Library › Part D · Article 32 in the series · Level 2 · 11 min read

Temperature, humidity and VPD

Standfirst: After reading this article you will be able to explain why each plant process has a minimum, an optimum and a maximum temperature, set day and night temperatures for lettuce, basil and fruiting crops from sourced ranges, and use the daily mean temperature to predict timing. You will also be able to read a VPD table, compute a dew point, and decide whether a grow room will drip at night.

Before you read: Water in the plant, Photosynthesis and respiration for growers.

How temperature acts on a plant

Almost every step of growth is a chemical reaction run by an enzyme, a protein that speeds up one reaction. Like most chemical reactions, enzyme reactions speed up roughly exponentially as temperature rises¹. Because enzymes are proteins, they also lose their shape (denature) when they get too hot; this frequently begins at about 40 to 50 °C¹. Each process therefore has a base temperature, an optimum and a ceiling. For development, Castilla calls the base the “vegetative zero” and puts it between 0 and 6 °C for most horticultural species⁴.

Photosynthesis has a broad optimum. At normal air CO₂, warming speeds up carbon fixation but also lowers the affinity of the fixing enzyme, rubisco, for CO₂, and the two effects partly cancel¹. Above the optimum, rates fall again¹. The optimum is not fixed: in lettuce, raising CO₂ to 1,200 µmol mol⁻¹ moved it from 25 to 30 °C³.

Respiration has no such brake. Between 0 and 30 °C its rate roughly doubles for each 10 °C rise, a temperature coefficient (Q10) of about 2¹. At night respiration burns sugar made during the day with no photosynthesis to replace it, so a warm night costs carbon (see Photosynthesis and respiration). For tomato, Castilla considers a night near 15 °C optimal because it limits respiration losses, and notes that around 75% of a greenhouse’s energy losses happen at night⁴. In one lettuce study the authors used a whole-plant Q10 of only 1.2, from their own earlier measurements³, so a warm night may cost less than the textbook figure suggests.

Timing follows the daily mean temperature

Growth is the gain in mass; development is progress through stages, such as leaf number or flowering. When day length and nutrition are favourable, the rate of development depends mainly on temperature, rising in a straight line from the base temperature to an upper limit, above which it falls⁴. Within limits, crops respond to the daily average temperature, whatever the split between day and night; Castilla adds that this is seen in developed crops with closed canopies⁴. The length of a stage depends on accumulated temperature even when day and night temperatures differ⁴.

That accumulated temperature is the thermal integral, or thermal time: the sum of each day’s mean temperature above the base, in degree-days (°C·d), where one degree-day is 1 °C above the base for 24 hours⁴. It only works where development rises linearly with temperature, between the base and the optimum⁴. Castilla’s example: Mediterranean greenhouse green beans, base 6 °C, needed 757 degree-days from sowing to the start of harvest⁴.

A controlled test shows the principle. Blanchard and Runkle grew dahlia, French marigold and zinnia with a 16-hour day and 8-hour night at 20/14, 18/18 and 16/22 °C, all averaging 18 °C, and at three pairs averaging 22 °C⁵. Flowering times were similar among treatments with the same mean daily temperature⁵. These are ornamentals; applying the result to food crops rests on Castilla’s general statement⁴ and is this article’s inference.

Shape is different. DIF is the day temperature minus the night temperature. With a positive DIF internodes are longer; with a negative DIF plants are more compact, an effect linked to the light-sensing pigment phytochrome⁴. In the same trial, plants grown at 20/14 °C were 10% to 41% taller than those at 16/22 °C⁵. Resh recommends a night about 5.5 °C below the day for transplants, and warns that too large a difference gives tomatoes long flower clusters that later kink⁶.

Temperature targets by crop

Cornell’s 24/19 °C serves lettuce pushed hard with extra light and CO₂²; the lower figures are Tesi’s optimum ranges as printed by Castilla, who adds that economic criteria must prevail when heating set points are chosen⁴. Lettuce, spinach and broccoli are cool-season crops that tend to bolt, go to seed or turn bitter in high temperature or high light⁷. Jones lists bolting, tipburn, loss of colour and poor germination above 25 °C⁷, and the FAO manual says warm nights above 18 °C cause lettuce to bolt⁸.

Basil sits at the warm end⁸ and is sensitive to cold: a 2023 study, citing a cold-storage trial on cut basil leaves, states that temperatures below 12 °C cause strong wilting, leaf discoloration and loss of aroma⁹. In that trial, leaves stored at 4 °C showed chilling injury and leaves stored at 12 °C did not (abstract only)¹⁴. Tomato and cucumber are also chilling-sensitive: plants grown at 25 to 35 °C and cooled to 10 to 15 °C show slowed growth and leaf lesions¹. Most Mediterranean greenhouse species greatly reduce their metabolic activity below 10 to 12 °C⁴.

The roots have their own optimum: in a Cornell master’s thesis (Thompson 1997, abstract reprinted in the handbook) that set air and pond water to 17, 24 or 31 °C, lettuce dry weight was highest at 24 °C air and 24 °C water²; Root-zone temperature covers the solution side.

Humidity as vapour-pressure deficit

Vapour-pressure deficit (VPD) is the saturation vapour pressure at the air temperature minus the actual vapour pressure of the air, in kilopascals; Water in the plant derives it¹⁰. The table applies the FAO-56 equation, e°(T) = 0.6108 exp[17.27 T / (T + 237.3)], and VPD = e°(T) × (1 − RH/100)¹⁰. The values are this article’s computation; Castilla prints the same equation⁴.

VPD in kPa, computed with the FAO-56 equation¹⁰.

What VPD does to the crop

Grange and Hand’s review (abstract only) concluded that between 1.0 and 0.2 kPa (55 to 90% RH at 20 °C), humidity has little effect on the physiology and development of horticultural crops; drier air causes water stress, and more humid air encourages disease and growth disorders¹¹. Castilla gives 1 kPa as that level in Northern Europe, while adapted Mediterranean crops may reach 3 kPa⁴. He also quotes a rule: avoid VPD above 1.1 kPa in winter or 2.7 kPa in spring⁴.

High VPD. Transpiration rises; if the roots cannot keep up, the stomata progressively close to avoid dehydration⁴. Guard cells can also lose water directly to dry air and close passively¹. Closed stomata admit less CO₂, so photosynthesis falls. A review of tomato climate reports that poorly controlled arid greenhouses reach 3 to 5 kPa, where growth is poor and blossom-end rot and stomatal closure are likely¹³.

Calcium. Calcium reaches young leaves and fruit almost only in the transpiration stream³, as Water in the plant explains. Humidity can cause lettuce tipburn from both sides: dry daytime air sends the water through the exposed outer leaves while the enclosed growing point stays humid, and very humid air slows the growing point’s own transpiration³. Low night humidity may lower turgor and so reduce the calcium that guttation pushes to leaf tips³. Frantz and co-workers removed tipburn by blowing air directly onto the growing point, which let them use two to three times the usual light and doubled edible yield at the highest light level³. Cornell applies the same idea with paddle fans blowing air down onto the crop at 140 cubic feet per minute per square foot of pond, an air speed of about 0.7 m s⁻¹ (our conversion); with the fans their lettuce took 17 mol m⁻² d⁻¹ of light, without them 12, for one cultivar². Diagnosis is in Nutrient disorders.

Condensation and disease. The dew point is the temperature to which air must be cooled to become saturated¹⁰. Any surface colder than the dew point collects water. As a greenhouse cools at night, RH rises and condensation starts on the coldest surfaces, such as the cover⁴. Leaves can be among them: losing heat by radiation, they can be 1 to 3 °C cooler than the air at night and frequently reach the dew point¹². Thermal curtains reduce that radiation loss, so leaves stay warmer and collect less condensation¹². In a water film, fungal spores can germinate⁴. A moderate VPD is no guarantee either: most fungi sporulate best in still air at 0.6 to 1.2 kPa¹².

VPD by growth stage

Seeds and cuttings have few or no roots to replace lost water. Grange and Hand name the propagation of leafy cuttings as a case where high humidity is desirable¹¹. The tomato review states that VPD is generally kept around 0.3 to 0.5 kPa for cuttings while they root, without citing a trial for that figure¹³. Cornell covers lettuce seed trays for the first two days to keep humidity high, and removes the covers once the root has emerged².

For established crops, the narrow targets often quoted, such as 0.8 to 1.2 kPa, are hard to trace. The same review compiled greenhouse recommendations spanning 0.2–1.0 to 0.47–1.27 kPa; its 0.5–1.2 kPa range came from a climate-control company and 0.75–1.06 kPa from a grower weblog, while the tomato trials it cites favoured about 0.8 kPa (Iraqi et al. 1995) or 0.5–0.8 kPa (Barker 1990)¹³. We found no peer-reviewed trial that sets 0.8 to 1.2 kPa as an optimum. The band a peer-reviewed review found to have little effect is 0.2 to 1.0 kPa¹¹, and Castilla adopts its 1 kPa upper end as the limit for Northern Europe⁴. The review places fungal disease and mineral-deficiency symptoms below 0.43 kPa¹³. Our reading of these sources: aim for day values in the middle of the band, and accept lower values at night, when higher humidity helps root pressure carry calcium¹¹. The night value in the worked example below, 0.35 kPa, is under the review’s 0.43 kPa line for fungal and mineral-deficiency symptoms¹³.

Leaf temperature is not air temperature

VPD is computed from air temperature, but the leaf has its own. The air spaces inside a leaf are close to saturation, at 99% RH in a textbook example¹, so the leaf’s own temperature sets the vapour pressure inside it. A leaf that transpires freely is usually cooler than the air¹. Insolated leaves that are not transpiring can be 2 to 8 °C warmer than the air¹², and leaves of plants short of water can rise 4 to 5 °C above the air in bright midday sun¹. A warm leaf therefore faces a larger VPD than the air reading suggests. Lamp heat on leaves belongs to Choosing grow lights.

Controls, from the mechanism

Every control moves temperature and humidity together. A heater adds no water, so the vapour pressure stays the same while the saturation value rises: warming 20 °C air at 85% RH to 24 °C gives 67% RH and a VPD near 1.0 kPa (our calculation with¹⁰). Ventilating with colder, drier outside air lowers RH, because that air gains heat faster than moisture⁴. A fan changes neither the room’s temperature nor its RH, but it thins the still air layer on each leaf (see Water in the plant), reduces gradients of temperature and VPD, and dries infection droplets¹². Shading can lower air temperature by 5 to 8 °C, and misting helps lettuce through hot periods, but the extra humidity reduces transpiration⁷.

Worked example: a grow tent by day and by night

A tent runs at 26 °C and 65% RH during a 16-hour light period, and at 20 °C and 85% RH during the 8-hour dark period. Saturation pressures come from the FAO-56 equation¹⁰; all results are our arithmetic.

1. Day: e°(26) = 3.361 kPa; actual vapour pressure 3.361 × 0.65 = 2.185 kPa; VPD = 1.18 kPa¹⁰. That is above the 1.0 kPa upper end of Grange and Hand’s band¹¹, which Castilla adopts for Northern Europe⁴.
2. If the leaf stops transpiring and runs 2 °C warmer than the air, the low end of the range above for non-transpiring leaves¹²: e°(28) = 3.780 kPa, so the leaf-to-air VPD is 3.780 − 2.185 = 1.60 kPa. A freely transpiring leaf runs cooler than the air and faces less¹.
3. Night: e°(20) = 2.338 kPa; actual vapour pressure 2.338 × 0.85 = 1.988 kPa; VPD = 0.35 kPa¹⁰.
4. Night dew point: invert the equation, solving 0.6108 exp[17.27 T / (T + 237.3)] = 1.988 for T. This gives 17.4 °C¹⁰.
5. A tent wall at 17 °C is below 17.4 °C, so water condenses on it¹⁰. If a leaf cooled 2 °C below the air, as greenhouse leaves can under a clear night sky¹², it would sit at 18 °C, only 0.6 °C above the dew point; tent walls, like thermal curtains, reduce that radiative loss¹².
6. Mean daily temperature: (26 × 16 + 20 × 8) / 24 = 24.0 °C.
7. We found no published thermal-time requirement for hydroponic lettuce that we could use. With Castilla’s green bean figures⁴, a 24 °C mean accumulates 24 − 6 = 18 degree-days per day, so 757 / 18 = 42 days to first harvest; at an 18 °C mean, 757 / 12 = 63 days.

The fix follows from the mechanism: raise the day RH or lower the day temperature, and dry the air or warm the coldest surfaces before lights-off.

Key numbers

Common mistakes

1. Lowering the night and raising the day by the same amount and expecting the same plant. The timing follows the mean, but stem length follows DIF^4,5.
2. Treating 0.8–1.2 kPa as a proven optimum. The traceable evidence is a broad band of 0.2–1.0 kPa with little effect^11,13.
3. Reading the air thermometer as leaf temperature. Leaves run warmer in sun when they do not transpire and cooler at night, which changes both VPD and condensation^1,12.
4. Drying the air day and night to stop tipburn. Very dry and very humid air can both cut calcium flow to the growing point³.
5. Letting a warm, humid room cool quickly at lights-off. The coldest surfaces fall below the dew point first and stay wet^4,10.
6. Keeping lettuce warm at night for faster growth. Warm nights cause bolting and raise respiration^1,8.

Summary

Temperature sets the speed of enzyme reactions, so every process has a base, an optimum and a ceiling. Photosynthesis has a broad optimum, while respiration keeps rising with temperature, which makes warm nights a cost. Development follows the daily mean temperature, which lets a grower trade day against night for timing, but the day-night difference changes stem length. Crop targets are ranges that depend on light, CO₂ and economics. Humidity is best read as VPD; a band of about 0.2 to 1.0 kPa has little effect on growth, and narrower published targets are hard to trace^11,13. Calcium delivery, stomatal closure and condensation all depend on VPD, and leaf temperature can differ from the air reading.

Next in the series: CO₂ and air movement.

References

1. Taiz, L. and Zeiger, E. (2002). Plant Physiology, 3rd ed. Sinauer Associates, Sunderland, MA.
2. Brechner, M. and Both, A. J. (2013). Cornell Controlled Environment Agriculture Hydroponic Lettuce Handbook. Cornell University CEA Program, Ithaca, NY. https://cpb-us-e1.wpmucdn.com/blogs.cornell.edu/dist/8/8824/files/2019/06/Cornell-CEA-Lettuce-Handbook-.pdf
3. Frantz, J. M., Ritchie, G., Cometti, N. N., Robinson, J. and Bugbee, B. (2004). Exploring the limits of crop productivity: beyond the limits of tipburn in lettuce. Journal of the American Society for Horticultural Science, 129(3), 331–338. https://doi.org/10.21273/JASHS.129.3.0331
4. Castilla, N. (2013). Greenhouse Technology and Management, 2nd ed. CABI, Wallingford.
5. Blanchard, M. G. and Runkle, E. S. (2011). The influence of day and night temperature fluctuations on growth and flowering of annual bedding plants and greenhouse heating cost predictions. HortScience, 46(4), 599–603. https://doi.org/10.21273/HORTSCI.46.4.599
6. Resh, H. M. (2013). Hydroponic Food Production, 7th ed. CRC Press, Boca Raton.
7. Jones, J. B. (2005). Hydroponics: A Practical Guide for the Soilless Grower, 2nd ed. CRC Press, Boca Raton.
8. Somerville, C., Cohen, M., Pantanella, E., Stankus, A. and Lovatelli, A. (2014). Small-scale aquaponic food production: integrated fish and plant farming. FAO Fisheries and Aquaculture Technical Paper 589. FAO, Rome. https://www.fao.org/3/i4021e/i4021e.pdf
9. Begum, F. U., Skinner, G., Smieszek, S. P., Budge, S., Stead, A. D. and Devlin, P. F. (2023). Improved chilling tolerance in glasshouse-grown potted sweet basil by end-of-production, short-duration supplementary far red light. Frontiers in Plant Science, 14, 1239010. https://doi.org/10.3389/fpls.2023.1239010
10. Allen, R. G., Pereira, L. S., Raes, D. and Smith, M. (1998). Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements. FAO Irrigation and Drainage Paper 56. FAO, Rome. Chapter 3. https://www.fao.org/4/x0490e/x0490e07.htm
11. Grange, R. I. and Hand, D. W. (1987). A review of the effects of atmospheric humidity on the growth of horticultural crops. Journal of Horticultural Science, 62(2), 125–134. https://doi.org/10.1080/14620316.1987.11515760
12. Albajes, R., Gullino, M. L., van Lenteren, J. C. and Elad, Y. (eds) (2000). Integrated Pest and Disease Management in Greenhouse Crops. Kluwer Academic Publishers, Dordrecht. Chapter 8.
13. Shamshiri, R. R., Jones, J. W., Thorp, K. R., Ahmad, D., Che Man, H. and Taheri, S. (2018). Review of optimum temperature, humidity, and vapour pressure deficit for microclimate evaluation and control in greenhouse cultivation of tomato: a review. International Agrophysics, 32, 287–302. https://doi.org/10.1515/intag-2017-0005
14. Fratianni, F., Cefola, M., Pace, B., Cozzolino, R., De Giulio, B., Cozzolino, A., et al. (2017). Changes in visual quality, physiological and biochemical parameters assessed during the postharvest storage at chilling or non-chilling temperatures of three sweet basil (Ocimum basilicum L.) cultivars. Food Chemistry, 229, 752–760. https://doi.org/10.1016/j.foodchem.2017.02.137

This article was drafted with the help of AI tools and reviewed and edited by PonicLabs. Sources are listed above; tell us if you find an error.