Quick answer: Hold hydroponic and greenhouse lettuce at about 50–70 % RH, with day air no warmer than about 24 °C and night air around 18 °C, and keep air moving across the crop at roughly 0.3–1.0 m/s (University of Kentucky). Air movement and steady transpiration carry calcium to the young leaves and reduce tipburn; drier air at night keeps lower leaves from staying wet and rotting. Mixed leafy-greens houses need the same basics, with extra attention to spinach downy mildew.
Quick answer
- RH: 50–70 %; temperature: up to 24 °C day, about 18 °C night.
- Air speed across the canopy: about 0.3–1.0 m/s to reduce tipburn.
- Night: no free water on leaves; dense heads hold humid air at the base.
Climate targets for lettuce
| Factor | Target | Source |
|---|---|---|
| Air temperature | Not above about 24 °C day, about 18 °C night; tolerates 10 °C nights with slower growth | University of Kentucky |
| Relative humidity | 50–70 % | University of Kentucky |
| Air speed at the crop | About 0.3–1.0 m/s to support transpiration and calcium uptake | University of Kentucky |
From University of Kentucky, Hydroponic lettuce. Virginia Tech's controlled-environment guidance covers the wider atmosphere of CEA facilities (Virginia Tech SPES-817).
Tipburn vs wet lower leaves: two different problems
| Tipburn | Wet lower leaves and basal rot | |
|---|---|---|
| What you see | Brown, dead edges on young inner leaves | Slimy or rotting lower leaves touching the substrate or raft |
| Cause | Calcium not reaching fast-growing young leaves | Free water and humid, still air at the base of the head |
| Role of humidity | Very humid, still air lowers transpiration from inner leaves | High night humidity keeps lower leaves wet |
| Fixes | Air movement over and into the canopy; steady growth; nutrient management | Lower night humidity, air movement at the base, avoid overhead wetting late in the day |
Tipburn is a calcium disorder: calcium is needed in fast-growing tissue, and steady airflow supports transpiration and calcium uptake. A dehumidifier does not fix tipburn on its own; air movement and growth rate matter more.
Mixed leafy greens
A house growing lettuce, spinach, kale and other greens shares one air system, but the crops differ:
- Spinach downy mildew is favored by cool, humid conditions and leaf wetness; Cornell describes it as a serious disease of spinach (Cornell Vegetables). Keep spinach blocks out of wet corners and give them the best air movement.
- Kale, chard and collards tolerate a wider range but still suffer from long wet periods.
- Cool-season greens in general grow best in cool conditions (Oklahoma State Extension); in high tunnels, ventilation is the main humidity tool (Cornell high tunnels).
Map the crop blocks, measure the wettest one at night, and control to that.
Hydroponic lettuce: where the water comes from
In NFT and deep-water raft systems, most of the water in the air comes from crop transpiration; open water surfaces, wet floors and channels add more. Cover reservoirs and channels, keep floors dry, and ventilate or dehumidify for the crop's transpiration.
Transpiration rises with plant size and light. A full-grown crop just before harvest can put several times more water into the air than a young one, so size equipment for the mature stage, at night when vents are closed.
Holding the targets
- Air movement first: horizontal airflow fans, or fans blowing down onto the canopy, at about 0.3–1.0 m/s at plant level.
- Vent and heat when outside air is drier.
- Dehumidify when outside air is humid, when vents are closed at night, or in indoor and vertical farms with no venting. In indoor farms, the dehumidifier must remove nearly all the water the crop transpires, plus the heat from lights; see grow room dehumidifier selection.
- Postharvest: harvested leafy greens need cold, humid storage to stay crisp (NC State, Produce guide: leafy greens); keep that separate from the growing area.
Our greenhouse dehumidifiers cover greenhouse installations, and ceiling-mounted and ducted dehumidifiers suit multi-tier indoor farms.
Choose dehumidification for lettuce when vents are closed at night or outdoor air is humid, alongside canopy airflow of about 0.3–1.0 m/s.
FAQ
Frequently asked questions
What humidity is best for lettuce?
About 50–70 % RH, with good air movement.
What temperature should hydroponic lettuce be grown at?
Not above about 24 °C in the day and about 18 °C at night; it tolerates cooler nights with slower growth.
Does high humidity cause tipburn?
It contributes by lowering transpiration from young leaves, which limits calcium supply. Air movement across the crop at about 0.3–1.0 m/s is the main fix.
Why are my lettuce lower leaves rotting?
Free water and humid, still air at the base of the head, often at night. Lower night humidity and move air at the base.
What humidity is best for spinach?
Moderate humidity with dry leaves and good air movement; long wet periods favor downy mildew.
Sources
- University of Kentucky Center for Crop Diversification, Hydroponic lettuce (CCD-CP-63).
- Virginia Tech Extension, Controlled environment agriculture facilities: atmospheres (SPES-817).
- Cornell University, Spinach downy mildew; High tunnels: greens.
- Oklahoma State Extension, Cool-season greens production.
- NC State, Postharvest produce guide: leafy greens.
