If you’re running a greenhouse covered by polyethylene film, then you know that a replacement job is probably coming up in the next few years. The integrity of your poly roof is critical to the success of your crops or your plugs or your trees–whatever your business is growing.
Despite its significance, the maintenance and replacement of poly roofs can often be kicked down the road until visible deterioration occurs. It’s not that poly is overlooked, per se, but as a going concern on the business’s budget, yes, poly can sometimes land on the backburner.
We’re here to demonstrate why that’s not such a good idea. Get out in front of your poly replacement needs, and season after season you’ll find greater efficiencies in the greenhouse.
The Impact of Timely Replacement
Alan Barnitz, assistant manager of the plug division at Bob’s Market and Greenhouses in West Virginia, took over the responsibility for the greenhouse coverings in the family-run business. He knows the importance of timely poly roof replacement.
Bob’s has 25 acres of greenhouse, and about half of that acreage is under poly, Barnitz says.
Typically, teams will replace poly every four years, maybe every three years if the material is looking dicey. Barnitz says the Bob’s team staggers this replacement work across their dozen or so structures; it’s a major investment of time and money, after all.
“We used to change all of our poly in-house, and it just got harder and harder to find the folks who are willing to get up and get on top of a greenhouse, because that’s a challenging job,” he says.
It’s perhaps made more challenging–or at least more consequential–because Bob’s is located close to a coal-fired power plant. This places a premium on that four-year mark for replacements; the soot in the region can degrade the poly material faster than in other environments.
“We’ve observed that not replacing the poly on time leads to not just reduced light transmission, which is crucial for plant growth, but also increased energy costs due to poor insulation,” he says.
Whether it’s the industrial Ohio River valley or other geographic areas with their own unique characteristics, it’s important for greenhouse operators to know their surroundings.
It may be tempting to push it and let next year’s budget deal with the poly cost, but risks tend to compound. By extending the use of old poly, you risk poor crop quality and higher operational costs in the long run due to increased heating needs and potential pest problems.
“By stretching out the plastic for one more year, how much does it actually cost operators in poor quality crops or in not making the sales or in heat loss?” Dave Marsh, foreman for LLK Greenhouse Solutions, says.
When you’re talking about poly, you’re talking about light.
Marsh adds: “If you have better light in your greenhouse and a healthier crop, those plants are going to be stronger and more prone to disease resistance.”
Technical Insights
Amy Lynch, strategic business manager at Berry Global, says that most greenhouse polyethylene is UV-stabilized and warrantied for four years, which is where the typical replacement cadence comes from. However, this does not mean all poly roofs can last that long under every condition.
Chemical exposure and geography can affect the performance. “Operators in the north might see a longer lifespan due to less intense sun exposure compared to their southern counterparts,” she says.
Still, that four-year benchmark is very helpful; just because the film physically hasn’t failed after four years does not mean operators are getting the most optimal growing environment out of it. Lynch says she visited a grower in the South, and the team was throwing away plants because they were getting too much stretch. On closer inspection, the poly film had been manufactured 12 years ago.
Curious about when your poly was manufactured? The film itself should bear a date code, depending on the manufacturer.
Time and Money
Increasingly, operators are looking at poly roof maintenance with different budgetary eyes. “What used to be a regular maintenance expense has now often been categorized as a capital expenditure, reflecting the significant upfront costs associated with poly replacement.”
Those costs come up typically in vendor transactions in the spring. Get out in front of the expenditures for the year and place your orders for timely delivery in later seasons. Most greenhouse operators will plan to do the actual replacement work in the fall.
“If you put poly on when it’s too hot, it will stretch and lay out really, really nice,” Barnitz says. “But the only downfall is when winter comes, it contracts and pulls really hard on your entire structure. [That contraction] makes it more difficult for vents to close. It’ll pull little gaps on your vent brackets, and then you have heat loss.” Autumn provides a good balance, when temperatures are around 70 degrees Fahrenheit.
The Bubble
Maintaining the inflation between the two layers of poly is crucial.
It’s important to regularly inspect and replace any old or deteriorated hoses from the blower to the plastic hookup. At Bob’s, the maintenance team consistently monitors the blower tubes to ensure the bubble remains inflated. This prevents the wind from pulling the poly sheet out from under the spring wire.
Proper inflation not only enhances insulation but also ensures the spring wire tightens securely against the poly, maintaining the structural integrity of the greenhouse cover.
This is not an area where you want to skimp on durability, Lynch says.
The exterior layer of a poly greenhouse serves as the primary defense against environmental elements like wind and hail. This outer barrier is essential for protecting the greenhouse structure.
In setups such as high tunnels or overwinter poly houses, where only a single sheet of plastic is used, the material is prone to billowing under windy conditions. This repeated movement can cause the plastic to continuously strike the structural supports or purlins, leading to wear and potential degradation of the film’s integrity. Hence another benefit of the bubble.
To enhance structural stability and prolong the lifespan of the greenhouse covering, a double-layer configuration is highly advantageous.
For operations like Bob’s Market, which focus extensively on plug growing, the choice of the inner layer is critical to manage internal condensation effectively. Opting for an inner layer that sheds moisture effectively can prevent detrimental dripping, thus maintaining optimal growing conditions and safeguarding plant health. This strategic approach to selecting poly materials ensures that each component of the greenhouse is optimized for both protection and productivity.
“For a young plant producer, a drip is like a tiny little missile that can damage an entire tray,” Barnitz says.
Short of a full replacement job, be mindful of tears and the need for occasional patching. Even the smallest encroachment of outside water or air can throw off a production cycle.
Bottom line: Be aware of the age of your poly. And double-down on monitoring its condition throughout those four-year cycles.
“If it’s been four years, it’s definitely time to replace it,” Marsh says, “but if you have a bunch of holes in the plastic or it’s not staying properly inflated, you might think about replacing it just to save on energy costs.”
Conclusion
Effective management of your poly film is an important maintenance routine and a strategic endeavor that significantly impacts the health and productivity of your plants.
Missing the mark on poly replacement has provided hard-won lessons for greenhouse operators across the industry. The main lesson: Don’t let budget constraints delay necessary updates that can prevent larger issues such as increased heating costs, poor crop quality, and structural damages due to environmental stresses.
Poly roof replacements, ideally conducted every four years, should align with the specific environmental challenges and operational demands of each greenhouse. Operators should consider the severity of local conditions—like proximity to industrial zones or high UV exposure areas—which can accelerate the aging process of poly roofs. Planning replacements in the fall, when temperatures are moderate, ensures the material integrity during installation, avoiding the pitfalls of contraction in colder weather.
Ultimately, maintaining the structural integrity and functionality of your greenhouse through timely poly roof replacements is an investment in your business’s future. It safeguards against operational inefficiencies and supports optimal plant growth environments, thereby enhancing overall productivity and profitability.
Sign up for the LLK Greenhouse Solutions newsletter below >>>
