Malfunctioning greenhouse unit heaters can cause crop problems

Ethylene from malfunctioning greenhouse heaters may lead to crop injury. Know the symptoms and how to check unit heaters to avoid the concern.

Now that the greenhouse season is in full swing, more space is being utilized and heaters that have not been used for a year are being fired up. Keep the following in mind as you open up more growing space.

Crops grown in greenhouses that utilize natural gas or propane fired unit heaters can be susceptible to ethylene injury. Ethylene (C2H4) is an odorless, colorless gas that acts as a plant hormone. Plants are very susceptible to ethylene injury at levels from 0.01 to 1 ppm or more. No other air pollutant causes a greater range of symptoms than ethylene gas. Symptoms range from shedding or shattering of flower petals, misshapen or malformed leaves and flowers, thickened stems, leaf yellowing or chlorosis, stunted plant growth, flower bud and leaf abortion to epinasty or twisting.

The effects on greenhouse crops will vary with the plant species and growth stage, temperature, length of exposure and the concentration of the ethylene. I have noted plant injury symptoms more often in plastic greenhouses compared to glass greenhouses, due to the airtight nature of poly-greenhouses.

An indicator plant to use for ethylene is a tomato plant. They are highly sensitive and will twist or wilt when exposed to ethylene. Tomatoes will exhibit injury within 24 hours if ethylene is present.

To avoid ethylene injury, unit heaters need proper ventilation and intake of fresh air from the outside. One square inch of vent cross section (of outside air) for every 2,500 Btu’s of heater output is recommended. Consider using a laundry dryer vent hose as a fresh air intake. Thus, if you have a 125,000 Btu heater, you would need an 8-inch diameter fresh air inlet pipe that would give you the 50 square inches you need. The newest high efficiency heaters have fresh air intake systems as part of the installation kit.

Also, unit heaters need to be maintained so that the heater itself is running properly, and the distribution tube, vent stack, ventilation louvers and fuel line are all functioning correctly. Try this checklist for proper maintenance.

Annual heating unit checklist


You should…

Heat exchanger

Check for cracks. While the furnace is running, inspect for light penetration.


Check for leaks. Place a smoke bomb or furnace candle within the firebox.

Gas lines

Check for leaks. Painting soapy water on the joints and seams.

Exhaust chimney

Check for leaks and obstructions.

Pilot light

Clean pilot and orifice.


Make sure the burner flame is clear blue. Yellow or orange flames represent impurities or a wrong setting.

If you suspect ethylene injury is occurring, contact your local MSU Extension floriculture educator. We can look at the crop and obtain air samples to verify if ethylene is the problem. Also, call your furnace maintenance firm to inspect the unit in question.

Most plants will recover from ethylene injury. However, those plants that bloom once (like lilies, tulips and hyacinths) that have been exposed to ethylene when they were in the flower bud stage will likely not bloom.

Remember, ethylene is an odorless, colorless gas that is a harmful pollutant to your greenhouse crops.

dosage effect of ethylene
The dosage effect of ethylene on impatiens. Plants not exposed to ethylene (A). Plants
exposed to 2 ppm ethylene for one day (B), two days (C), and three days (D). Initially
only open flowers abscised, then buds began to abscise. After three days of exposure
all flowers and buds had been shed. Photo credit: “Ethylene Contamination: Symptoms and
Sources in the Greenhouse
,” Dr. Michelle Jones, The Ohio State University.

Related Events

Related Articles

Related Resources