Greenhouses are the dream possession of every gardener, and rightly so. Greenhouses have many more advantages than disadvantages, and a gardener who has one will never want to be without. There is something special about spending hours in a warm, sunny greenhouse when the temperature outside is cold and blowing, yet inside it is toasty warm and the smell of damp soil is aromatherapy like nothing else.
Gardeners are limited by what commercial greenhouses offer for plants. They may offer only two colors of impatience, and the color you want isn't one of them. By having your own greenhouse, you can grow exactly the types and colors to please yourself. Browsing seed catalogs can give the gardener a wider choice of varieties and colors.
Not only can you grow exactly what you want, you can start seedlings well in advance of planting season. If a particular packet of seeds doesn't germinate well, you still have time to try again. By having time on your side, you can experiment by starting seeds of plants you can't find in a greenhouse.
As the outside air warms, move your seedlings to cooler areas in the greenhouse, which helps them "harden off" which will make their transfer to the garden easier once planting time comes.
Of course, if you live in a cold area, you will need to heat your greenhouse to keep your seedlings from falling below freezing temperatures.
In a greenhouse, the gardener can alter the amount of heat, light and air movement for optimal growth. Once seedlings are started, they can withstand cooler temperatures which makes them strong. A common misunderstanding about greenhouses is that direct sunlight is an advantage; actually, diffused sunlight is optimal. If you can see shadows in your greenhouse, you need to diffuse the light by hanging fabric or netting to break up the direct sunlight. Strong direct sunlight makes seedlings leggy instead of stocky.
Air movement is also important. By using fans to keep air moving around the plants at all times, you break up pockets of stale air that encourage mold and mildew growth and making sure the seedlings get fresh carbon dioxide.
Imagine growing fresh flowers, herbs and vegetables for harvest at any time of the year. It is possible in a heated greenhouse that also has full-spectrum grow lights. The grower will have to extend the daylight to mimic the longer days of summer and monitor the temperature to allow vegetables to ripen as if it were summer.