I am still quite the rookie, being that I lack a lot of experience, my grow style is pretty much keep it in the sweet spot according to the charts, and then largely LITFA.
This has been working for me, but now I want to up my game a tick.
My questions are where to target within the range of acceptable VPD and DLI.
For example, to achieve a VPD of 1.0 I can run 84f at 75%rH, or I can run 67f at 55%rH. Those are 2 very different conditions from an environmental control POV. Which end of the range is better for the plant? I would expect the lower temp/rH would be less energy consumed, but aside from that, is there any plant related reason to move towards one end of the range vs the other?
Similarly, for DLI I can achieve DLI of 41 many ways. Running higher ppfd for less hours, or running lower ppfd for more hours. With LEDs I doubt there is much difference in energy consumption between the two. Assuming you are respecting the photo period requirements of the plant, I am currently growing autos so that is a wide open range, which end of the range is better. For example, maybe better light penetration with the higher ppfd???
This chart tells me when to increase/decrease my nutes, I ignore MFG recommendations for the most part. Pretty cut and dry, no big range to speak of, just follow the chart.
If you havenât found this video yet, Iâd highly recommend at least popping in some ear buds and giving it a listen.
Itâs Dr. Bruce Bugbee (Director of the Crop Physiology Laboratory at Utah State University) talking about the exact thing youâre asking about. There are several videos featuring him, mostly about the cannabis/hemp plants relationship with light. Theyâre all very informative, mostly easy to comprehend with a basic understanding of the subject. And he uses a dry erase board for visual aid, bless his heart.
If that link doesnât work I can try to do it another way. But if you just search his name on YT tons of videos will pop up. Heâs even the featured guest on an episode of Garden Talk with Mr. Grow It from early this year. Hopefully you find the answers youâre seeking. Itâs a deep dive getting into DLI, at least it looks like itâs deep from the edge of the pool.
Correct from my understanding and experience. Of course theres a maximum point of positive transpiration as well. No matter where your at in growth cycle, its best to keep within vpd ranges if you can. The only acception being toward end of harvest where you want the rh a little lower to protect from mold and bud rot.
I think this can be a misnomer. That is mentioned everywhere but with good quality LEDs now like hlg quantum boards, i donât believe it is accurate. My leaf temps are the same as canopy ambient temp. Any temps over 78 degrees F at the canopy in the tent, will cause the flowers to foxtail for me. Below that and they stay nice and tight.
I think anyone saying you need 80+ degrees with LEDs is either unaware or actively sabotaging your grow.
There are nine components that, work together determine cannabis yield. These levels of lighting, and the other factors below are only possible when they are all optimized in unison.
At the top, physically and in importance, is light. Optimal yield will be under white light shining at 900-1000 ppfd during flower delivering approximately 60 DLI in a 12/12 daily schedule. Modern âWhite Lightâ LEDâs add deep red photons which are beneficial. The lights-off period should be as perfectly dark as possible.
Air Temperature in the grow room should remain between 77 - 86 degrees F for optimal yield.
Humidity between 40 - 60% relative humidity to allow healthy transpiration while avoiding mold and other problems.
Air flow is important at all stages of growth, enough to gently move the leaves is adequate.
The carbon dioxide level in the room should be at least 400ppm which is generally found in ambient air. This can be improved by increasing the fresh air circulation in the grow room if necessary.
Root Temperature should be in the same range as the air temp: 77 - 86 degrees Fahrenheit.
Water on a schedule that allows the plant to dry out until the pot âfeels light.â When you water use enough to ensure approximately 10% runoff. Adjust the pH of all water to about 5.5.
Soil and Nutrients are best handled by using a 50/50% mix of Vermiculite and Peat. Added to amend the soil are Silica, which is provided by the Vermiculite, Dolomitic Limestone at a rate of 40 grams per cubic foot of soil, and Gypsum (CaSO4) at a rate of 10 grams per cubic foot.
The optimal fertilizer mix is a 20-10-20 NPK mix added to provide a conductivity level of 1.0 - 1.5 mS/M concentration in veg (about 650ppm) increased to a maximum of 1200ppm (2.4 mS/M). Example: add approximately 0.6 grams per liter of 20-10-20. The most common problem is overfeeding and the recommended dose will only be appropriate when all the other components are dialed in. You should feed your plants with every watering, no flushing is normally necessary or desirable.
Oxygen is essential for plant health so make sure the grow room has a good supply of fresh air throughout the grow.
Thatâs a pretty handy checklist to work through for any grow and a good place to start when problems appear.
I noticed that he didnât even mention VPD, or increased CO2, perhaps he considers them to be above the level of basic growing? Itâs also surprising that he recommends a 20-10-20 fertilizer throughout the grow period. The âcommon knowledgeâ is that we should decrease nitrogen and increase phosphorous during flowering.
That surprised me too, but we need to remember that all of his parameters need to be optimized for this to work. And, if only in that condition can we ramp up to these levels.
Iâm pretty sure that another of Dr, Bugbeeâs videos mention 40 DLI as a maximum under âNormalâ conditions.
My tent hits the low 80s sometimes during a warmer day. Trying to maintain > 75% humidity makes me skirmish as I see times when there is liquid (H20) forming where leaves touch each other, which I assume is a result of transpiration. Visible moisture âfeelsâ like a red flag to me. I have 2 fans blowing, not radically, but constantly dancing the leaves every 15 seconds. When I see âwetâ leaves I open the tent and shake the plants to knock any drops, then readjust the fans. I often dont see the moisture though unless I open the tent and move the leaves around, then I see it.
I know that I âfeelâ more at ease at 75f and 65-70%.
Again, many paths to the top of the mountain, but I hear a lot folks keeping rH around 60 and running upper 70s or low 80s which pretty much throws VPD out the window.
I latched onto VPD because it was a clear target I can measure, and appeared to be based on sound science. It just seems that achieving VPD in a warm environment pushes the edge of other potential moisture issues.
The problem with those charts is they only show one range of VPD. I use a VPD of abput 1.2 in veg, but up it to 1.5 in flower (This is based of the Pulse recommendations). This chart seems ro be targeting 0.8 or so, which is way too low IMO.
Oh I see the legend, still lower than I would feel comfortable with. That being said, Iâve had successful grows with 30-40% RH, so VPD really is more of a guideline than a requirement.
You know, the more I think about it, the more I believe you are right about VPD just being a guideline.
It stands to reason that the ideal rH would be determined by the genetics more than anything else. In nature some strains adapted to dry climates, others to humid climates. 1 strain from a low lying equatorial zone might thrive in a higher rH than one from the Kush mountains.
I think I may have been giving these VPD charts too much credenceâŚ
Edit to add:
I think what I am theorizing is that different strains may have adapted to different transpiration levels. With landrace or heirloom you might be able to get a clue what the strain likes. With the modern hybrids, who knows.
I.e. going by the numbers for VPD is not cut and dry, which may explain why the various charts from different sources are so different. Looks like it is a touchy feely thing I have to develop an intuition for.
