VPD by Day, Dew Point by Night

VPD Vapor pressure deficit The gap between how much moisture the atmosphere holds and how much it could hold. When VPD collapses, transpiration stalls and calcium stops reaching developing tissue. Full entry → is the number you steer, and there are only two inputs to steer it with: how warm the room is and how much water the air is holding. The same VPD can come from a warm dry room or a cool humid one, and those are not the same room.

Dew point Dew point The temperature at which saturation occurs and water condenses. Sizing dehumidification to a target dew point is more reliable than targeting a humidity percentage. Full entry → is the one that does not move when the temperature does. It depends only on how much water vapor is actually in the air.

Setpoints by stage

These are the numbers we run: a sealed room with supplemental CO₂ and a cool finish for color, which is the configuration that makes the trade-offs hardest. Day and night are split because the dark period is a different control problem from the photoperiod Photoperiod The hours of light a crop receives in a 24 hour cycle. Cannabis flowering responds to the length of the uninterrupted dark period rather than to the light hours themselves. Full entry → , and it is where disease risk actually lives. Stages before the taper Taper The scheduled wind-down over the final days before harvest, bringing CO₂, canopy temperature and humidity down together rather than one at a time. Full entry → count forward from the flip. The taper counts backward from harvest, so on a 9-week cultivar Cultivar A specific cultivated variety maintained by propagation. It is the precise term for what is loosely called a "strain". Full entry → it begins partway through week 8.

The day column is aimed at transpiration Transpiration Water moving up through the plant and evaporating out through the stomata. It drives nutrient uptake, and carries calcium, which moves almost entirely by that flow. Full entry → . VPD is the pull that draws water up the xylem Xylem The vascular tissue that carries water and dissolved minerals from root to leaf. Flow is one way and pulled by transpiration, so anything that slows transpiration slows delivery. Full entry → , and that water carries calcium into the shoot tips, so the day figures are a growth input rather than a disease guard. Propagation gets the lowest figure because an unrooted cutting can take up very little. Veg through flower runs higher: enough to keep calcium moving and the stretch in check, not enough to close the stomata Stomata Adjustable pores on the leaf surface, each framed by a pair of guard cells, that let CO₂ into the leaf and water vapor out. The plant opens and closes them constantly. Full entry → , which is what CO₂ enrichment depends on. During the taper the day target comes back down with the temperature.

Both humidity columns are outputs. At night the rule is one interval: the dew point sits 15 °F (8 °C) under the night setpoint, which holds relative humidity Relative humidity The share of moisture air is holding against the most it could hold at that temperature. Because it moves with temperature, cooling air raises RH with no water added. Full entry → six to seven points under the 65 percent ceiling at every night temperature in the schedule. By day the rule is one deficit: at each stage temperature, humidity sits wherever the leaf VPD target needs it. Both come out through the Magnus equation Magnus equation An empirical curve fit giving saturation vapor pressure from temperature alone. Not derived from first principles: its constants come from fitting measured vapor pressure. Full entry → for saturation vapor pressure Saturation vapor pressure (SVP) The ceiling on how much water vapor the atmosphere can hold at a given temperature, in kPa. It climbs steeply as a room warms, so the same VPD needs a higher relative humidity in a warmer room. Full entry → .

How to run it

If your controller accepts dew point, enter the figure from the night table and leave it. It stays valid as the room steps down through the taper, which is the whole reason we hold it.

If it accepts only relative humidity, which is the common case, use the implied humidity column. It sits near 59 percent the whole way down, because the dew point steps down with the room. That percentage is only true at the temperature it was computed for, and that is what the margin is for: a room set to 73 °F (23 °C) that actually runs at 70 °F (21 °C) takes the same 58 °F (14 °C) dew point from 59 percent to almost 66 percent, over the ceiling, with nothing wrong but 3 °F (2 °C) of undershoot.