greenhouse structures By Bridget White

Greenhouse technology and equipment have been evolving at a staggering pace in recent years. What were once mostly small, family-owned businesses are now automated, computer-

controlled, highly technical greenhouse operations. And just to keep things exciting, new products are being introduced every day.

One of the more interesting ideas floating around research labs is greenhouse coverings that alter the light spectrum. These coverings have actually been around for more than 20 years, when Spanish researchers invented the first glazing product. Of course, the concept has changed dramatically from that time to include such things as anti-condensation films and films that disperse light. The newest form of greenhouse covering, photoselective films, block certain portions of the light spectrum, lowering greenhouse temperatures and providing a host of benefits to greenhouse growers.

The attraction to photoselective films came about during the energy crisis of the 1980s when these coverings were marketed as energy saving devices. Currently, photoselective plastics are being evaluated for their potential to filter out the near-red portion of the infra-red spectrum Ð the light that plants use to sense when they are crowded, which leads to stem elongation or “stretch.”

Peg McMahon, assistant professor at The Ohio State University, found that a very wide selection of plants (most bedding plants, mums, Easter lilies, poinsettias and others) grown under infra-red blocking plastics are shorter, more compact and darker green than plants grown on open benches without any additional shade. Such a plastic would also be of interest to greenhouse vegetable growers because daminozide (Altar), once the primary chemical for controlling vegetable height, has been banned in the United States.

The implication of McMahon's research for growers is that infra-red, photoselective coverings could be used to cover greenhouses and retard stretch without the use of, or with a great reduction in the use of, growth regulators. If a product of this type could be successfully developed and marketed, it could mean vast differences in the way most of the horticultural industry functions.

What's Available?

The type of greenhouse coverings available really depends on how much technology you want. A host of companies, both domestic and foreign, make what are loosely called “photoselective glazes.” Most of these products are either anti-condensation films or films that disperse light. Though several companies, the most aggressive of which is Green Tek, are working toward a product, Klerk's Plastic Products Manufacturing Inc. currently makes the only commercially available greenhouse film that truly selects and blocks certain wavelengths of light.

The two products available from Klerk's are Kool Lite 380 and Kool Lite Plus. Both versions of Kool Lite contain something called pearl lustre pigments Ð an additive developed years ago by Merk and used in such varied things as lipstick and car paint Ð to select which portion of the spectrum will be reflected.

“Our initial goal,” said Jim Ralles, Klerk's division manager of greenhouse and ag films, “was to figure out how to reduce heat in the greenhouse without reducing the quality of the light. We figured out that we could use pearl lustre pigments to reflect colors of light that plants don't use, which would reduce heat going into the greenhouse because all light has heat. We discovered that certain portions of the spectrum can be blocked without hurting the plants. In other words, we reflect out the bad or useless light and keep the good light.”

Kool Lite 380 was developed about eight years ago and is hailed by growers as one of the best anti-condensation, heat reducing films on the market. According to Ralles, Kool Lite Plus retains the same properties as Kool Light 380 but adds the ability to reflect light in the near-red portion of the infrared spectrum. It should be noted that Kool Lite Plus is not marketed as a growth regulator substitute, but is only touted as a low-heat, high-light film that also blocks light in the infra-red portion of the spectrum.

Evaluating photoselective glazing

After talking to several growers who use Kool Lite products, it seems that the jury may still be out on photoselective glazes. The biggest problem seems to be that the products have not been on the market very long. Kool Lite 380 was patented eight years ago, and Kool Lite Plus less than two years ago.

Justin Morotta, Possum Run Greenhouses, Lucas, Ohio, is known for being an innovative and aggressive grower. He is currently using Kool Lite 380 on all of his greenhouses, 52,000 sq. ft. He admitted to being curious about the growth regulator possibilities with Kool Lite Plus but said that he replaced the film on his greenhouses last year, before Kool Lite Plus was introduced. It will be years, Morotta said, before he even starts thinking about whether or not he would or could use an infra-red blocking film.

Intertwined with the novelty of Kool Lite Plus is the fact that growers still don't completely understand the product and are hesitant about investing money in something that may or may not work for them.

“I use Kool Lite 380 on my greenhouses,” explained Chuck Allen, Blossomtime Greenhouse, Grand Rapids, Mich. “The reason I don't use Kool Lite Plus is because I still haven't figured out how to use it. I'm also a Klerk's distributor, so I received samples of Kool Lite Plus as soon as it came out. I took one look at it and said this just looks like AC [anti-condensate] film. And, yes, it might be the best thing since sliced bread, but I just haven't figured out how to use it or where it fits into my program.”

The surprising thing about users of Kool Lite 380 is that they seem to be enjoying the benefits of an infra-red blocking film i.e., Kool Lite Plus, without actually using one. That is, they are seeing some reduction in stretching without the use of growth regulators.

“We have some decreased use of fungicides and growth regulators,” reported Rick Ouding, Kalamazoo Specialty Plants, Kalamazoo, Mich., “but it's kind of hard to quantify without doing side-by-side tests. Plus, we've just made the switch to 100 percent coverage with these plastics the last couple of years. Our crop mix changes and the weather changes so much from year to year that it's really tough to make a comparison in such a short amount of time. I can say that it's entirely possible that these films retard growth. I can say that since the temperature in the greenhouses is lower, there's less stress on the plants and we're watering less. I can say that we don't have to spray for Botrytis very often. Now, whether it's because of the film or something else, I don't know for sure.”

Grower after grower seems happy with the Kool Lite 380, which Klerk's claims only blocks green light. While their “real world” experiences tell them that plants stretch less with this product and that they have less disease with it, there are no experiments to back these results. And, their results are an unexpected benefit from a covering that was supposed to simply reduce heat in the greenhouse.

One explanation has to do with lowering stress. While the amount of infrared light that hits plants causes them to stretch, so does over-watering or increased stress. One grower theorized that the decreased amount of stress in Kool Lite greenhouses could be responsible for the compact plants.

Ralles confirmed that crops grown in greenhouses covered with Kool Lite 380 need to be watered less than those grown under other coverings. In fact, when the product was first introduced, growers reported an increase of stretching. Investigations showed that this was because the growers had not adjusted their watering schedules for the new covering. Decreasing water tables eliminated the stretching.

Allen says the same is true of fungal diseases. Many growers using Kool Lite 380 report a decrease in incidents of Botrytis. Less stress from disease means less tendency toward stretching as compensation.

The question that any grower would have to ask about a product like this is how much does is it? Certainly a product that offers more is going to cost more. On the retail end, it costs approximately $90 per thousand. But Ralles argues that the film pays for itself.

“The cost of the product is definitely offset by the money you'll save,” Ralles said. “If you only save 10 percent off your fuel bill, which is a sure-thing with infra-red films like these, you'll easily be able to pay for the extra cost of the covering within three years. The rest of the time, you'll just be saving money. And that's just the fuel bill. If you start saving money on fungicides or growth regulators or watering, you will pay for your film earlier and start saving money earlier.”

So, even though the Kool Lite products are pricey, and they are pricey, they more than pay for themselves over the life of the film.

Even given a certain amount of hesitation because of price, the real problem with photoselective films seems to be skepticism. People aren't sure that they work, mainly because the films have not been on the market long enough to convince most people.

The future of photoselective glazing

“I am very excited about using greenhouse coverings in the place of growth regulators,” said Ouding. “I expect that a few years from now we will be using plastics that have much better properties than we have now, in terms of keeping things shorter or helping them flower. We will be even further controlling temperature and making light manipulations. I definitely think that things will be changing to make our lives easier and more comfortable in the greenhouse, whether that's done with pink coverings or a pigment that's added to help the plants grow.”

While many growers see innovations like photoselective glazes as the future of the greenhouse industry, they also recognize that there are some limitations that must first be addressed before these products will enter the mainstream.

The main concern for most growers is that they raise a variety of crops, with a variety of needs, over a variety of climatic conditions and growing seasons. What's good for pansies in Michigan in August might not be good for lilies in Florida in February. Growers question how one product will address the needs of such a varied industry.

“I think it [an infra-red blocking film] has more advantage for someone that grows only one crop,” said Morotta. “They would probably benefit from that product more than someone who has many, many cultures that would respond differently.”

“Our light intensity and quality changes drastically throughout the year,” added Ouding. “In Michigan, the day length during the summer is about 16 hours, and during the winter, it's about eight hours. We would want to make sure that a product would work for us and work on a variety of crops before we put it on our greenhouses.”

Recent European studies show that thrips are sensitive to certain portions of the spectrum, implying that this persistent problem could be controlled with the right film. A new study from Israel confirms that Botrytis can also be controlled by blocking certain wavelengths of light.

Pest and disease control, growth regulation, environmental control. When linked with accomplishments like this, there is little doubt that photoselective glazes will continue to be a part of greenhouse coverings, if not the future of greenhouse coverings. But a number of problems remain before the full transition is made from research lab to average grower.

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Bridget White

Bridget White is associate editor of GPN.

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