Hello,
I am a few days into week 3 of flowering, and I started to notice some brown spots on the stems a couple weeks back. I thought it was a fluke or them rubbing against my netting, but want to get everyone’s opinion.
Lit Farms: Rainbow Marker (photoperiod)
Light cycle: 12/12
Nutrients: Athena Blended
Substrate: Tupur Coco in 3 gallon buckets
EC in: 2.5
EC out: 5-6 (within Athena’s recs)
pH in: 6.2
pH out: 5.6 (lower than I want but Athena said it’s typical for this stage)
ThinkGrow Model H Plus: 800 PPFD
RH: Day 65-72%, Night 62-65%
CO2: 1200-1400 ppm
Temp: Day 79-82F, Night 77-79F
VPD: 1.0-1.2
Other observations: a little nute burn when I wasn’t getting enough runoff, otherwise they seem healthy. The brown spots appear cosmetic so no soft spots or weakened structure. I cut a stem open and it appears to be only on the surface.
Thank you!
I wouldn’t concern myself with the random spots on the stems and or stalk. I would be concerned about your temps and humidity. Humidity is pretty high. It is early in flower but I’d recommend taking steps to dropping it. If you stay above 80° get it under 55-60% I noticed you mentioned cO2? You should be pushing lights much harder than 800ppfd to get any benefit from that, your temps should be higher. How are you measuring ppfd?
Hello, thank you for the reply. I am having an issue with my light/controller at the moment. Trying to push it to 1200ppfd, but it is having trouble. So I normally would be keeping it at 85F. Calling the manufacturer this afternoon to troubleshoot.
I have been keeping RH at a max of 72% until the buds bulk up more. I drop it down to 50-55% the last 3-4 weeks of flower.
I measure PPFD with the TM Par meter.
It is a true hermie. Good eye! Found it after taking the pics. Similar to what the first responder, Low, mentioned above, I had multiple things out of scope and stressed her out. Unfortunately, I noticed some brown tipped pistils on the other plants after closer inspection so I think they all pollinated. I’m going to chop them down and try to fix some of my issues.
Sanitize it well. 
Sorry man.
Hey! I have a few more sessions under my belt now, have learned a lot, but still learning in my opinion. I have done a lot of research since this post, and I am starting to see what you are saying about the co2/temps/humidity. I am a scientist by trade, so I treat this like doing variables in an experiment. Not concerned with yield, just want to learn.
I’ve dialed a lot of things in, started to monitor drybacks, bought a stronger led that can achieve a much higher PPFD, etc.
You seem very knowledgeable. My question is what do you think the optimal PPFD, temps, and humidity are to utilize that trio for a better outcome? Also, what stage would you do this? I am in the stretch/flowering phase and do not want to stress them due to herms. I’m thinking it’s best to increase temps to 85F, RH 75%, and PPFD to 1200 at the end of stretch.
Do you think this is accurate or a good idea in terms of those numbers and timing I stated?
Just want to make sure I’m utilizing these tools for optimal performance. Thank you!
I think 1200 PPFD is a bit too high. I wouldn’t go much beyond 1000, and even then, the priority should be your average canopy PPFD, not the peak. Some areas will naturally come in higher or lower. With that said if you are supplementing cO2 in which case depending on your set up you could go as far as 1500 but at this point you wouldn’t see much gain. Photosynthesis efficiency generally tops out around 1200-1300. Again at the average canopy.
Personally, I focus on the fundamentals: basic nutrients, a controlled root zone, a stable environment, and consistent PPFD. Consistency is key. Once the plant adapts to steady conditions, it begins to rely on them. That balance allows it to take in everything it needs, rather than having to prioritize certain elements due to a lack of others.
If you manage the basics well, the results will reflect that. You’ll get healthier plants and a better final product. That baseline stability matters more than any extra additives or inputs.
Throughout the entire run, keeping those parameters consistent is important, especially in flower. During veg, things are a bit more forgiving as long as temperatures are appropriate. The plants can also handle a bit more light in that stage.
It sounds like you’re using VPD as your guide, which is a solid approach. Just remember that VPD should be controlled by humidity, not temperature. You want to set your target humidity, then adjust temperature to support it for the stage you are in. Based on what you described, your humidity by the end of stretch would be running too high.
Even when supplementing with cO2, it’s important to manage humidity just as carefully as you would without it. Adding cO2 increases photosynthesis, which causes plants to transpire more and raise humidity levels in the grow space. If humidity isn’t kept in check, it can throw off your VPD, reduce nutrient uptake, and increase the risk of mold or mildew. Plants operating with cO2 run at a higher metabolic rate, so a stable environment becomes even more critical. To get the full benefit of cO2, you need to maintain proper humidity, temperature, and airflow throughout the grow.
The temperature conversation around cO2 is mostly about the need for a sealed environment, where limited exhaust means heat builds up more easily. Without active venting, managing temperature becomes more critical since you’re relying on internal climate control. When you increase PPFD to take advantage of cO2, you’re also increasing light intensity and overall wattage, which naturally generates more heat. So the combination of higher light output and restricted airflow in a sealed space means you need to stay on top of both temperature and humidity to keep everything in balance and maintain the benefits of cO2 supplementation.
Hopefully answered your questions or concerns, however if I didn’t please expand on the questions at hand when you have the time. I will do my best to get back to you.
Tag me @Low so I don’t miss it.