenhouse. When the lid was on it was held to the structure with hooks and eyes. I enjoyed using that to harvest winter greens out of. It had fence wire under the plastic covering, so that I could take the plastic off and the wire would still protect the plants from rabbits. The mini-greenhouse has a lot of advantages. Leaning into it to harvest greens was no problem, however it was inconvenient to plant seed seeds or do close weeding. Rather than the 8′ long lids, a series of 2′x4′ lids would be better, hinging 2 panels together and folding one on top of the other for venting. When they are removed they can be stacked to take up less space.
When it comes to season extension you need to think through exactly what you want to do. The 12-Month Gardener by Jeff Ashton has a lot of good ideas for cold frames. Consider carefully the plants you put in them. Although these structures provide some protection, you will be growing cold hardy, not tender crops throughout the winter. Four Season Harvest can help with crop choices and timing. Territorial Seed Company pays particular attention to crops for winter harvest. They even put out a winter catalog in July. There are many ways to extend your gardening season. I hope you make use of some of them so that you can eat from your garden all year.