Most ideal veg/flower temp/vpd to prevent mildew

Hey guys. The vpd target range is huge. I know jungle boys runs their veg room like a jungle. Fogs up their glasses. So like 80 degrees and correlated humidity. Anyone know if that is more or less likely to induce mildew or what the science or real world experience says?

If you’re using the default templates in Pulse, 80° in seedling stage will get you to a pretty high humidity but there’s no foliage at seedling stage so powdery mildew is not an issue.

Check out the temp/RH combinations in veg and flower and the RH that goes with ≈ 80° is below where mildew becomes an issue.

A few thoughts…

Powdery mildew blooms when the tent temp gets to the dew point. PM is always in the air but, when there’s water on the foliage and it there’s insufficient air flow, it has the opportunity to settle and bloom. Stay away from the dew point and your odds of getting PM drop.

If there’s good ventilation, you’ll tend to not have moisture on your leaves. I use a USB fan underneath each plant, blowing up into the canopy. In addition, I run a variable speed fan across the top as well as an inline fan. This is in an open tent so there’s already lots of air circulation.

Dr. Bugbee, in one of the videos he did with the people from Cornell, IIRC, said that adding silica to your nute mix will pretty much eliminate PM. It was an off the cuff, “this is the simple truth” sort of comment. I was pleasantly surprised to hear that since I’ve use Botanicare’s SilicaBlast religiously. If only that kept thrips at bay, as well…

ATM, my grow is in mid-flower and the temp in the tent is 82 with an RH of 54%. I have any qualms about that temp re. PM.

An issue for me is that higher temps increase net photosynthesis but temps > 80°F in flower may reduce the production of terpenes. As a result, I’ve been trying to keep temps <= 80° but it’s not easy because my SO parks her car about 2’ from the tent. As of a few days ago, I decided to let temps rise and adjust the RH as best I can to hit a VPD of 1.4. It’s a bit of a struggle since my tent is in an unheated garage in SoCal so it’s not easy to control temp and RH.

My rationale that decision? I grow for THC not terpenes damn the torpedoes, bring on the heat! :slight_smile:

Regarding running your rooms like a jungle - the reason that growers do that is if they supplement with CO2.

If you’re supplementing with CO2, it raises the ideal temp for photosynthesis so something between 80-90F. To hit VPD targets at those temps, you need to have really high humidity.

High humidity does lead to mold problems, so all of these people running high temp & humidity rooms probably do IMP or run an extremely clean room with positive pressure, etc.

My mildew tips: prevent soil and dust from getting on leaves, that’s where the original spores come from. I.e., don’t bring plants from outside, inside, and use filters on your hvac.

Then, avoid dewpoint at all costs. Dew indoors is most often caused by spikes in humidity (like when an AC cycles off) or rapid dips in temperature; but definitely don’t be running your room close to the dewpoint. The other way dew is caused is by leaves sitting on each other; the preventative for this is consistent airflow across the room and avoiding a crowded canopy.

Bud rot typically doesn’t happen for me unless there is plant injury or bug damage or if I got my plants dirty like splatter from spraying the soil or kicking up dust from a dirty room. Very dense buds can get rot if you wait too long to harvest and they start to die. Once you have bud rot in the room just take it down as soon as you are willing.

Powdery mildew, once it shows up, is practically impossible to eliminate during that crop cycle. If you get it on veg plants, use a horticultural oil spray and cover every square millimeter of the plant. If you get it on flowers… just keep your humidity on the lower end and pray it doesn’t get too bad by the time you harvest.

Whether you get mildew/rot or not, every time you harvest clean, dust, and sanitize your room & change filters and make sure the entire room has been warm (or even hot) and dry for a few days before restarting it.


Very good tips.

Another spray suggestion for PM is something that changes the PH of the leaf surface, either an acid (like vinegar) or a base (like baking soda) mixed with a bunch of water. Make sure not to over-do it, make sure you don’t overspray onto the soil a lot, and do a little to start to see how your plants tolerate it.