I’ve had a handful of calls in the past few weeks asking me to identify poor air quality damage on spring bedding crops. Even if you have never had problems, the following is a good refresher on why proper maintenance of greenhouse heating systems is important.
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.
These impurities can leave your crop looking a bit down (Literally! It’s called epinasty, see more below), and it can happen in as little as 24 hours. Plants are much more sensitive to these impurities than humans or other animals, so while a CO monitor might not be detecting anything out of the ordinary, your plants could be dying for some fresh air. I’ve summarized some common signs of damage here.
Ethylene damage:
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. Later in the season, the plants will look as if a growth regulator was applied, and if the stress was short your customers may actually enjoy this!
Sulfur Dioxide damage:
All fuels contain traces of sulfur, and during combustion sulfur can combined with oxygen to form sulfur dioxide. Levels from 0.5 ppm and up 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. The burn is often first present at the edges of the leaf.
So, what can you do to prevent damage? Annual servicing of your heating system and keeping it in good repair is a good starting point. Schedule annual maintenance by a licensed contractor before winter and early spring when heating is necessary. Properly maintained systems are more likely to burn clean, giving off fewer harmful impurities.
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. Remember, the shorter the exposure time, the better outcome for your crop.
If damage has occurred, now is the time for a contingency plan. 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. If plants must stay in the environment, many species will recover if the stress was fairly mild. 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 to have very delayed flowering or no flowering at all, and to generally look like several doses of plant growth regulator (PGR) were applied.
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.
Our thanks to the growers who contributed pictures for this blog post!
References:
https://ag.umass.edu/fact-sheets/problems-with-using-unvented-greenhouse-heaters