January 27, 2022 - Ron Allen, UC Master Gardener, Mariposa County - By using a greenhouse, a home gardener can extend the growing season into the chilly winter months. In Mariposa County, a typical small greenhouse can provide temperatures that are 10-15°F warmer than outside ambient air. This is a sufficient bump in warmth to keep late season vegetables growing and protect sensitive landscape and potted plants as the cold arrives. A greenhouse is also useful for getting started on seed propagation before warmer spring weather finally arrives.
Home greenhouses consist of an aluminum frame, transparent polycarbonate panels, straight walls, a gable roof, typically come with small roof vents, and they have a sliding entry door. You can walk into them to work. They cost anywhere from $1,000 up, depending upon the size you buy. They take some patience when assembling, and you need to pay careful attention to the user manual’s instructions and figures, but they work well. The panels should last about ten years—which is pretty impressive for plastics exposed to the harsh Sierra Foothills summer sunlight!
It is crucial that you provide a level spot for the greenhouse. Any out of plumb aspect to the foundation will translate into misalignments at the top of the structure when it is assembled. If the base is not square, then the side panels will fail to fit the frame. Avoid foundation short-cuts.
If you have a smooth, flat site, then you can set a greenhouse directly on the ground. You will need anchor stakes to hold it down. Use a six-foot bubble level with a straight 2x4 stud to check that the base is level.
If your site is not level, then I recommend a wood frame base. You will lay out a level, square frame of 4x4-inch (or 4x6) treated beams supported by landscape stones, which matches the bottom dimensions of the greenhouse frame. Inside the foundation frame, you can put down a weed block (commercially-available cloth or cardboard), a metal gopher screen of half-inch hardware cloth, and a pea gravel floor. Install sturdy benches and shelves to support sheltered plants (photo).
You can improve your chances of winter gardening success by selecting plants appropriate to the weather. Good choices for cold weather vegetables are Swiss chard, spinach, cauliflower, broccoli, Brussels sprouts, cabbage, kale, bok choy, collard greens, and turnips. And remember that a greenhouse allows you to grow carrots, beets, and peas year-round in the Sierra foothills.
Fig. 1. A small greenhouse with pea gravel floor and simple, but sturdy benches and shelves for potted plants and seedling trays. Photo credit: Ron Allen.
For assistance, contact our Helpline at (209) 966-7078 or at email@example.com. We are currently unable to take samples or meet with you in person but welcome pictures.
Updated: The U.C. Master Gardener Helpline is staffed; Tuesdays from 9:00 A.M. - 12:00 P.M. and Thursdays from 2:00 P.M. - 5:00 P.M.
Clients may bring samples to the Agricultural Extension Office located at the Mariposa Fairgrounds, but the Master Gardener office is not open to the public. We will not be doing home visits this year due to UCANR restrictions.
Serving Mariposa County, including Greeley Hill, Coulterville, and Don Pedro
Please contact the helpline, or leave a message by phone at: (209) 966-7078
By email (send photos and questions for researched answers) to: firstname.lastname@example.org
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Master Gardener Office Location:
UC Cooperative Extension Office,
5009 Fairgrounds Road
Mariposa, CA 95338
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