VPD for indoor growers: the humidity metric that matters more than RH
Relative humidity gets all the attention. Growers monitor it, adjust humidifiers and dehumidifiers to hit target numbers, and wonder why their plants still show stress at "70% RH." The problem is that relative humidity doesn't account for temperature, and the combination of temperature and humidity is what drives plant transpiration.
VPD (vapor pressure deficit) does account for both. It measures the drying power of the air, which is what the plant actually experiences.
What VPD measures
Every air temperature has a maximum amount of water vapor it can hold (the saturation vapor pressure). Warm air holds more; cool air holds less. Relative humidity is the percentage of that maximum currently in the air.
VPD is the difference between the saturation vapor pressure at the current temperature and the actual vapor pressure in the air. In other words, it measures how much more water the air could absorb before it's saturated.
Low VPD (under 0.4 kPa): the air is nearly saturated. Transpiration slows because there's no gradient pulling water out of the leaves. Nutrient uptake via transpiration stream slows too. Fungal risk increases because moisture lingers on leaf surfaces. Calcium deficiency symptoms (tip burn in lettuce, blossom end rot in tomatoes) are more common at low VPD even when the solution has enough calcium, because calcium only moves into the plant through transpiration.
High VPD (above 1.6 kPa): the air is dry relative to temperature. Transpiration is excessive. Stomata close to prevent wilting, which shuts down CO2 intake and slows photosynthesis. Leaf edges brown. Growth rate drops.
The target range for most crops: 0.8-1.2 kPa. Within this range, stomata stay open, transpiration is steady, nutrient transport is efficient, and fungal risk is low.
Why RH alone misleads
At 25°C and 70% RH, VPD is about 0.95 kPa. Good.
At 30°C and 70% RH, VPD is about 1.28 kPa. Getting high. The plants may start closing stomata during the warmest part of the photoperiod.
At 20°C and 70% RH, VPD is about 0.70 kPa. Getting low. Transpiration is sluggish and calcium delivery to new growth may be inadequate.
Same humidity percentage, different temperatures, different plant responses. This is why "keep humidity at 70%" is incomplete advice.
Adjusting VPD
The VPD calculator takes temperature and humidity and returns the current VPD. To move VPD into range:
VPD too high (dry air, high temp): Increase humidity (misting, humidifier, wet towels on intake fan) or decrease temperature (shade cloth, AC, night-cycle fans). Increasing humidity is usually cheaper than cooling.
VPD too low (humid air, cool temp): Decrease humidity (dehumidifier, exhaust fan, increase air exchange rate) or increase temperature. Dehumidifiers also add heat, which moves VPD in the right direction from both sides.
Night vs day. VPD naturally drops at night as temperature falls. This is fine; plants transpire less in the dark anyway. The VPD target applies during the photoperiod when transpiration is active.
Crop-specific targets
Lettuce and leafy greens: 0.5-1.0 kPa. Lower range because lettuce is prone to tip burn (calcium deficiency from reduced transpiration) at high VPD, and it tolerates high humidity without fungal problems better than most crops.
Tomatoes and peppers: 0.8-1.2 kPa. Standard range. These crops transpire heavily and want moderate drying to pull calcium and other nutrients into the fruit.
Strawberries: 0.6-1.0 kPa. Susceptible to botrytis (gray mold) above 1.0 kPa? No, the opposite: botrytis thrives at low VPD (high humidity). Keep VPD above 0.6 to manage mold, but below 1.0 to prevent water stress.
Seedlings and clones: 0.4-0.8 kPa. Young plants have small root systems and limited transpiration capacity. High VPD dries them out faster than they can drink. Humidity domes over seedling trays keep VPD low until roots establish.
Practical monitoring
A thermometer and hygrometer (combo units are cheap) in the grow area, at canopy height, give the two inputs the VPD calculator needs. Fancier setups use a continuous data logger that records temperature and humidity every few minutes and flags when VPD drifts out of range; the Inkbird IBS-TH2 and SensorPush are popular options for home growers.
The key insight: VPD changes throughout the day as temperature changes. Checking once in the morning and once at peak temperature (usually 3-4 hours after lights-on for indoor grows) catches the two extremes.