Screens…Back to Basics
By Les Evans, Technical Advisor, LS AMERICAS

I find it enormously distressing when growers ask, "What are screens?" because it means we have failed to educate them on the basics of what screening can do. Growers take on the blame, and they state that they probably have been too busy to look into screens. Such should not be the case. The guilt lies solidly with the manufacturer and it is time for us to stop and educate growers on the current technology.

In defense of the manufacturer, we often get caught up in how to improve and change products, rather than letting the grower know the basics. In a recent survey conducted by LS Americas, it was found that a lot of growers do not know very much about the benefits of screens. They do not know what screens can do for the quality of their crops. It hit us like a ton of bricks, as we felt for sure that the word was out about screens. Such is not the case. So, here is another attempt at explaining what screening means to the grower.

What is screening?
If you look ‘screen’ up in the dictionary you will find that it is defined as that which shelters, protects, conceals, or shields; a kind of sieve. That’s a start, but the greenhouse definition is a little different. Screens used in the horticulture industry vary in the amount of shading they provide from a low of 20% to a high of 99.9 %. (Notice how nothing ever gets a 100% rating?) Screens do more than filter light; they create a controlled climate that results in temperature and humidity stability, fewer plant diseases, lower energy costs and a more efficient use of labor.

What are the different types of screen?
Screens have been developed for a variety of applications. There are energy screens, blackout screens, solar protection screens, moveable internal screens, moveable open external screens, insect screens and decorative screens.

Do thermal screens make a difference in plant temperature?
Yes, according to James E. Faust, Royal D. Hein and Paul Kiefer ; there is a reduction of heat loss during the nights when thermal screens are used, which is due to a reduction of greenhouse volume heated. Screens actually provide an insulating barrier between the greenhouse air that is heated and the greenhouse glazing.Elevated above the crops, thermal screens are made of either polyester, polyethylene or aluminum materials. By separating the main greenhouse air from the non-heated air located at the top of the house, thermal screens reduce heat loss thus, thermal screens decrease the volume of air to be heated. The screens also help keep the heat from rising from the crop area to the peak of the greenhouse.During the day, the screens have an important role as well. Greenhouses are built to allow for plenty of light to enter the roof so the plant underneath can realize its maximum growth potential. Plants need the light for photosynthesis, but the same light can become harmful if it becomes excessive. By excessive we mean that either the plant is absorbing its maximum amount of light already and anything over this may become harmful, or that the light is being converted into heat, and too much heat can also be dangerous for the plant. The same screen that acts as an insulation barrier at night is also protecting during the day. Aluminum strips in the screen reflects this unwanted light away from the plant below, preventing any plant damage from excess light, and cooling the overall greenhouse temperature by not allowing a certain percentage of light to be converted to heat. The greenhouse environment has become more controlled and the overall result is a plant that is less stressed and less susceptible to disease.

Do screens reduce energy costs?
Yes. Screens have been proven to reduce energy costs. There are many studies concluding that the savings on heating bills alone pay for the cost of screen without adding in other benefits. And even better is the fact that growers report that they have realized overall energy savings. According to Fred Kornerup, President of Rock Valley Greenhouse & Garden Center (Rockford, IL), they realized approximately 50% reduction of heating costs alone by using a thermal screen. The screens also reduce daytime energy costs, the costs associated with cooling the greenhouse. No matter what the method of keeping the greenhouse climate cooler during the day, the screen will greatly improve the effects and at a lower cost. For example, a fan and pad system will not have to run as long because the temperature has been regulated by the screen, saving the grower money on electricity costs.

What are rolling screens?
Rolling screens are used in quonset greenhouses. Basically, they are manually operated systems that move the screens. Hand cranks allow the screens to be positioned wherever they are needed; to completely cover or partially cover the quonset to shade either side separately for coverage over the quonset top only or to cover either one or both of the side walls. Some systems even offer an option of utilizing two different shade factor screens on a single system to maximize growing space.
Growers can choose from open-structures for maximum air circulation or the closed structure for greater weather protection or over-wintering Quonset houses. Screen shade factors range up to 85%, allowing growers to customize their desired environment. The systems are very simple and normally easy to install.

We keep hearing about different ways to move screen – what’s the difference?
Yes, there are many different ways to move screening. Screening can be moved by motors located at the endwall or sidewalls and recently we are seeing what is referred to as hanging systems that utilize hooks and moving cables. There are also push-pull systems that utilize rack and pinion mechanisms. Both the new hanging systems and push/pull systems put less stress on the whole system and are very efficient. Screen systems are very adaptable and once installed easy to maintain. Due to the many different styles of structures, of course, a screen system that is right for one grower may be different than that for another grower.

When are outdoor moveable environmental screens needed?
External screens are specifically designed for protecting outdoor crops against adverse weather conditions such as excess sunlight, strong winds, hail and driving rain. A combination of aluminum strips and open shade with a structure of UV stable HDPE yarn produces a cloth that reflects extreme sunlight yet permits adequate air flow. During the day the cooling effect is unequaled and by night the heat is retained under the curtain to keep crops warm. The result is a controlled growing environment which allows crops to be grown in extreme weather conditions.

What are the benefits of insect screening?
Developed due to the more stringent government imposed laws regarding chemical applications, insect screen is an excellent choice for pest management that does not depend on pesticide application. The laws are getting stricter and the paranoia surrounding chemicals more intense. Consumers prefer chemical-free plants and vegetables, while workers’ protection plans are addressing issues regarding handling chemicals. The reasons for insect screens are compelling and the growers are finding it an alternative to pesticides. The National Greenhouse Manufacturers Association has recently published an excellent document on greenhouse insect screen installation consideration for greenhouse operators that is available at no charge by calling 800-792-NGMA and requesting a copy.

What about moisture control with the screens?
Excess moisture stimulates fungus and disease growth on plants. Because the screen retains heat, its temperature remains higher than that of its surroundings, preventing dew drops from forming on the screen and falling on the plants below. Screens also help maintain higher leaf temperatures which prevents dew condensation.

What are blackout screens?
Blackout screens are designed for giving the grower control of the length of a day without heat buildup under the screen. Aluminum is often found on the upside of the screen to reflect unwanted sunlight away from the crop. The underside of the curtain is black polyester to deflect necessary heat back into the growing area to maintain constant plant, soil and air temperatures. Even a grower that does not require daylight control can benefit from the energy savings provided during the night with a blackout screen. A good blackout screen should not allow condensation drops to form to avoid spotting and disease on the crops. A grower can block out up to 99.9% of light without heat buildup by using a blackout screen.

What are decorative screens used for?
Decorative screens are just what they sound like. Available in a variety of colors, they are used in greenhouses as an attractive way to reflect the sun’s rays while offering a comfortable environment for the general public. Thus they are often found in retail garden centers. Basically the screens are made of a combination of aluminum and transparent strips so that they reflect sunlight and retain heat radiation at night. The result is a pleasant interior climate at all times for both plants and visitors, while the grower or retailer realizes savings in heating and cooling costs. The screens are flame retardant so in the event of a fire, they will prevent the fire from spreading.

What about fire hazards?
The ULS series screen comes in the flame retardant REVOLUX version. ULS REVOLUX has all the same benefits and features as the standard ULS but is 100% flame retardant. With the increasing use of screens in greenhouses there is also the increase of risk of fire. ULS REVOLUX will not catch fire and if a fire starts elsewhere in the greenhouse, the REVOLUX material will prevent the fire from spreading to other parts of the facility.

In Conclusion
While long overdue, this article only covers the basics, but hopefully has answered most of your questions. We are always looking to the future for product improvements and thus constantly creating questions and inquiries. It’s our job to inform and learn from the grower and we welcome the interaction. Afterall, the more we know about each other the better.