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Is Your Greenhouse or High Tunnel Heating System Damaging your Plants?

Posted: May 1, 2014

Heated high tunnels and greenhouses where air pollution from the heating system is present, can damage crops. Diagnosing plant injury based on heater-generated pollutants can be confusing as the damage can mimic other symptoms and the pollutants are gases that can pool in areas of reduced air circulation.
Damage caused by heating system malfunction

Damage caused by heating system malfunction

Tomatoes are very sensitive to ethylene, sulfur dioxide and nitrogen oxides. All of these chemicals are part of the brew that is generated in the combustion chamber of our heating systems through burning coal, heating oil, propane, natural gas, or wood. A prolonged cold period in spring will start to wear crops down in situations where either the heating system is poorly vented, not vented at all or poorly engineered. This is especially so in high tunnels where heaters are often a second thought and only used to prevent temperatures from dropping below 45°F at night.

Plants vary substantially in their sensitivity to air pollution and to their sensitivity to specific gases. There are variations between varieties and certainly among species. Light intensity, age of plants, time of day, humidity, watering and nutrient status all impact how a plant reacts to pollutants. High humidity in well-watered plants increases the likelihood of damage as the plants stomata are open allowing pollutant gases to enter.

Although there are many pollutants generated by heater combustion; ethylene, sulfur dioxide and nitrogen oxides are the most common problems.

  • Ethylene at very low concentrations will cause epinasty or the twisting and bending (generally downward) of stems. As the level of ethylene increases, the damage looks more and more like broadleaf herbicide damage with twisted and distorted stems. Ethylene can also cause flower abortion.
  • Sulfur dioxide causes leaf and fruit burns. At lower levels, the damage may be chlorosis or flecking on leaves and fruit. Moderate levels may cause leaves to drop off.
  • Nitrogen oxides cause similar damage to sulfur dioxide at higher levels. At lower levels, the leaves may simply appear darker or have downward curving leaf margins or both.

Take the time to inspect your heating system regularly. This is especially important with older systems (6-8 years +).

  • In propane or natural gas fueled heaters, carefully inspect the heat exchanger for pinholes using a bright flashlight.
  • Higher efficiency units may have a blower to help remove exhaust gases, is this functioning properly?
  • Does your exhaust pipe leak?
  • Is your chimney high enough to prevent downdrafts? Your chimney should extend 3’ over the peak of your greenhouse.
  • The high humidity in a greenhouse is hard on heaters and even the best made heater ages in this environment. Used heaters may not be of any value. This is especially so if they were not carefully stored.
  • Even though propane and natural gas are considered ‘cleaner’ fuels, burning them generates plenty of pollutants.
  • Non-vented, free standing temporary heaters may provide temporary heat, but at what cost to your plants?
  • Smaller heating oil fueled systems are very similar to propane and natural gas. Larger systems typically run central boilers that distribute hot water, so generally are of little concern.

The safest way to run wood and coal fueled heaters is to have them outside of the growing area and use hot water from the boiler pumped through fin tubes and radiators to heat the planting area. It is very challenging to properly run a wood or coal stove inside of a growing area. Make sure that the stove gaskets are in perfect condition, the chimney is leak-free, and that the stack extends at least 3’ above the peak of the growing area. Manually fueled wood and coal stoves require frequent refilling to maintain a constant flow of exhaust gases up and out of the growing area. These heaters are especially likely to be pollution challenges when starting up and refueling due to the potential for downdrafts.

Prevent crop damage before it occurs. Inspect your heaters regularly, follow manufacturers maintenance instructions, keep your chimneys in good condition and avoid the use of temporary, free-standing heaters such as salamanders.

Prepared by Steve Bogash, former Extension Educator

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Is Your Greenhouse or High Tunnel Heating System Damaging your Plants?

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