Discussion on Vapor Pressure Deficit

I decided to start this thread and am hoping tp learn more.

I believe @Renfro posted a chart at some point.


I thought at 50% rh with temps at 76º-79º was ideal but from what I am reading my VPD is still too high.

@Renfro please feel free to chime as well as @Carolina Dream'n

I try to follow VPD in veg and early flower. Late flower you risk mold with the high humidity you will want
LED vs HPS heat up leaf surfaces differently. Take an IR thermometer and figure out the leaf surface temp difference compared to the room temp

my led room has a -2degree leaf surface difference so I use that vpd chart

Nizza said:

I try to follow VPD in veg and early flower. Late flower you risk mold with the high humidity you will want
LED vs HPS heat up leaf surfaces differently. Take an IR thermometer and figure out the leaf surface temp difference compared to the room temp

my led room has a -2degree leaf surface difference so I use that vpd chart

Click to expand...

So what are the effects of different lights? In my case 315cmh vertical Hortilux bulbs.

Thanks

ilovetoskiatalta said:

So what are the effects of different lights? In my case 315cmh vertical Hortilux bulbs.

Thanks

Click to expand...

CMH has more IR than LED but less than HPS. HPS it's like 60% of the output is far red.

As to VPD:

Leaf surface temps (LST) are more important that room temps IMO. Different lighting sources will have varying levels of IR and therefore will effect LST differently. A VPD chart is a general guideline but you really have to figure out where the goldilocks zone is for your strain under your lights, and perhaps with your CO2 levels.

For most growers it's just your gut and a few data points like room temp / humidity and IR thermometer LST readings. Tools like FLIR will give a much better picture of what is going on with your plants. Another new tool that is starting to gain traction is spectral imaging where we can see things like when the plants are stressed, diseased... This technology is going to change how commercial gardens detect problems.

Running high humidity to keep your VPD in line can be playing with fire. Just a few degrees or %RH can put you out of the goldilocks zone and cause problems like PM and botrytis. Where is your goldilocks zone? It's not as simple as just a chart because of the variables like strain and grow lights. Products like the AirROS will help mitigate problems caused by swings outside of the target range.

Yeah I find that keeping humidity at 30-45% is just fine at 77-79 degrees. I would rather remove virtually any chance of mold and decrease yield by 5-10% than risk losing everything to mold.

Looking at that chart you would think these would all be dead.

Great thread! I've been working all day on a similar thread Id think you'd enjoy op. Hope you check it out.

Relationship between ambient conditions and plant health and growth