RELATIVE HUMIDITY is a familiar term quantifying the amount of water vapor in the air compared to how much it can hold. Warmer air has a greater water–holding capacity than colder air. Our orchids are comfortable in the same humidity range we enjoy, somewhere in the 40–70 percent RH range. Low–humidity air dries out our skin and our orchids. Moderate humidity levels are desirable, but too high humidity can cause problems.
HUMIDITY On a hot day, we sweat to cool our bodies through evaporation. Plants also cool themselves as water vapor evaporates from the leaf surface. The evaporation rate slows as humidity levels increase because water cannot vaporize as quickly from the small pores (stomata) on leaves in a high–humidity environment. High humidity is also conducive to the conditions that favor the growth of rots, molds and fungal infections. Summer is a very challenging time of the year when high temperatures make us want to water more frequently, thus increasing the opportunity for fungal and bacterial infections.
Diurnal Changes. Typically, the humidity is highest around sunrise and lowest around midafternoon. As the air is warmed by the sun, it can hold more moisture. With each 20 F (11.1 C) increase in temperature, the amount of moisture the air can hold roughly doubles. Dew point, the temperature at which the air is saturated with water vapor, is a common meteorological term from which the relative humidity can be inferred. As the dew point temperature approaches the ambient temperature, the relative humidity approaches 100% because the air has become saturated with water. When there is a big difference between air and dew point temperatures, the humidity is low. Higher nighttime humidity levels are typical because the cooler night air holds less moisture.
[1] We often think that our orchids need more water in the heat of summer, but it’s the humidity rather than the temperature that determines how frequently you should be watering your orchids. When humidity levels are very low, pots dry very quickly, and it is difficult to keep orchids hydrated. Orchids may require water every day or two in very low humidity.
Seasonal Changes. There are seasonal humidity patterns that you can use to adjust your watering habits to match the evapotranspiration rate as well as understand when your plant is vulnerable to diseases. You can get the hourly data from weather service sites to create track trends in your area or hang a humidity sensor in your growing area.
[2] On a typical spring day in St. Augustine, the temperature and humidity levels are both moderate. The diurnal pattern for a random day in April shows the nighttime humidity is in the 50 to 70% range, dropping below 50% during the day. You notice your pots dry out much more rapidly at these low humidity levels, so you increase your watering frequency as a result. Nighttime humidity levels are usually not excessive unless we have a period of gray gloomy weather that is more reminiscent of winter.
VAPOR PRESSURE DEFICIT Though humidity is a useful and familiar measure, there is a more precise way to express the driving force of water loss from the leaf: vapor pressure deficit (VPD). VPD is a measure of the evaporative forces at the leaf surface, given in pressure units such as millibars (mb) or kilopascals (kPa). It is defined as the difference between the pressure exerted by water vapor in saturated air and the water vapor actually present in the air. At a VPD of zero, the air is at its dew point, so there is no moisture gradient between the plant leaves and the air, that are both saturated with moisture. A low VPD indicates that air is near saturation, so the transpiration rate is negligible. A high VPD means the air is drier, and the moisture gradient between the leaf and the atmosphere encourages a higher transpiration rate.
[3] In summer, both temperatures and relative humidity can be excessive, often not dropping below 80 F (26.7 C) at night while humidity levels can climb above the 85% danger level. This environment encourages fungal pathogens because the water lost through the stomata is only slowly evaporated so leaves remain wet longer. During periods of high disease pressure, applications of precautionary fungicide sprays and drenches may be warranted. Pots do not dry rapidly, so you may water less frequently than you do in the spring.
We could find no recommended VPD levels for orchids, but some guidance is offered in an article by Wollaeger and Runkle (2015).
Growers should aim to have fairly low VPD, for example, 0.3 kPa, when rooting cuttings in greenhouses. This will reduce the drying of young plants, thereby reducing the frequency of misting and watering required to keep plants hydrated. However, Michigan State Extension recommends maintaining a greater VPD (greater than 0.5 kPa) in greenhouses while finishing plants, especially when there is a dense plant canopy. Plants will be able to transpire, cool themselves and be less stressed while the environment is less conducive to disease
[4] When the temperature and humidity mediate in the fall, you notice a growth spurt in your orchids. The daytime humidity levels are low, so pots dry out more rapidly and you find yourself watering more frequently than you did during summer. Nighttime humidity levels are usually not too excessive, until the tropical storm season. The leaf wetness and lack of drying that accompanies these extended periods of cloudy, rainy weather are invitations to disease. If your plants cannot be sheltered from this weather, spray before and after with anti–fungicide and anti–bacterial products.
Greenhouse vegetable growers harvesting fruits should be aware that one study, “Vapor Pressure Deficit (VPD) Effects on the Physiology and Yield of Greenhouse Tomato,” reported that a VPD of 0.8 kPa during the day and night increased photosynthetic rates and tomato fruit yields compared to plants grown with a VPD of 0.5 kPa. A too–dry environment can also cause problems. For example, another study, “High Vapor Pressure Deficit Influences Growth, Transpiration and Quality of Tomato Fruits,” showed that a very high VPD of 2.2 kPa could cause plant stress and fruit cracking in tomato.”
[5] Our winters typically include 10 to 14 days of cold to freezing weather when both the temperatures and humidity are very low. More enjoyable are periods when it warms up during the day. Cool nighttime temperatures and high humidity encourage condensation on plant leaves and that is a recipe for both mesophyll cell collapse from cold water damage as well as rots and bacterial problems. The dreaded flower blighting from Botrytis is common during the high humidity evening hours, requiring either more air movement to dry leaves and flowers or higher temperatures to drop humidity.
Plant metabolic processes require the plant to absorb carbon dioxide through the stomata in the leaves to produce food, and water to be absorbed largely through the roots and drawn into the plant through transpiration, exiting the stomata as water vapor. A moderate to high transpiration rate encourages the uptake of mineral nutrients through the roots. Calcium uptake in particular requires a strong transpiration rate.
[6] A healthy root system and strong transpiration stream facilitate calcium uptake, without which deficiencies can occur.
When the vapor pressure deficit is in the optimum range, the plant has a moderate transpiration rate so water and mineral nutrients can be absorbed from roots, water loss through the open stomata is not excessive and carbon dioxide can be absorbed through the open stomata to produce food. We suggest the optimum range to be around 0.5 to 1.2 kPa for many thin–leaved orchids using the normal photosynthetic pathway. Thick–leaved orchids with crassulacean acid metabolism (CAM) have adapted to endure higher VPD deficits, opening their stomata only at night when humidity is higher.
[7] Excess water that cannot be transpired can result in a swelling of plant cells that produce a blister on the leaf, edema, typically during cool, cloudy weather.
In an overly dry atmosphere, where the vapor pressure deficit is high (perhaps over 1.5 kPa), moisture is evaporated rapidly through the open stomata, sometimes so much so that the plant will shut its stomata to limit water loss. With closed stomata, leaves cannot cool themselves, and carbon dioxide cannot be absorbed from the atmosphere. Spider mites also thrive in a dry environment.
[8] Cooler winter temperatures and high humidity/low vapor pressure create the conditions favorable to the growth of Botrytis
In an overly wet atmosphere, where the vapor pressure deficit is low (perhaps below 0.4 kPa), the stomata can remain open for carbon dioxide uptake. Moisture is slowly evaporated from the foliage, though the transpiration rate is slowed. Edema, the physiological response to a plant’s inability to shed water, can cause leaf blistering. Disease pressure is high because of the potential for excessive leaf wetness.
[9] The VPD in spring tends to be moderate to high during the daylight hours, often with values over 1.2 kPa. C3 plants will lose water rapidly through evapotranspiration so watering frequency should be increased. Nighttime VPD values are moderate so CAM plants should grow well with warming temperatures, increased sunlight and healthy VPD.
Plants respond to the VPD by increasing or decreasing the stomatal opening, and this in turn affects the ability of the plant to absorb carbon dioxide, water and mineral nutrients as well as cool themselves during hot weather. Some orchids normally have their stomata open during daylight (C3 plants, often thin–leafed orchids, such as oncidiums). These plants absorb carbon dioxide and photosynthesize during daylight hours and can cool their leaves through evaporation. Other orchids have adapted to an epiphytic lifestyle by opening their stomata in late afternoon and throughout the night (CAM plants, such as cattleyas and other thick–leafed orchids) when humidity is higher as a water conservation measure. These orchids absorb carbon dioxide at night when the humidity is higher and store it until the daylight hours when photosynthesis occurs.
[10] The summertime VPD tends to stay in the moderate range during daylight hours but drops to low levels at night. C3 plants can cool themselves during the heat of the day while additional shading, additional air movement or under bench wetting may be necessary for CAM plants that cannot cool themselves through evapotranspiration by day. Nighttime VPD levels are low, so nutrient uptake, particularly of calcium, is low and disease pressure is high.
Temperature and Humidity Monitors
The simplest and most foolproof way to monitor temperatures in your growing area is a max–min thermometer. This is a mercury–filled thermometer with a U–tube, which will register the current temperature, as well as the lowest and highest temperatures experienced since the last reset. Some have a button reset; others use a magnet to reset the temperature. The mercury travels in a U– tube inside the thermometer, rising to the maximum in the channel to the right during the day and dropping to the minimum in the channel to the left at night.
You will find yourself resetting these thermometers often during the hottest days of summer and coolest nights of winter, as you monitor the temperature extremes. You can position them in different areas of your growing area to find the microclimates most suitable for growing different varieties of orchids. They work consistently, and they do not require batteries that seem to always fail on the coldest night.
Of course, mercury thermometers do not have all the bells and whistles that some of the digital devices offer, like remote monitoring, alarms and phone notifications. I have a box of failed temperature and humidity monitors, but this year discovered one that seems to work really well: a Govee H5179 thermometer–hygrometer combination that measures both temperature and humidity. This is a battery–powered sensor that communicates with your phone via a 2.4 GHz WiFi connection using the Govee Home app, for either Android or Apple devices. You can get the instantaneous readings on your phone using their Widget, as well as view the data from the last hour, day, week, month or year on your phone. You can check the temperature in your growing area, whether you are sitting in front of the TV at home or up in the mountains of North Carolina. You can set high and low temperature alarms, so a notification is sent to your phone in the event temperatures are outside of your preset acceptable range. Another nice feature is the low battery warning light to let you know it is time to switch out the three AAA batteries. I also have the earlier models 5075 and 5100 that have less functionality and range, but the 5179 model is the only one I would buy today.
The device also calculates the dew point and vapor pressure deficit, a great feature that many of the other monitoring devices lack. If you are interested in having remote sensors in your growing area to keep you apprised of the temperature and vapor pressure deficit, try the Govee H5179 sensors, available on Amazon for around $40. As long as you have a suitable WiFi network, your phone will give you constant feedback on conditions around your orchids.
— Sue Bottom (email sbottom15@ hotmail.com)
The max–min thermometer on the left is a great way to monitor the temperatures in your growing area. The Govee 5179 sensor on the right takes temperature and humidity readings every few minutes and transmits the information to your phone by a Wi–Fi Connection. It can also calculate the dew point and vapor pressure deficit.
Indoor growers and growers in the arid southwest must take additional steps to prevent plant stress from excessive vapor pressure deficits. Our orchids grow well outdoors in St. Augustine, Florida during much of the year, except during the cold winter periods and wet tropical storms in late summer and fall. Understanding the vapor pressure deficit that occurs during each season will help guide you in your watering and fertilizing frequency. It also explains why additional shading may be required in summer for cooling, and when disease pressure may be high.
[11] The fall and spring diurnal patterns are moderate, with the difference being that temperatures are decreasing as are the hours of daylight. You will water a little more during the fall growth spurt particularly for those plants that enjoy the cool weather; others are preparing themselves for the winter rest.
[12] We enjoy moderate temperatures on many winter days, and the vapor pressure deficit is often in the low range day and night. This means our C3 and CAM orchids can absorb plenty of carbon dioxide without excessive water loss but transpiration rates are low reducing the need for frequent watering and mineral nutrition.
Acknowledgements
istakes, ask questions and open my mind to new thoughts. Thank you, Anna Meigs, for introducing me to the topic of vapor pressure deficit and its implications for orchid growing. Many thanks to Dr. Courtney Hackney for his review of this article and suggestions for improvement.
Additional Reading
Leonardi C., S. Guichard and N. Bertin. 2000. High Vapour Pressure Deficit Influences Growth, Transpiration and Quality of Tomato Fruits. Scientia Horticulturae 84(3–4):285–296.
Monteith J.L. and M.H. Unsworth. 1990. Principles of Environmental Physics. Edward Arnold, Sevenoaks.
Murray F.W. 1967. On the Computation of Saturation Vapor Pressure. J. Appl. Meteorol. 6. 203–204.
Prenger J.J. and P.P. Ling. Greenhouse Condensation Control: Understanding and Using Vapor Pressure Deficit (VPD).Ohio State University.
Wollaeger H. and E. Runkle. 2015. Why Should Greenhouse Growers Pay Attention to Vapor–Pressure Deficit and Not Relative Humidity? Michigan State University Extension and MSU Department of Horticulture.
Zhang D, Q. Du, Z. Zhang, X. Jiao, X. Song and J. Li. 2017. Vapour Pressure Deficit Control in Relation to Water Transport and Water Productivity in Greenhouse Tomato Production During Summer. Scientific Reports. 2017;7:43461. doi:10.1038/srep43461
— Sue Bottom started growing orchids in Houston in the mid–1990s after her husband Terry built her first greenhouse. They settled into St. Augustine, Florida, Sue with her orchids and Terry with his camera and are active in the St. Augustine Orchid Society, maintaining the Society’s website and publishing its monthly newsletter. Sue is also a member of the AOS Editorial Board (email: sbottom15@gmail.com).
