Using VPD to Guide Temperature, Humidity, and Light
When it comes to dialing in an indoor grow, I’m a big believer in following VPD.
In short, VPD (Vapor Pressure Deficit) is a measurement that helps quantify how efficiently a plant is able to transpire, essentially how well it can “breathe.” Transpiration drives nutrient uptake, cooling, and overall plant function, which makes VPD one of the most useful tools we have for managing the grow environment.
Why VPD Matters More Than Temperature Alone
One important thing to understand is that VPD is based on leaf surface temperature, not just room temperature. This is especially relevant when comparing HPS and LED lighting.
HPS fixtures emit significantly more infrared radiation, which warms the leaf surface directly.
LEDs produce much less radiant heat, so leaf temperatures tend to be closer to ambient air temperature. Because of this, temperature targets differ slightly between HPS and LED grows even when aiming for the same VPD.
The recommendations below are based on VPD targets first, with temperature and humidity adjusted to support those targets.
A Quick Note Before We Begin
A Note on Ranges, Not Rigid Set points
You’ll notice that the recommendations below are presented as ranges for temperature, relative humidity, CO₂, and PPFD. This is intentional.
In real world grows, we often have one environmental factor that becomes the limiting constraint or that one variable that’s hardest for us to control. For example, in the Winter, it’s common to struggle with keeping humidity high enough when plants are small and not transpiring much. If you can’t push humidity high enough, let’s say 55% without adding a humidifier, one option is simply to lower temperatures so that 55% relative humidity still lands you in an ideal VPD range, or at least as close as you can.
In other words, you don’t always need more equipment to improve your environment. While adding controllers, humidifiers, or HVAC can certainly help, not everyone wants, or is able, toinvest more money into their grow. That’s why it’s so important to understand which levers you can pull to keep your environment in a range where plants can easily and efficiently transpire.
Nailing your environment isn’t about hitting a perfect temperature or a specific humidity number.
It’s about balancing temperature and humidity together so the plant can breathe properly. This becomes even more important as light intensity increases. If you’re pushing higher PPFD, your plant must be able to transpire fast enough to meet that increased demand.
If you do have strong environmental control and the ability to elevate both CO₂ and light levels, you can safely run your room hotter and more humid and push plants harder. Just be observant.
Different genetics can respond very differently to the same environment, and what works well for one cultivar may stress another.
Think of these setpoints as a guide, not a rulebook. They’re meant to help you dial in your own grow more quickly and with less frustration.
You don’t need to be perfect. If you’re reasonably close to these targets, you’ll be in a good place.
Here is a VPD chart I just searched for online quickly that you can reference to get your temps and humidity in the right balance. It gives you the ability to move sliders and see how things change as you change different environmental factors. In regards to Leaf Surface Temperature, you need an IR Gun to get an exact reading. But if you don’t have one you can just assume Leaf Surface Temperature is around 0-2 degrees of the ambient temperature.
—> https://vpdchart.com/#F,66,83.3,82.4,0 <—
LED Grows
Vegetative Stage (LED)
Temperature Ranges
- Day: 75–84°F
- Night: 70–75°F
Relative Humidity Ranges
- Day: 65–75%
- Night: 60–70%
VPD Range
- 0.8–1.1
- I personally like staying around 0.8 or 0.9CO₂ Range (if applicable)
- Day: 800–900 ppm
- Night: ~600 ppm
- 400ppm minimum at all times
PPFD Range
- Start around 150 PPFD (clones)
- Gradually increase to 600 PPFD as plants mature and environment is ideal
- 300 PPFD Rooted Cuttings - 450 PPFD Early Veg - 600 PPFD Established Veg
- Without CO₂ supplementation or ideal environment, cap closer to 500-600 PPFD
Flower Weeks 1–3 (LED)
Temperature Ranges
- Day: 80–86°F
- Night: 75–77°F
Relative Humidity Ranges
- Day: 65–75%
- Night: 55–60%
VPD Range
- 1.1–1.2
CO₂ Range (if applicable)
- Day: ~1200 ppm
- Night: ~600 ppm
400ppm minimum at all times
PPFD Ranges
- Gradually increase to 900–1200 PPFD with CO₂ and ideal environment
- Without CO₂, remain around 600-750 PPFD but if environment is ideal you can push a bit higher up to 800-900 PPFD with certain genetics
Stretch Control Tip
- To reduce stretch, run day and night temperatures the same
- To aggressively limit stretch, run night temps slightly higher than day temps
Flower Weeks 4–Finish (LED)Temperature Ranges
- Hold around 82°F initially
- Gradually drop to 68–70°F by the final week
- Can drop 2-3 degrees each week
Relative Humidity Ranges
- Start around 65%
- Gradually drop to 45–50% by the final week
- Be sure to match temperature and RH to maintain VPD
VPD Range
- 1.2–1.5
- I personally like staying around 1.2 or 1.3
CO₂ Range (if applicable)
- Final 2 weeks: reduce to ~600 ppm
- Otherwise maintain earlier flower levels
- 400ppm minimum at all times
PPFD Range
- Keep consistent through finish
HPS Grows
- Because leaf temperatures are higher under HPS, air temperature targets are slightly lower while maintaining the same VPD.
Vegetative Stage (HPS)
Temperature Ranges
- Day: 76–82°F
- Night: 73–77°F
Relative Humidity Ranges
- Day: 68–74%
- Night: 60–65%
VPD, CO₂, and PPFD Ranges
- Same targets as LED
Flower Weeks 1–3 (HPS)
Temperature Ranges
- Day: 78–82°F
- Night: 75–77°F
Relative Humidity Ranges
- Day: 55–60%
- Night: 50–55%
VPD, CO₂, and PPFD Ranges
- Same targets as LED
Stretch Control Tip
- Same day/night temperature strategies apply as with LED
Flower Weeks 4–Finish (HPS)
Temperature Ranges
- Begin around 82°F
- Gradually drop to 65–70°F in the final week
- Can drop 2-3 degrees each week
Relative Humidity Ranges
- Start around 60%
- Gradually drop to 40–45% by the final week
- Be sure to match temperature and RH to maintain VPD
VPD, CO₂, and PPFD Ranges
- Same targets as LED
Final Notes & Best Practices
- Ideal VPD Targets
- Vegetative: 0.8–1.1
- Flower: 1.2–1.5
When lowering temperatures in the final weeks of flower, always reference a VPD chart to maintain proper balance.
Aim for a 5–10°F day/night temperature difference during late flower.
Lower temperatures in late flower will help bring out color but be sure you can dry your air enough to drop temperatures. Cold and wet are perfect conditions for botrytis, aka “bud rot”
A Note on Light Intensity and CO₂
If you are not supplementing with CO₂, there is very little return on investment pushing PPFD beyond ~800–900. In many cases, excessive light without adequate CO₂ simply creates stress rather than increased yield.
Higher light intensity demands:
- Higher temperatures
- Higher humidity
- Increased transpiration
All of this is required to keep stomata open and functioning properly. Without CO₂
supplementation, plants simply can’t make effective use of extreme light levels.
