It has been a joy to see so many new gardeners and to observe many other gardeners step up their game to do more vegetables and flowers this year. Many are at work on their fall vegetable garden and soon will be planting hardy mums and pansies, along with trees and shrubs, this autumn.
This surge in gardening activity has led to lots of questions from folks thinking about adding a cold frame or hobby greenhouse to further expand their gardening interests and growing season.
A cold frame is some kind of shelter or enclosure, generally without heat and using passive ventilation to extend the growing season in the fall and to get an early start in the spring. A short simple cold frame with a vent or windows that open or lift works great for lettuce, some herbs and other cool-season crops to get an extra three to six weeks of production in late fall to early winter. There are premade kits or get plans to build your own. Cold frames usually stay 5 to 10 degrees warmer than outdoors depending on design and construction.
Hobby or estate greenhouses are generally a smaller version of the commercial greenhouses you visit at garden centers or conservatories. They vary a lot in strength, design, appearance and performance. A greenhouse is really a piece of equipment to create the conditions you need to grow and care for plants. If you are going to use a greenhouse to start vegetable transplants in the spring, you will need different heating, cooling and shading from what you might need if you are planning to raise orchids or overwinter tropical plants you have out on the patio or by the pool.
After you decide what plants you plan to grow and know the conditions you need, then you can decide on the greenhouse frame, the type of cover or glazing you want to use and the equipment you need to complete your greenhouse and make it functional.
Greenhouse frames for years were built of wood and still can be, but most are now built of steel or aluminum — although some inexpensive kits have plastic assemble-yourself frames. The plastic frame kits rarely last long in our high winds, so wood, steel or aluminum frames work best in Oklahoma.
Check with your local building code folks to see if you can just build your greenhouse or need a permit and load engineering. Many communities don’t require permits on personal greenhouses under 200 square feet, but you still need to check.
Think about how tall you need the sidewalls to handle the plants you plan to keep in the greenhouse. Greenhouses are trending taller, but you probably want 6- to 10-foot-tall sidewalls. Do you want a traditional greenhouse frame with a peaked ridge, or do you want a Quonset or arched-roof greenhouse? Steel or aluminum frames can be made of round pipe, square pipe or formed shapes like “C” channels or other roll-formed or extruded shapes.
You need to decide on your greenhouse frame and glazing together as not all glazings work on all greenhouse frames. Greenhouse copolymer film is the least expensive glazing and does best on Quonset or barrel-shaped roofs. Greenhouse plastic comes in one- and four-year grades of film and you can install one layer or two layers of film. The two layer film roofs with air blown between the two layers will reduce energy use by about 30%.
The most popular glazing for hobby greenhouses are the more rigid polycarbonate panels. Polycarbonate is available in single-layer corrugated panels and in twin layer panels that resemble rows of square soda straws that again save about 30% on heating expenses. Polycarbonate is available in clear, diffused and kool-lite panels and should last 10 to 20 years before you need to change glazing depending on the panels you choose.
The further upscale glazing choices include corrugated or twinwall acrylic panels or single or insulated glass. The acrylic and glass panels should only be installed on heavier weight, more engineered greenhouse frames. Many of these frames and glazing have long lead times so you will have limited choices if you want a greenhouse ready for the first freeze in early November, and you will have more choices if your target is to be ready to start spring crops next February. Like most things in gardening you get your best results by thinking and planning ahead.
After you decide on your greenhouse frame and glazing it leads to better decisions on heating, cooling, benches and shading. Before you visit with your greenhouse supplier or local extension office to plan out your greenhouse, you will need to determine what fuel you have available for heating — natural gas, propane or electric. For ventilation and cooling, do you have single- or three-phase power source and what voltage — 120, 240 or some other voltage?
Many gardeners dream of a greenhouse to support their hobby and to make gardening fun year round. If you have been thinking about a greenhouse, maybe this is the time to jump in and enjoy the warmth of solar energy in your greenhouse in the winter, to enjoy the scents and color of growing plants. It is extra special to watch plants grow and bloom in the greenhouse while the outdoor world is gray and bleak for the winter.
Rodd Moesel serves as president of the Oklahoma Farm Bureau and was inducted into the Oklahoma Agriculture Hall of Fame. Email garden and landscape questions to rmoesel@americanplant.com .