A greenhouse is a structure that holds plants in optimal conditions. Greenhouses let in light and hold in heat. Greenhouses also hold in moisture and humidity---and come in all shapes and sizes to accommodate the needs and space availability of gardeners.
Tabletop greenhouses are the smallest greenhouses available. A sturdy box tray with a clear plastic and tight-fitting lid maintains the warmth and humidity necessary to germinate seeds and keep seedlings healthy.
Patio greenhouses, also called grow racks, are small structures intended for outdoors. Grow racks are usually about 2 feet long and a foot or two wide with two, three, or four shelves. Gardeners place potted plants on the shelves. Grow racks come with a fitted plastic covering that zips open and shuts for easy access to plants.
Hobby gardeners with little space often use the tabletop and patio greenhouses for up to three seasons. They are temporary structures with no base, and must be protected from strong winds which may blow them over.
The term "cold frame" refers to an outdoor box to grow plants in. Greenhouses that do not have automatic or energy-powered heating and cooling systems are essentially walk-in cold frames.
A cold frame can be bought assembled and placed in a sunny spot outdoors. Simply fill with potting mix and seeds or with young plants.
Some gardeners build raised beds a couple feet high. They fashion a cover for the raised bed, turning it into a stationary cold frame. The cover can be as simple as clear poly sheeting available at garden supply centers and some hardware stores, or a fancier flip top.
Lean-To and Hobby Kits
A lean-to greenhouse is another good option for gardeners with limited space. A lean-to is essentially half a greenhouse, and is attached to the side of the home. Most are fashioned after the home has been built, and the door to the lean-to is outside.
Greenhouse kits come in two main varieties, portable and permanent structures. Portable varieties are set up in early spring and taken down in fall. They are much less versatile and can be blown down in increment weather.
When a gardener is ready to invest in a greenhouse, the best bet is to measure the sunny area where the greenhouse will be, and build a permanent structure from scratch or purchase a steel greenhouse kit complete with base.
Reputable greenhouse kit manufacturers include Juliana, Rion, and Sunshine.
Things to look for in a greenhouse kit include a base, a metal or wooden frame, and polycarbonate panels. Polycarbonate does not break like glass, but lets in light and maintains warmth as glass does.
Polycarbonate panels are available in 4mm, 6mm, and 10mm thicknesses. For gardeners who live in northern climes, 6mm or 10mm are necessary to keep out cold and frost in early spring and late fall.
Design and Build Your Own
Some gardeners choose to build their own greenhouses out of various materials, both bought and salvaged. For example, old plastic bottles can be fashioned into greenhouse panels, while old windows can be salvaged and connected to make a greenhouse. You can also use polycarbonate sheeting, which can be purchased at home and garden stores and stretched over a tubular greenhouse frame.