November December 2023 - 15

LIGHTING
A pressurized carbon dioxide tank
sits outside a greenhouse used for
strawberry production. The carbon
dioxide is vaporized and released
into the greenhouse when the
greenhouse concentration drops
below a set point value.
by high-intensity discharge fixtures. Return on investment
calculations are needed to determine whether it makes
sense to invest in the latest lighting technology.
CARBON DIOXIDE CONTROL
In addition to light (energy) and water, carbon dioxide
is the third input for photosynthesis. Therefore, each
of these three inputs can become the limiting factor for
photosynthesis. The concentration of carbon dioxide in the
air is very small (on average 420 parts per million as of this
writing) and plants growing in an enclosed environment
such as a greenhouse can quickly deplete the available
carbon dioxide.
Because carbon dioxide is colorless and odorless,
we need sensors to tell us what the concentration
is. Such sensors are especially important when we
use supplemental lighting. If the carbon dioxide
concentration is insufficient to maintain a high rate
of photosynthesis, running the supplemental lighting
system will not result in more plant growth because the
carbon dioxide concentration is too low. Increasing the
carbon dioxide concentration can be accomplished by
burning natural or propane gas in specially designed
burners, or by vaporizing liquefied carbon dioxide.
Growing crops that have a large amount of biomass
during the winter months when ventilation rates are
often low usually requires carbon dioxide enrichment so
that the rate of photosynthesis remains high (assuming
light and water are not the limiting factors). Research
has shown that enriching the greenhouse environment
with carbon dioxide to a concentration of 1,000-1,200
ppm can be beneficial during times when the ventilation
system is inactive. At those concentrations, there is
no negative impact on humans working in such an
environment, but for safety reasons, an alarm system
should be installed that can warn people when the
concentration exceeds 5,000 ppm.
INTERACTIONS
It would be nice (from a control point of view) if the
environmental parameters impacting plant growth and
development were independent, but that's not the case.
Changes in air temperature impact the humidity and the
rate of photosynthesis, as do changes in light intensity
and carbon dioxide concentration. Because greenhouses
are designed for maximum light transmission and are
therefore not well insulated, the indoor environment
experiences substantial fluctuations that have to be
counteracted by the control system.
The more we impose stringent control set points,
the more expensive it will be to achieve them. If, on the
other hand, we can relax the control approach (e.g., by
aiming for an average temperature over a specific time
period as opposed to a fixed temperature, or by moving
supplemental lighting to times of the day with lower
electricity rates), we may be able to save substantially on
the overall operating costs.
I recommend trying novel approaches in a small
growing area before deciding on implementing changes
for an entire operation. Consulting with crop advisors,
researchers, Extension personnel, equipment suppliers
and fellow growers can provide useful insights.
A.J. Both is a professor and Extension specialist for the
Department of Environmental Sciences at Rutgers University.
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November December 2023

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