For growers considering upgrading their operation’s greenhouse coverings, the importance of lighting should be a main focus according to Tom Manning, a project engineer in the Department of Plant Biology and Pathology at Rutgers University.
“It depends on what the crop is. If lighting is somewhat less of a concern, double polyethylene is the least expensive option in almost all cases,” he says. “The energy efficiency is much better than single glass, approximately a third less costly. And compared to rigid double glazings, it’s maybe five to 10 percent less efficient, but it is less expensive.”
According to Manning, there are many options — single layer film, acrylic and single layer polyethylene, for example — that growers should avoid.
“They just don’t make sense in most environments,” Manning says. “If you have high efficiency or are thinking about getting it, there are triple-layer and five-layer materials you can get for greenhouses that will have some impact on light, but not that much. That’s something you have to crunch the numbers on and see if it really makes sense based on your energy costs because they are more expensive and the incremental gains of adding a layer [of glazing] decreases with each layer. Going for one to two layers has a great benefit, from two to three [has] less and so on.”
He also has a specific rule for determining if new greenhouse coverings are necessary.
“First, determine what you should be using,” he says. “And if you’re using more than what seems reasonable, then you start looking at why.”
Manning also says that there are signs growers should look for that could indicate new coverings are needed. For one, growers have an estimate of how much energy they’ll be using because it’s often different from operation to operation. Secondly, Manning says rely on local cooperative extensions or even other growers in the area and of similar size.
He also recommends looking for leakages, damaged coverings and maintaining a detailed service plan to mitigate any potentially compromised structures. He also notes that it’s important to know how long each type of covering can be expected to last. Polyethylene coverings, for example, last four to six years on average.