Maybe it's just me, but the term vapor pressure deficit or VPD immediately
sounds like something bad. You know, big ly bad. You're not going to love it. Like
a 347 billion dollar trade deficit, or something you need to treat with a
special ointment. Either way, you're going to live and learn, amigos.
Seriously, vapor pressure deficit or VPD is neither intrinsically good or bad, or
infectious for that matter- thank God. It's just an advanced grow room and
greenhouse metric that combines air temperature, leaf temperature, and
relative humidity into a single handy unit of pressure. I'll get into the
pressure part in just a moment, but for now just note that VPD expresses how
temperature and relative humidity actually affect your plants. Trust me,
when you understand VPD, you are fast on your way to becoming a more intuitive
and ultimately successful grower. Before you know it you'll be writing "Master
Grower" on your résumé. Good for you buddy. You're going to need a min/max
thermometer and hygrometer to measure your grow rooms temperature and relative
humidity, and ideally an infrared thermometer so that you can measure the
temperature of your leaves. Not all VPD calculators take this into
account, but I think it's a good idea. If you don't have an infrared thermometer,
position your thermometer's remote probe at canopy level and take the reading after
a few minutes. Good to go; let's do this.
So, VPD is all about your plants and how they feel in your grow room. Aw!
Scientifically-minded growers favor VPD, as it provides some handy absolutes in
the constant shifting world of the air's moisture-holding capacity at any given
temperature. You already know that plants primarily take up water to keep
themselves cool. We covered this in Humidity 101: they suck up water via
their roots, only to spew the vast majority of it back into the air as
water vapor through tiny pores in their leaves, called stomata- AKA transpiration.
Now, if you could shrink yourself down and teleport yourself inside a leaf,
you'll find conditions in there to be very moist and humid indeed, and we're
still talking about leaves. Gasses inside a healthy leaf are 100% saturated
with moisture, and this creates pressure, referred to as a saturated vapor
pressure. We'll give this the catchy title: "VP Sat." Okay, so now we have this pressure
inside the leaf, but what about the stomata? The tiny little leaf pores-
think of these guys as a gateway between the super moist and humid internal world
of a leaf, and your grow rooms environment. When humid and dry air meet,
what happens? They try to equalize and balance out,
of course. So, if your grow room is hot and dry water, vapor will exit your
leaves more quickly, increasing the transpirational demand on your plants.
Whereas the air in humid grow rooms isn't so different to internal leaf
conditions, so less water vapor can escape. Are you with me? Okay, think
of it like this: VPD is a measure of the drying effect of the air around your
plants. A high VPD number means a major drying effect; a low number means a
minimal drying effect. But, more scientifically: VPD is the difference, or
deficit, between the saturation vapor pressure inside your plants' leaves (the VP
Sat) and the vapor pressure of your grow room (VP air). Okay, it's time for a VPD
calculation example. Note that I've made a VPD calculator available too, in the
description below. Say your grow room's lights have been on for a few hours and
the air temperatures 25 degrees Celsius, or 77 degrees Fahrenheit. The relative
humidity is 50%. That is, your grow room's air is holding half of its maximum
capacity. Using an infrared thermometer, you can measure the temperature of a
leaf at the canopy to be 73.4 Fahrenheit, or 23
degrees Celsius. You now have all the data needed to calculate your VPD. Grow
room air temperature, leaf temperature, and relative humidity. Your VPD is the
difference between the saturated vapor pressure inside the leaf (VP Sat), and your
grow room's vapor pressure (VP Air). To calculate VP Sat, you take your leaf
temperature in degrees Celsius, and plug it into the following equation where T
is your leaf temperature. And hey, to two decimal places, we see that the VP Sat
equals 2.81 kilo Pascals. Next, we calculate the vapor pressure of
your grow room's air. Basically, it's the same equation as before, save an
adjustment at the end for your grow room's relative humidity. T is now your
grow room's air temperature, and H is the relative humidity. In our example, our
grow room's air is 25 degrees Celsius. Blah, blah, blah, and after that
quick math excursion, we calculate the vapor pressure of our grow room's air to
be 1.58 kilo Pascals. Thus, our vapor pressure deficit is VP
Sat minus VP Air, or 2.81 minus 1.58, making 1.23 kilo Pascals,
ladies and gentlemen. Okay, so, what does this mean
in practical terms? Well, let's say that you're cultivating
fast-growing, heavy-fruiting annuals; the sort of plants that a lot of you appear
to be growing by the questions and comments I receive. For propagation and
early vegetative growth, you should be shooting for a low
VPD, so your tender, new plants with underdeveloped root systems aren't
forced to transpire too much. A VPD of 0.4 to 0.8 kilo Pascals is ideal.
Once your plants are established, 0.8 to 1.2 kilo
Pascals is recommended for healthy transpiration. This would apply to both
vigorously-growing plants, and annuals in transition, or the early budding or
flowering stage. In mid to late flowering, your plants need to transpire the most:
1.2 to 1.6 kilo Pascals is a sensible range. This reflects the fact that lower
relative humidity, around 40 to 45%, is often recommended for the
flowering stage, as it tends to create a higher VPD. Anything below 0.4 could lead
to mold and plant disease issues, as well as poor nutrient uptake- especially
calcium. Basically, your plants aren't transpiring enough. VPD's over 1.6 kP
put your plants in danger of over-transpiring, and becoming stressed. This can
lead them to taking up too much nutrient, and resulting toxicity issues. Obviously,
you need to weigh your VPD against your plants' lifecycle development stage. Some
species, cultivars, and phenotypes, can tolerate wider VPD ranges, and others are
a lot more fussy, and need to be dialed in over several crop cycles. Note that
your leaf temperature should read lower than your air temperature. A 2 degree
celsius gap, around 3.5 degrees Fahrenheit, is healthy. If your
leaf temperature is equal to, or higher than your air temperature, then your grow
lights are probably positioned too close. I hope it makes sense; let me know if you
have any questions. If you're worried your VPD is too high, or too low, check
out my next video. But for now, this is Everest transpiring heavily- I mean,
steadily. Bye-bye.
you