Improperly ventilated heaters & ethylene damage

Spring is on its way, and with cold nights and warmer days we are seeing a common spring problem – poor air quality damage on spring bedding crops.  Symptoms, solutions and preventative measures are included in this 2017 update to a previous post.

Natural gas and propane are popular choices when it comes to heating a greenhouse.  The products of burning fuel are carbon dioxide (CO2) and water (H20); both compounds we know are good for your plants.  However, combustion is often (if not always) incomplete, and impurities such as carbon monoxide (CO), sulfur dioxide (SO2) and ethylene (C2H4) are also released leading to poor air quality if your heater is not properly vented.

Typically symptoms from ethylene damage and sulfur dioxide damage can been seen fairly quickly after exposure.

Figure 1. Signs of ethylene damage include leaf curling and epinasty, seen here in A) New Guinea Impatiens and B) lettuce seedlings.

In the short term (a few hours to a few days), ethylene damage results in leaf curling, epinasty (leaves bending downwards from the petiole) and flower drop.  If the stress continues over a longer period (several days to a week or more), plants can take a long time to flower, or not flower at all.  Ethylene levels as low as 0.01 parts per million (ppm) can create symptoms in sensitive species. Levels are usually highest near the heater and can be diluted by air circulation.

Figure 2. Sulfur dioxide damage on Petunia. Burn can be seen on leaf edges within 24 hours.

Upon recovery later in the season, plants will look as if a growth regulator was applied, highly branched and even a bit darker green.  On the bright side, if the stress was short your customers may enjoy this side effect!

All fuels contain traces of sulfur, and during combustion sulfur can combined with oxygen to form sulfur dioxide. Levels greater than 0.5 ppm can cause injury on sensitive species. Once the sulfur dioxide enters the plant through the leaf pores (stomates), it reacts with water to produce sulfuric acid that causes leaf burn and chlorotic spots, which first appear at leaf edges.

Help – I think I’ve got a problem!

Step 1 – Confirmation: Plants that are more sensitive to ethylene and sulfur dioxide damage can be used as indicators of a potential problem.  Place a few pots containing sensitive species (petunia, cuphea and tomato are all good examples) close to the heater and check them daily for signs of damage.

Step 2 – Contingency plan for this season: Remember, the shorter the exposure time, the better outcome for your crop. Venting the greenhouse to dilute the impurities in the air is a good option, however, keep in mind that you’ll need more heat to compensate.  If possible, move the plants to a different location with a different heating system.  Many species will recover if the stress was fairly mild, and new leaves should help hide some of the damage.  Some species, such as coleus, are able to withstand high levels of ethylene stress with no symptoms.  That said; expect the majority of a long exposed crop (more than a few days) to have very delayed flowering or no flowering at all, and to generally look like several doses of plant growth regulator (PGR) were applied.

Step 3 – Fix the problem for next year: Growers that have experienced substantial damage may want to consider replacing the heating system entirely with one that doesn’t vent indoors.  If you are considering installing a heating system that does vent indoors, ask the company for greenhouse customer references and check with them for any concerns or problems before moving forward. Remember to ensure any new system (and maintenance on an existing one) is installed by licensed contractors.

Remember, an ounce of prevention is worth a pound of cure!  Schedule annual maintenance of your heating system before you need it.  Properly maintained systems are more likely to burn clean, giving off fewer harmful impurities.

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