The use of long-life, greenhouse-grade plastic has reduced the frequency of re-covering greenhouses. Still it is a chore that has to be done every four or five years to give a clean and strong glazing that will protect your plants throughout the year.
Getting a 40- x 100-foot sheet of plastic over the greenhouse frame can be difficult, especially if there is even a slight wind. It is best to select a calm morning with a temperature around 50°F. Several growers have been lifted off the ground trying to keep a sheet of poly from blowing into the next county on a windy day.
If you have enough help and the greenhouses are at least 10 feet apart, it may be easiest to unroll the plastic on the ground along the sidewall and pull it over the frame from one side to the other using ropes. Care should be taken so that it doesn’t catch on braces or other obstructions and tear.
There has to be a better way. Most manufacturers suggest that the easiest way to apply poly is to unroll it over the ridge and unfold it. The poly is packaged with a gusset fold for this reason. To aid growers in lifting the roll of plastic above the ridge, a number of devices have been developed.
Plastic roll support. A front end loader or forklift can be fitted with a roll support that can handle one or two rolls (Fig.1). The roll support can be made to detach easily. The rolls are loaded at ground level and then lifted above the ridge. This type of device could also be designed to reroll the poly when it needs to be removed by adding a gear motor powered by the hydraulic system.
Aerial work platform lifts. A motorized scissor or telescoping lift can be used to aid in the positioning of the plastic and the installation of the fasteners. A roll support can be added to aid in unrolling the plastic. This is the best piece of equipment for attaching the plastic to roof vents or on endwalls. It is also great for doing installation or maintenance in the ridge area. Select a machine with large wheels for use on gravel surfaces. Used machines are available from equipment dealers or on the Internet.
Gantry roll support. Mechanized equipment for covering gutter-connected greenhouses usually consist of a gantry that spans the roof between the two gutters and carries one or two rolls of plastic (Fig. 2). On narrow bays, plastic in tube form is used so that both layers are applied at one time. After one end of the plastic is attached to an endwall, the gantry is moved down the gutter, unrolling the film. Employees walking in the gutter tighten and attach the plastic to each gutter with extrusions. An endwall cross-track system will help speed- move the gantry between bays. An experienced crew of six people can cover about an acre of greenhouse per day. Some manufacturers make gantries to fit their greenhouses. Some of these units can be used to apply shade compound or wash the glazing. A few growers have built their own gantry.
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.
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