Maximum Yield Cannabis USA April/May 2018 | Page 28
grow
V
“When we
talk about
humidity,
we’re talking about
the amount of
water in the air in
the form of vapor.”
apor pressure deficit (VPD) has become a common term
among cultivators. Various bloggers and cultivation experts
have been weighing in on VPD and why it’s important for growing
healthy, high-yielding plants. In this article, we’re going to clarify
what VPD is, why it’s important, and what you can do to ensure
you’re achieving the optimal environment for cultivating cannabis.
To understand VPD, it is important to understand how humidity
works. When we talk about humidity, we’re talking about the
amount of water in the air in the form of vapor. There are two ways
to measure humidity—absolute humidity and relative humidity (RH).
Absolute humidity tells us exactly how much water vapor is in the
air while relative humidity tells us how much water is in the air as a
percentage of how much water the air can hold at that temperature.
This is where things can begin to get confusing temperature and
humidity have a complicated dynamic. As temperatures increase,
the air is able to hold more water vapor than at lower temperatures.
This means that a room will see a drop in RH when the absolute
humidity has not changed, but the temperature has increased.
So, why is this important? All cultivators know managing humidity
is important. For example, high humidity levels can lead to mold
growth, damaging crops and affecting yields. However, RH levels
also dictate how much a plant will transpire, and thus, how much
they will grow. This is because the water we give to plants merely
acts as a vehicle for nutrients. Once the nutrients have been
metabolized, plants transpire water back into the air and are able
to bring in more water and nutrients again. But when RH levels are
high, plants have a harder time doing this.
The reason for this is water vapor in the air creates a certain
amount of pressure, pushing back on plants as they try to transpire
water. As you can imagine, more water vapor in the air (i.e. higher
RH levels) means more pressure, causing plants to have a harder
time transpiring. This brings us to what VPD actually is. As the
name indicates, this is a measure of a deficit, or difference between
the pressure that could be exerted by the water vapor at 100 per cent
RH at a given temperature and the actual pressure exerted by water
vapor at your current RH at the same given temperature. Essentially,
this is the way the plant would feel measuring the difference
between the pressure inside the leaf and the pressure of the air
outside, giving us an idea of how easy or difficult it would be for
the plant to transpire. Vapor pressure deficit is measured in units of
pressure and is essentially RH and temperature in a single value.
Much research has been done to find the ideal VPD for cannabis
transpiration, ensuring plant s drink slowly enough to be able to
metabolize nutrients but fast enough to ensure they are taking up
enough. Depending on the stage of growth, flowering plants should
have a VPD of between 10 kPa and 15.5 kPa.
It’s clear VPD is important for
healthy plants but requires strict
attention to detail and equipment
that can ensure consistent humidity
levels and temperatures. Unlike
common HVAC systems, there is
climate control equipment designed
specifically for cultivation.
Stephen Keen is a 14-year veteran of the
cannabis industry and life-long tinkerer.
He saw a need for cannabis-tailored
climate control solutions, which led to his
co-founding of Hydro Innovations—now
Surna Inc—in 2007. Surna’s mission is to
design efficient, reliable, and intelligent
equipment for climate control for
cannabis cultivation.
28
grow. heal. learn. enjoy.
myhydrolife.com