Most VPD charts hand you a single number and call it a day. That is fine for a poster on the wall, but flower is not one stage. A plant in week 1 stretch is doing something completely different than a plant in week 8 with dense, resin-heavy colas. If your VPD target never moves across the cycle, you are leaving transpiration control on the table.
This is a walk through the flower cycle, week by week, from one grower to another. We will cover transition and stretch, mid-flower bulk, and the late-flower dryback that keeps botrytis out of your best buds. If you want the full background on what VPD actually is and how to calculate it, start with our complete VPD guide.
A Quick Note on Leaf VPD vs Air VPD
Before the numbers, one thing matters more than any chart: the target that counts is leaf VPD, not air VPD. Leaf temperature under LEDs often runs a couple degrees cooler than air temp, and that gap shifts the actual pressure gradient the plant feels.
Every range below is written as a leaf-VPD-aware target. Verify against a real chart or calculator with your own leaf temp offset before you dial your room. We break down the difference in Leaf VPD vs Air VPD, and you can run your exact numbers on the free VPD calculator.
Weeks 1 to 3: Transition and Stretch
The first three weeks of 12/12 are about establishing the canopy, not maxing out transpiration. Plants are still stretching, building the frame that will hold your yield. Push VPD too hard here and the data tends to show slowed growth and tighter, more defensive stomatal behavior.
Aim for roughly 1.0 to 1.2 kPa (leaf VPD) through the stretch. This keeps stomata comfortably open so the plant moves water and nutrients without fighting the environment. Rodriguez-Morrison, Llewellyn, and Zheng (2021) found cannabis photosynthesis and yield respond strongly to environment when stomata stay open and functional, and a moderate gradient early supports exactly that.
Keep humidity in the 60 to 65 percent range at typical flower temps to land in that window. The goal is steady, unstressed establishment. You are not trying to force anything yet.
Weeks 4 to 6: Mid-Flower Bulk
This is where the flowers pack on weight, and it is where a well-tuned VPD earns its keep. Transpiration is running high, the plant is pulling water and feeding hard, and the canopy is at full demand.
Target the 1.2 to 1.4 kPa band (leaf VPD) through the bulk phase. This slightly firmer gradient keeps transpiration strong, which drives nutrient uptake and calcium movement into developing tissue. If the data shows stomata closing (leaf temp climbing, transpiration flattening), the gradient has been pushed too far and the plant is protecting itself.
Practically, that usually means holding humidity around 55 to 60 percent as temps sit in the high 70s to low 80s. Watch your dryback and runoff, not just the wall sensor. If the numbers say the plant is drinking well and growing, the environment is working.
One trap to avoid in bulk: chasing a tighter VPD by dropping humidity too fast. If transpiration outruns what the roots can supply, the data shows tip burn and stalled feeding even when your kPa reading looks textbook. The gradient and the root zone have to move together. A perfect number on the sensor means nothing if the plant cannot keep up with it.
Every room is a little different, so verify your exact target against a printable VPD chart you can keep near the controller.
Weeks 7 and Beyond: Late Flower and Ripening
Late flower is the highest-risk stretch of the whole cycle. Buds are dense, humidity gets trapped inside the canopy, and that is exactly the microclimate Botrytis cinerea wants. Bud rot thrives in cool, humid, still air around tight flower clusters, and once it takes hold in a dense cola it spreads fast.
This is why the late-flower dryback strategy exists. As buds densify, you raise VPD to push transpiration and pull moisture out of the canopy, lowering the humidity that mold needs to establish. Aim for 1.2 to 1.5 kPa (leaf VPD) in the final weeks, leaning toward the higher end if your buds are especially dense or your room holds humidity.
Zheng’s work on controlled-environment cannabis reinforces the same principle growers see in the room: managing humidity and keeping air moving is central to reducing gray mold pressure in dense canopies. A firmer VPD is one of your cleanest levers for that.
VPD does not work alone here, though. Dehumidification and canopy airflow do the heavy lifting alongside it. Raising VPD tells the plant to transpire, but you still need the equipment to pull that moisture out of the room and keep air moving through the interior of the canopy where rot starts. Think of VPD as the setpoint and your dehu and fans as the muscle behind it.
The Dryback Tradeoff
Here is the honest part. A more aggressive late-flower VPD is a tradeoff, not a free win. Push too high and the data shows stomata closing, transpiration stalling, and ripening slowing down. The plant stops drinking and effectively hits the brakes.
The move is to raise VPD deliberately, not blindly. Nudge it up, watch transpiration and dryback for two or three days, and confirm the plant is still moving water. If transpiration holds and humidity drops, you have found the sweet spot. If the plant clamps down, back off. Let the data tell you where the edge is instead of guessing.
The Week-by-Week Cheat Sheet
Keep these as starting points, all leaf-VPD-aware, all worth verifying against your own leaf temp:
- Weeks 1 to 3 (stretch): 1.0 to 1.2 kPa
- Weeks 4 to 6 (bulk): 1.2 to 1.4 kPa
- Weeks 7+ (ripening): 1.2 to 1.5 kPa, leaning high for dense buds
Notice the trend: the gradient rises as the cycle progresses. Early flower stays gentle to protect establishment, and late flower firms up to protect the harvest from rot. That upward drift, tuned to what your plants are actually doing, is the whole game.
Where the Numbers Come From
None of this works if VPD is the only thing you look at. A target of 1.4 kPa means nothing if your leaf temp offset is off or your sensors are drifting. The growers who nail this are the ones checking VPD against transpiration and dryback, not just chasing a number on a chart.
That is the difference between reacting to problems in week 8 and preventing them in week 4. The data is there in every room. Most operations just are not watching it closely enough to act in time.
Run Your Own Numbers
Growgoyle is software that runs your grow. It watches your environment and your plants together, so the week-by-week VPD shifts above stop being guesswork and start being decisions backed by your own room data. METRC tracks your grow for the state. Growgoyle tracks it for you.
Start with the free VPD calculator to dial in your leaf-VPD targets for each stage of flower. Then, if you want the full picture across the whole cycle, start your free 30-day trial.
You don’t need to wait for a new batch. Got a room in flower right now? That’s all you need.
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