What you need to know about replacing your polyethylene covering

When growth slows in your crop – many problems can get blamed, says Steve Froelich, an engineer and technical consultant for CropKing. However, one of the biggest reasons this can happen is that the plants aren’t getting enough light to utilize what growers are feeding it to grow efficiently. And if the cause of delayed growth is a lack of sufficient light, it may be time to replace your covering.

How covers wear down

Surely snow, wind, hail and other natural elements can cause tears in your covering. But more than likely, your covering will be damaged from particles in the air, Froelich says. Whether you’re in a rural area where other agriculture is taking place, or your structure is placed near a bustling metropolitan area, dust and dirt blowing around in the atmosphere can become embedded into the plastic, which can darken it, reducing your light transmittance.

The manufacturer of the cover will always provide warranty information that includes its expected longevity – but you should be proactively checking for these types of damages, he adds.

“How I do it is – for double poly – I pick a day and time in the year when I put my cover on. I will go inside at that particular time on that particular day and take light readings with a hand-held light meter,” Froelich says. “Then I will do those same readings on that same day at that same time a year later, and so forth. You’ll be able to see a pattern of the amount of light in your greenhouse starting to decrease.”

Generally, a 1-percent decrease in light levels can lead to a 1-percent decrease in potential production, given that all other controlled elements, such as CO2, temperature, humidity and more are at the appropriate levels. Froelich says that when he has about a 15-percent reduction in light, it’s time to replace the cover. “And that will vary from year to year,” he adds. Some years it will be drier, causing more dust to build up – while during rainy years, this isn’t as much of an issue.

However, if you do see tears in your covering, you can do patch repairs with tape; however, “If you’ve got rips and tears over the bulk of the cover, it’s probably best to replace it,” Froelich says. “The light transmittance through the tape isn’t anywhere nearly as good as the light transmittance through the original cover.”  

When to measure and replace

Froelich recommends conducting the light meter reading in the early fall season. While there’s no big reason to conduct during this time, scientifically, it could be a good time to set yourself the reminder that with winter coming, you may not get another opportunity to make the replacement until springtime.

“Generally, I don’t like to replace covers in the middle of the summer,” he says. During this time, it can be difficult to find a day where it’s not too windy – because even the slightest amount of wind can make it difficult to pull the cover over the top of your structure evenly. Plus, the temperature will be better in September or October. “If you’re covering a poly house on a day that it’s 95 degrees out, the poly is going to expand and stretch. And when you hit real cold temperatures, the poly will have shrunk – and really, you’re [over]-stretching the plastic instead of having it [placed] tightly on there,” he says.

Finding a day with ideal weather coupled with the schedules of your help to get the job done is probably the most difficult obstacle to replacing the covering, Froelich says, adding that it will typically take about six people to install the new covering.

However, if it doesn’t get done, it can set you back significantly financially.  

“If you’re growing lettuce or tomatoes – a time [during] the year when other costs are extremely high – you could easily be in a situation where it’s not cost-effective to continue to grow in that facility,” he adds. 

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