So, you have a high VPD problem. Well done. Admitting it is the first step.
Seriously, the preliminary question is: should we treat the causes of your VPP
issue, or do we just try to alleviate the symptoms? The answer is: both.
Really? Yes. Both. Remember: high VPD means there's a big difference between the
saturated vapor pressure inside the leaf and the air vapor pressure in your grow
room. The high vapor pressure deficit is what's forcing your plants to uptake
lots of water, and solutes due to the resulting high transpirational demand
on your plants. Let's dive straight into the example. Back in May, I erected a lot.
I erected a Sun Hut blackout 100 grow tent in my attic. The ambient relative
humidity was low, hovering around 25%, at which the air temperature was 26 degrees
Celsius, or about 79 degrees Fahrenheit. Here are some tomato cuttings looking
happy in their deep water cloner. This is just a regular Super Sprouter Aquaclone
filled with a 50-50 mix of tap water, and RO water, adjusted to pH 6. Note that I
didn't add any nutrient,s and I measured the leaves temperature to be around 24
degrees Celsius, or 75 degrees Fahrenheit. Using my trusty VPD calculator, I see
that gives us a DPD of 2.14 kiloPascals.
Yikes, this is way over my suggested range for propagation, and early
vegetative growth: 0.4- 0.8 kiloPascals. So, why are my cuttings even
alive let alone looking pretty healthy? Answer: well yes, the VPD is way too high
and yes these cuttings would thank me for a humidity dome, but they're not
exhibiting any symptoms of high VPD because they're just sucking up water
with virtually no dissolved nutrients in there. As soon as i add nutrients to the
equation by transplanting them into a rich soil based potting mix, still
feeding with water, then bam! Look how sick they get! At this point, a novice
grower might well point an accusatory finger at the potting mix and march
angrily to his local Hydro store, demanding a refund. But there's nothing
intrinsically wrong with the potting mix. It's my growing environment. Take a
closer look at those leaves: chlorosis, necrotic spots. Just horrible. So what
sequence of events has led to this situation? Think of hi VPD as just the
start of the story, not the epilogue. A high VPD causes our freshly rooted
cuttings to over transpire, as too much water is being sucked out of them by the
dry air. This in turn, forces our tender young
plants to uptake lots of water from the potting mix. This water carries solutes,
or dissolved nutrients from the potting mix, and I'd wager that the preliminary
culprit here is calcium: an immobile element that relies on transpiration to
transport it through the plant. So, if transpiration rates are too high, guess
what: too much calcium is taken up. This, in turn, causes other nutrients to be
locked out: potassium, magnesium, manganese, and iron. Now we have what I will
politely call a chain reaction. Iron and magnesium lock-out means no more
chlorophyll can be produced, hence the pale leaf color. Potassium and manganese
deficiencies cause their own set of problems, including stomatal regulation.
So what causes high VPD? Essentially high temperatures and low humidity are two
antagonists. Their effects can be exacerbated by too much air movement
around the leaves or radiant heat from grow lights being too close to the
leaves, especially grow lights that emit a lot of infrared like HPS. Move your
grow lights up, then dim them down, or change to a light source lower in
infrared, like LED. Alternatively, consider switching to an air-cooled hood so you
can safely dial down your gardens' ventilation in an effort to guard some
transpired moisture and elevated relative humidity. You can also add a
humidifier, but I find that they don't work well in small grow tents. Better to
humidify the room the grow tents are situated in. Use humidity domes to create
a localized, high-humidity zone around your plants. Or, if your plants are larger
and in mid-flower- that is, too big to cover with humidity dome- then you're
going to have to keep the PK booster on the shelf, my friend, and reduce your
nutrient concentration. Here's an example: if you're shooting for a maximum VPD of
1.6 kiloPascals, and you find yourself to be around 2.0, then you're 0.4
kiloPascals too high. That's 25% over! Try to offset this by lowering the strength
of your nutrient solution by at least the same percentage. So, if you're feeding
at 1,000 PPM, reduce to 750 PPM while
you try to fix the causes of the high VPD. If your feeding schedule includes
CalMag, try reducing that first, and like I said, forget about using any PK
boosters until your high VPD pressures are resolved. Finally, greenhouse growers
sometimes wet the greenhouse floor or crank up the evaporative cooling in an
effort to alleviate high VPD. Similarly, try spraying the inner walls
of your grow tent; trays of water on the floor to increase humidity. Okay, I'll
cover dealing with low VPD's in a follow-up video soon. Take care, amigos.
And don't forget to post any questions or comments down below. You know I love
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Thanks for watching! This is Everest VPD OUT.