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A covered hoop house, backlit and glowing from the evening light, sits in a grassy field in front of backdrop of trees at Bluebird Farm in Morganton, NC.

Dear Tom—

Our spring was very cloudy. What can we do to compensate for low light in our greenhouse?


—Bill in Highlands



A direct approach is to add lights to your greenhouse but they can cost tens of thousands of dollars for a large greenhouse. Some lighting experts recommend against lights at our latitude since the light fixtures are likely to block more light than they provide.

A few years ago, Melissa Brechner from Cornell University spoke to the NC Greenhouse Vegetable Growers Association on greenhouse lettuce production in NY. One comment in particular caught my attention: “I can turn a cloudy day into a sunny day with CO2.” Some botanists believe that plants evolved at a time when carbon dioxide levels were higher than now. While plants grow well (by normal standards) at current CO2 levels, photosynthesis is more productive at higher levels. R G Hurd did light and CO2 studies on tomatoes indicating that CO2 at 1000 parts per million (ppm)—300–400 is typical—produced tomato growth similar to adding 30% more light.

Gray CO2 monitor with an interface that measures CO2 in parts per million, temperature, and humidity range with colored lights that indicate low (red), normal (yellow), and ideal (green)

Plants use carbon dioxide in the presence of sunshine to produce sugars that are turned into plant tissues and eventually into tomatoes. Plants are more efficient at using limited light when carbon dioxide levels are higher than normal.

We started exploring CO2 supplementation on our farm by buying a CO2 monitor in the $100–150 range. We start our tomatoes in November in our walk-in cooler with lights and a heater added to replace the normal cooling function. When I checked on the plants in the morning, the first thing I noticed in that closed space was that CO2 levels were down to about 200 (420 ppm is normal for outside in Western North Carolina). After working in the cooler on grafting plants for a few hours the CO2 levels rose to 900 ppm from my breathing.

In Norway, a location with famously low light in winter, an early study found that CO2 levels of 1000 ppm produced 30% more tomato weight and 19% more tomatoes than ambient levels Some studies show an effect even at night by reducing respiration with higher CO2.

CO2 supplements can be overdone. Damage to plants can occur above 1500 ppm. Occupational safety limits are around 5000 ppm for people. On a side note about human wellness, CO2 can exceed 1000 ppm in closed residences from breathing and alertness. Test-taking success drops off when lots of people are in a closed room due to CO2 levels, e.g., SAT tests.

Several systems exist for providing extra CO2 in a greenhouse. The main choices are bottled CO2 or propane burners to generate CO2. Incomplete combustion products are very bad for tomatoes, particularly ethylene, so some advisors recommend against burners for vegetable production. CO2 bottles are available from hydroponic suppliers like Fifth Season or Lotus in Asheville, as well as welding gas suppliers. A refill is about $20 and the bottle deposit is about $50. Rate of use depends on the size of the space. We use more in cloudy weather, but about one bottle a week for a 26 x 96-foot greenhouse might be typical.

CO2 Tank attached to a digital controller

Three types of controls are used. The cheapest is no control—just point the gas bottle outlet away from plants and carefully open it for an amount of time determined by trial and error. CO2 is heavier than air so aim high. Behind internal circulating fans might be a good target. A more sophisticated approach is a “sniffer” that senses CO2 levels in the enclosure and releases gas to hit a programmed target. They can work with fan controls to prevent releases while fans are on. Some models with a sensor and regulator are available for around $300. The mid-range approach in terms of effort and cost is a timer that controls a valve on the CO2 tank. Some experimentation may be needed to get the flow rate right, but in general, the timer is set to start gas flow when fans are typically off at the end of the day and before they start up in the morning. With a timer, manual operation is needed on cloudy days. A timer and gas regulator cost about $125.


On cloudy days, plants use up CO2 inside greenhouses and growth drops off. Turning on fans periodically will solve this problem, but probably will increase heating expense. Adding CO2 to atmospheric levels will counter that effect also. Adding CO2 to about three times outside levels on cloudy days will also increase plant growth as if the day were 30% brighter. Supplemental CO2 early in the day, late in the day, at night, and on cloudy days will increase growth rate roughly 60% compared to CO2-deprived plants on cloudy days. The cost of a mid-range system is about $200 plus $20 per week for CO2.

If it takes 15 weeks to take tomatoes from sowing to harvest, CO2 supplementation could reduce that time by five weeks, perhaps elongating winter break for overworked farmers. Faster growth means less heating expense, as well as a shorter time using fans and irrigation. It seems likely that a CO2 system could pay for itself in the first year, but our farm is not set up to do that experiment.

I hope this is helpful. Thanks for your question.




Tom Elmore

Tom Elmore

Tom Elmore is co-owner and operator of Thatchmore Farm in Leicester NC. He has grown certified organic fruits and vegetables for 25 years and serves on the Boards of the NC Greenhouse Vegetable Growers Association and the Organic Growers School.