For flower, start around 1.0–1.2 kPa leaf VPD during stretch, 1.2–1.4 through bulk, and 1.2–1.5 in late flower. Those are working ranges, not a recipe every cultivar has to follow. A plant in week 1 and a room full of dense week-8 colas need different attention.
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 humidity control that helps protect dense 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 stretch as a starting point. Confirm the plant is establishing well and the root zone can meet demand. This is a practical starting range, not a week-specific optimum established by a controlled cannabis trial.
Do not set RH from a range alone. Measure canopy air temperature and RH, check representative leaf temperatures, then calculate the leaf VPD. The same humidity can land in a different kPa range as leaf temperature changes.
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 leaf-VPD band through bulk as a starting point. Check leaf temperature, substrate moisture, irrigation response, and plant condition together. A warmer leaf or slower dryback can prompt investigation; neither one alone proves stomata have closed or identifies the cause.
Watch dryback and runoff along with the canopy readings. Record the room setpoint and measured leaf offset so the next person checking the room is working from the same assumptions.
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.
In the final weeks, 1.2 to 1.5 kPa leaf VPD can be a useful working range when the crop and root zone tolerate it. Manage the actual humidity and air movement around dense flowers. Raising the temperature simply to make the VPD number larger does not remove water from the room.
Room humidity control and substrate dryback are different jobs. Dehumidification removes water from the air; dryback describes declining water content in the growing medium between irrigations. Do not dry the root zone harder on the assumption that it will prevent bud rot.
VPD does not work alone here. Dehumidification, air movement within the canopy, sanitation, and scouting all matter. Check the wettest parts of the room and the lights-off transition. A good reading at one wall sensor cannot guarantee conditions inside a dense flower.
The Water-Demand Tradeoff
A more aggressive late-flower VPD is a tradeoff. Increasing atmospheric demand does not guarantee that the plant will move more water; root supply and stomatal response can limit it. If the crop starts showing stress, investigate the environment and irrigation together.
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 working range rises as flower develops, but the crop gets the final vote. Keep the setpoint only if the leaf readings, irrigation response, and plant condition support it.
Where the Numbers Come From
These stage ranges are practical starting points. A 2025 controlled humidity study found poorer flowering and cannabinoid outcomes under very high RH and low VPD in the tested plants. It did not establish the exact weekly targets above. Your cultivar, leaf temperature, light, root zone, and disease pressure still matter.
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 keeps environment readings with room notes, crop photos, scheduled work, maintenance, and harvest history. Direct connections are available for Pulse, TrolMaster, and AROYA, with CSV imports for other compatible data. That lets the team review a VPD change alongside what was happening in the crop, instead of leaving the reasoning on one grower’s phone.
Start with the free VPD calculator. Save the target and measured leaf offset with the room record, then compare what happened with a similar run. A difference is a lead to investigate, not proof that VPD alone caused the result.
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