It’s time to brighten things up

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.
 

Now that we have taken care of the easy upgrade to the greenhouse, let’s look at some other areas where steps can be taken to increase the light getting to plants. (By the way, if you don’t have one, purchase a light meter and check your greenhouses for light transmittance, so you are able to determine the areas inside the greenhouse you need to address.) Here are some of the steps you might want to consider taking:
 

Inside the greenhouse

• Place water and heat pipes and conduits below ground, along the post line under the gutter or along the sidewalls.
• Install shade and energy blanket systems that retract into a small space. When retracted, some shade systems create more than 10 percent shade.
• Locate overhead heaters near the north wall of the greenhouse when possible.
• Use a glazed louver system rather than motorized metal shutters.
• Remove insect screening and evaporative pads located on the south side of the greenhouse during the winter.
• Remove overhead hanging basket supports for the winter. Eight rows of 1-1/4-inch support pipe create more than 5 percent shade in a 21-foot bay.
• Replace fan-jet poly ducting with clean material.
• Paint structural members, bench surfaces, and walkways white to reflect light. White paint will give about 80 percent reflection whereas aluminum reflects about 60 percent of the light that strikes it.
  
   

Outside the greenhouse

• Remove all remnants of shading and grime from the glazing before winter. Use shade remover or a power washer.
• Remove nearby trees if they create shadows on the greenhouse. Observe the shadows during January and February.
• Take action, now.
   

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 jbartok@rcn.com.