Should you replace your greenhouse plastic every year? I’ve been asked that question many times, and the answer is “That depends.”
Here’s what you need to know. Modern greenhouse plastic has a guaranteed life of at least four years. And some growers may get a year or two more out of it, but by doing so they are risking the damaging effects of nature. Do you really want to have to call your insurance agent?
Research shows that greenhouse film loses 2 to 5 percent of light transmittance each year due to the effect of ultraviolet light. Additives such as HALS (hindered amine light stabilizer) and BASF’s Tinuvin light stabilizers and UV absorbers slow this degradation, but there is still a gradual loss over four years. The effect of this is reduced plant growth and lower quality.
Depending on where your greenhouse is located, dust, dirt, and air pollutant accumulation may cause an even greater light reduction. A 5 percent reduction per year is likely in some areas near large East Coast, Midwest, and West Coast cities. Industrial areas also tend to have greater smog (smoky fog) concentrations. It may not be as bad as Beijing, where people wear gas masks outside, but smog is still common in many places in the U.S. Air pollutants tend to stick to plastic, making them difficult to remove with plain water. Adding soap may work better.
For air-inflated greenhouses, replacing yearly may be economic. During the summer, light levels are high enough that the reduction in light may not affect plant growth. It is during the late fall, winter, and early spring when this is of most concern. A rule of thumb is that “1 percent increase in light equates to 1 percent increase in plant growth.”
Let’s look at an example. The cost of replacing the outer cover on a 30- x 100-foot hoophouse is about $750 ($0.25/square feet for plastic and labor x 3,000 square feet = $750). It would take an increase in production of just a quarter-pound per tomato plant at a wholesale price of $2 per pound to pay for the new plastic. Increased yields of 0.5 to 1 pound extra are not uncommon when new plastic is installed.
Although more expensive and having a longer payback, old fiberglass, polycarbonate, and acrylic glazings that have deteriorated should be replaced.
I was in a greenhouse recently where the light at the bench level was just 35 percent of outside light. The glazing was about 30 years old, and there were many overhead obstructions in the way.
Inside the greenhouse
Outside the greenhouse
While all these changes aren’t necessary for most operations, taking the time to consider what makes sense for your greenhouse is worth the effort. Bottom line: a few changes may increase your light levels significantly.
Bartok is a regular contributor to Greenhouse Management and an agricultural engineer and emeritus extension professor at the University of Connecticut. He is an author, consultant and a certified technical service provider doing greenhouse energy audits for USDA grant programs in New England.
Have a question? You can write John at email@example.com.