What Percentage of Shade Cloth Should You Use in a Greenhouse?
Greenhouses are built to let the sun in, not only so that plants can use the sunlight for photosynthesis, but also to trap heat from the sun to keep plants warm in colder climes and in winter. However, if the weather is warm, the latter can definitely become too much of a good thing, prompting greenhouse owners to open air vents and drape shade cloth over the roof to cut down sunlight.
Available Cloth Types
Shade cloth can be either woven or knitted, comes in a variety of materials and colors--though black or dark green are most common--and is graded based on the amount of shade it offers. This is quantified as a percentage number that describes the density of the cloth. The higher the percentage of the cloth, the less light it will allow in. A 20 percent shade cloth, for instance, allows 80 percent of the light to come through, while a 60 percent cloth lets in only 40 percent.
Choose Adequate Shade Cloth
Plants need sun to carry out photosynthesis, but too much heat will slow down photosynthesis in some plants and stress others, such that they wilt and fail to thrive. Too little light is just as bad, often resulting in plants that are pale, spindly and more susceptible to pests and diseases. Your challenge is to cut enough sunlight to prevent stress, while allowing in enough light to promote good photosynthesis and healthy plants.
The day-to-day sunlight requirement for many plants is lower than what they would get in, for instance, Southern California or Arizona. In general, a 30 to 40 percent cloth is a good rule of thumb for most mixed use greenhouses and varies from there based on your crop, your greenhouse ventilation, your climate, and your greenhouse's orientation to the sun.
For instance, 40 percent shade is recommended for summer greenhouse tomatoes in Michigan, and no shade in winter, while a California grower may use up to an 80 percent cloth in summer and 50 percent in winter.
If your greenhouse is oriented north-south, it will collect less heat in both winter and summer, allowing you to use a less dense shade cloth, or no shade cloth at all. If it is oriented east-west, you will always have sunlight on the broadside of the greenhouse and it will collect more solar heat.
Shade cloth comes in varying levels of UV resistance. UV resistance increases the life of the product because the sun's UV rays can be highly destructive to many materials. A high rated cloth might seem more expensive, but do the math and decide for yourself whether the cheaper cloth is really cheaper when useful life and replacement cost are factored in.
If you plan to concentrate on intensive growing conditions for a single crop, learn its light needs, both intensity and duration, and tailor your shade accordingly. There are inexpensive devices that can measure the intensity of light in your greenhouse, so you can compute exactly how much to cut.