Small Garden Greenhouse: Part 1
Treat your plants to a safe transition and extend the growing season.
Figure 1: The completed greenhouse.
Composite Illustrations By Nate Skow
When I decided to build my first greenhouse, I wanted a place for my vegetable plants to continue growing after they had become too large for the lighted shelves where I started them. I wasn’t sure I wanted a greenhouse at all and didn’t, therefore, want to put much money into one to use for a trial. Buying a kit or a complete structure involved more than I wanted to spend, so I decided to build my own.
Since I wasn’t sure where I might locate the greenhouse, I made it light and sturdy enough to move around, and covered it with an elaborate array of laminated arc-shaped beams supporting two layers of hardware store plastic. These plastic-covered laminations have survived winter storms, thunderstorms and some small hail, but creating the beams was tedious and time consuming so I designed a more conventional structure for this project. I also chased pieces of the cheap plastic I used as a cover (the first time) all over the neighborhood one winter, so I recommend using film made for greenhouses – it will last for several years.
When I leave my plants in the greenhouse at night, a small thermostatically controlled electric heater keeps the temperature above 50°. During the day, heat is always a hazard so ventilation is essential. On a cold but sunny day, the temperature in the greenhouse tops out at about 140° with the door closed. I try to keep the temperature below 80° by propping the door open, but during my first house’s second spring, I accidentally left the door shut at noon. After lunch and a brief nap, I returned to plants that looked as if they had been sprayed with brush killer the day before. Opening the door and watering didn’t revive them. To help prevent that happening again, I’ve included a window and a heat-activated opener in this design. Commonly available radio-transmitter systems have alarms to alert you if the temperature inside the greenhouse gets too hot or cold. I’m going to set mine up this spring.
The wall and roof framing used in this greenhouse is appearance-grade 1-by-2 pine. The base is untreated construction grade plywood and pine 1-by-4s and 2-by-4s. The skids are 10-foot, treated, construction-grade 2-by-4s. If you intend to water by general spraying, you should consider using treated or naturally rot-resistant lumber, such as cedar, redwood or walnut heartwood. Other possibilities are Osage orange, mulberry and black locust. I’m not concerned about rot because I moisten flats by pouring water into a corner I have lifted, and pots I water just enough to keep the soil damp.
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