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Plastic greenhouse covers extend the gardening season by Rhonda Nowak for the Mail Tribune Sunday, February 17th 2019 A A <br><p>{/p} “Every time I come into this shrubbery I am more struck with its growth and beauty.” — Jane Austen, “Mansfield Park,” 1814 Literature scholars have written much about Jane Austen’s extensive use of plants and nature in her novels to highlight human behavior. For example, this line comes from a chapter in which a character named Fanny remarks on the changes that have taken place in the parsonage garden. “Three years ago, this was nothing but a rough hedgerow along the upper side of the field, never thought of as anything, or capable of becoming anything; and now it is converted into a walk, and it would be difficult to say whether most valuable as a convenience or an ornament; and perhaps, in another three years, we may be forgetting — almost forgetting — what it was before. How wonderful, how very wonderful the operations of time, and the changes of the human mind!” Fanny is really describing her own transformation since arriving at Mansfield, having grown from, as one author described, “a fresh transplant inadequate to the demands of her new environment” into a thriving evergreen shrub with “roots that have grown deep beneath the foundations of the estate.” Austen’s work was a forerunner to a whole genre of literature that has been called “greenhouse romanticism,” in which many heroines (and a few heroes) develop strength of character within the controlled moral and social environments of their time and circumstances. Common among the novels and poetry of the genre are parallels that are drawn between the nature of plants and people. In the 19th century, greenhouse romanticism served as an expression of women’s budding sense of self-identity. In the 20th and 21st centuries, the genre evolved into ecofeminism and ecocriticism. Greenhouses, too, have evolved since Austen’s day. Two hundred years ago, they were called conservatories and were ornate, glass-paneled structures in which only the very rich could afford to grow exotic plants. Today, most greenhouses are more practical and less decorative. Their primary use is to extend the growing season and to grow particular crops in places where the climate is unsuitable. I have grown plants in a 10-by-20-foot greenhouse for the past seven years, and now it’s time to replace the plastic cover. Jerry and I are also planning to build a 20-by-40-foot hoop house (also called a high tunnel), so I’ve been researching cover options. A major transformation of greenhouses occurred after World War II with the advent of plasticulture. In 1949, professor Emery Myers Emmert was the first to build a plastic-covered hoop house at the University of Kentucky. Emmert’s designs took off in Europe and Asia, but not so much in the U.S., where produce can be more easily transported. Since the 1950s, a variety of polyethylene films (think sandwich baggies) have been developed for greenhouses that deflect ultraviolet rays, thus slowing down degradation of the plastic and prolonging the life of the cover. Some plastic films have chemical properties that reduce condensation within the greenhouse, reduce absorption of dust particles, and reflect sunlight and heat back inside. The most cost-efficient and commonly used type of greenhouse plastic is clear polyethylene, available in sheets of various thicknesses called mils (a mil equals one-thousandth of an inch). Utility-grade polyethylene is 4 or 6 mils with a lifespan of two to four years, and commercial-grade polyethylene is available in thicknesses up to 12 mils that last five to seven years. Clear polyethylene allows about 90 percent of the available sunlight to be transmitted within the greenhouse. Polyethylene is a highly flexible material, so it withstands wind and temperature fluctuations better than rigid panels. Twin-walled polyethylene sheets are double-layered with air injected in between the layers for added thermal protection. Woven polyethylene covering is also available in thicknesses from 6 to 12 mils. The reinforced design of woven films makes them more resistant to stretching, tearing, puncturing, and turning brittle. The weave diffuses sunlight throughout the greenhouse, which reduces shadow spots, but the transmission of sunlight is slightly reduced to about 85 percent. Jerry and I have decided to replace the greenhouse cover with more 12 mil woven polyethylene, costing about $300. We’ll cover the hoop house with 6 mil clear polyethylene that has UV, condensation and dust protection, which will cost about $500. Although commercial agriculture in the U.S. was slow to embrace the use of hoop houses, the USDA’s Natural Resources Conservation Service began the High Tunnel System Initiative in 2010 as part of its Environmental Quality Incentives Program. The initiative provides financial assistance to small and large-scale farmers who want to build high tunnels to grow crops. For more information about the program, see www.nrcs.usda.gov. Greenhouses and greenhouse coverings have come a long way since Austen wrote “Mansfield Park.” I’m hopeful that one day greenhouse plastics will be made from renewable resources. As Austen wrote, “How wonderful, how very wonderful the operations of time, and the changes of the human mind!” Rhonda Nowak is a Rogue Valley gardener, teacher and writer. Email her at [email protected] or visit her blog at http://blogs.esouthernoregon.com/theliterarygardener/. News In Photos Loading ...