Floriculture Greenhouse Update

Nature seems to be getting more violent in recent years with frequent earthquakes, increased numbers of hurricanes and record breaking snowstorms.  Insurance damage claims have increased considerably.  The International Building Code has revised upward its wind and snow loading requirements for some areas of the U.S.  Each year there are reports of greenhouses that have been damaged by weather and natural events.  Greenhouse design is different than conventional farm buildings in that the structural profile has to be small to allow maximum light to reach the plants. Most farm buildings are over designed to handle severe weather conditions.Storm damage to greenhouses can include racking of the frame, bending of the hoops, broken glass or torn plastic and uplifted foundation posts.  Preparation ahead of time can minimize the damage.Wind loadingWind forces that act on a greenhouse are influenced by numerous factors including the basics wind speed, building orientation, exposure, height and shape of doors or vents that may be open.  The wind passing over a greenhouse creates a positive pressure on the windward side and a negative pressure on the leeward side.  These can combine to create a force that wants to collapse or overturn the building.  An 80 mph wind can produce a pressure of 16 pounds per square foot (psf).  For example, the 10' by 100' sidewall of a gutter-connected greenhouse would have to resist a 16,000 pound force.Wind can also create a force similar to an aircraft wing that wants to lift the greenhouse off the ground.  An 80 mph wind blowing perpendicular to the side of a 28' x 100' hoophouse can create a lifting force of 220 pounds per foot of length or 22,000 pounds of uplift on the whole structure.  When you consider the total weight of materials and equipment in the greenhouse is about 6000 pounds, the foundation must have a withdrawl resistance of about 300 pounds each.  This is why building inspectors frequently require that the posts be surrounded by concrete.Although you have no control over the force or direction of severe winds, here are a few tips to help minimize storm damage:

• Check the area for loose objects.  Anything that can be picked up and hurled through the glazing should be secured or moved indoors.  Metal chimney (stove pipe) sections should be secured with sheet metal screws.
•  Inspect for dry or weak tree limbs that could fall on the greenhouse.
•  Close all openings including vents, louvers and doors.  The effective force of the wind is doubled when it is allowed inside the building.  The wind on the outside puts a pressure or lifting force on the structure.  The wind inside tries to force the walls and roof off.
•  On air inflated greenhouses, increase the inflation pressure slightly by opening the blower’s intake valve.  This will reduce the rippling effect.  Check to see that the plastic is attached securely and that any holes are taped.
•  Disconnect the arm to the motor on all ventilation – intake shutters and tape the shutters closed.  Then turn on enough exhaust fans to create a vacuum in the greenhouse.  This will suck the plastic tight against the frame.
•  Windbreaks can reduce the wind speed and deflect it over the greenhouse.  Conifer trees (hemlock, spruce, pine, etc.) in a double row located at least 50' upwind from the greenhouse can reduce the damaging effects of the wind.  Wood or plastic storm fencing can be used as a temporary measure.

John Bartok, Extension Professor Emeritus and Ag Engineer, UConn

Fact Sheet: Reducing Storm Damage to Your Greenhouses (includes snow loading also)

Note: With wind, rain and cooler temperatures, growers are advised to continue inspecting plants and use preventative sprays for diseases such as Downy mildew on basil and Chrysanthemum white rust on garden mums.  Leanne Pundt, UConn

We came across this short video (15 minutes long) that might interest some growers. A Minnesota farmer, Steven Schwen explains how his thermal-storage greenhouse works. He uses diagrams and talks a little about costs. This project was funded in part by SARE.

http://cookingupastory.com/sustainable-energy-thermal-banking-greenhouse-design 

Paul Lopes and Tina Smith University of Massachusetts

Occasionally growers want to use an unvented heater as a back up heat source. The following was taken from an article that John Bartok, Agricultural Engineer wrote on using unvented heaters. Tina Smith, UMass

An unvented heater is one that is designed without a flue connection so that the heat and products of combustion are exhausted into the greenhouse. Dumping these flue gases into the greenhouse may improve the overall efficiency rating as compared to a conventional heater but the pollutants and added moisture from combustion may put your plants in jeopardy. Unvented heaters can be fired with natural gas, propane or kerosene. These fuels are advertised as clean-burning. This is generally true if the burner is clean and has been adjusted to top efficiency. A heater that has been serviced in the fall will probably not be at peak efficiency as spring approaches. Inefficient combustion can emit unwanted and harmful pollutants into the greenhouse that can affect the plants and the people working with them. All fuels contain traces of sulfur, some more than others depending on its source. During combustion sulfur in the fuel is combined with oxygen to form sulfur dioxide. Levels as low a ½ part per million (ppm) can cause injury on some plants. Once the sulfur dioxide enters the plant through the stomates, it reacts with water to produce sulfuric acid that will cause leaf burn, flecking and general chlorosis. Tomatoes and white petunias are very sensitive to this and will show signs in as little as one hour. They therefore make good indicator plants. Ethylene gas is another pollutant formed during combustion. It can also be formed from ripening or rotting plant materials. Levels as low as 0.01 ppm can create symptoms such as malformed leaves and flowers, stunted growth, bud abcission, epinasty and flower senescence. Levels are usually highest near the heater and can be diluted by air circulation. Care should taken if you install unvented heaters. Be sure to have an adequate makeup air supply and provide frequent maintenance. Use indicator plants near heaters or commercially available indicator tubes to warn of excess levels of pollutants.

For the complete article see: Problems With Using Unvented Heaters

The payback on energy conservation is only 6 months to 2 years according to John Bartok at a recent Connecticut Greenhouse Growers Program.

Energy conservation is an important first step even if you are considering changing to alternative fuels. So, take some time to do a walk thru energy audit of your greenhouse operation and see web sites below for links to more information on energy conservation and renewable energy to assist in your decision making and planning for the future.

For more information:

Connecticut Greenhouse Growers Program: Dealing with Energy

Fact Sheet: Greenhouse Energy Conservation ChecklistGreenhouse Energy Website

University of Massachusetts:Energy Choices for Farms and Greenhouses

North Carolina State University Database: State Incentives for Renewables and Efficiency

Leanne Pundt, University of Connecticut

Bare heating system pipes waste a considerable amount of fuel each year in areas such as boiler rooms and headhouses where heat is not needed. This heat loss continues every day the system is operating. Adding 1" thick fiberglass or foam insulation to a ¾" pipe will save about $2.25/linear foot and on a 2" pipe about $5/linear foot over the heating season in northern climates. The payback usually takes less than two years. Installation is simple and can be done by unskilled workers in slack time.

John Bartok

Mechanical thermostats tend to loose accuracy over time. You can easily check the accuracy of a thermostat. Start by checking the accuracy of a good thermometer by inserting it into an ice bath. The reading should be 32°F. After allowing it to come back up to room temperature, place it next to the thermostat you want to check.. Slowly move the dial until the heater turns on. The reading should be the same as the thermometer reading. If not, mark the thermostat accordingly. Next time the heating system is serviced, have the service person recalibrate it. If the thermostat setpoint is 1°F too high, a 30' x 100' double poly covered hoophouse will use an additional $300 of fuel for the heating season. This is based on maintaining 60°F inside where the average winter temperature is 25°F outside with fuel oil at $2.00/gallon, natural gas at $1.37/therm or propane at $1.17/gallon.

John Bartok

One factor that influences heat loss from a greenhouse is the amount of glazed area. In a 30° wide hoophouse, the glazed area from the ground to bench height is about 15% of the total surface area. Insulating this area with an inch or two of polyurethane or polystyrene can reduce total heat loss over 10%. Use a closed cell insulation board and not beadboard as this absorbs moisture reducing its insulating value.

For more energy saving tips see: Greenhouse Energy Conservation Checklist

John Bartok

In some greenhouses, cold air infiltration adds considerable to the cost of heating. Cracks around doors, vents and shutters that don't close tight, broken glass and tears in the plastic are typical examples. For example, a 4 foot square shutter that fails to close fully and leaves 1/2 inch gaps will allow about 12,000 cubic feet of cold air to enter each hour. To heat this amount of air over a 24 hour period to 60°F when the outside temperature is 0°F require almost 4 gallons of fuel oil. Most infiltration leaks can be corrected with minimal cost. Weatherstripping and foam insulation work well on small gaps. Shutters not needed for cooling should be covered with a sheet of film plastic or one inch of polystyrene or polyurethane insulation board. A few hours spent in tightening your greenhouses is well worth the effort.

Fact Sheet: Greenhouse Energy Conservation Checklist

John Bartok