Humidity is very important and effects your plant growth. I am doing RDWC and find humidity very low at the start of flower and for the next 4 to 5 weeks than it starts to drop off from there. Temps and humidity play a vital roll, the Stomata open and close depending on humidity levels. 60%+ is the best as the Stomata are fully open and feasting on Co2 which runs the plant.
[FONT="]Plantworks: Part 1 Humidity and Vapor Pressure Deficit[/FONT]
[FONT="]By [/FONT]
[FONT="]Urban Garden Magazine[/FONT][FONT="] [/FONT][FONT="]⋅[/FONT][FONT="] July 12, 2010 [/FONT][FONT="]⋅[/FONT][FONT="] [/FONT]
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humid, [/FONT]
[FONT="]humidity[/FONT][FONT="], [/FONT]
[FONT="]Issue 11[/FONT][FONT="], [/FONT]
[FONT="]vapor pressure deficit[/FONT][FONT="] [/FONT]
[FONT="]Think like a plant.[/FONT]
[FONT="]Have you ever been given this odd-sounding advice? Even when we are encouraged to try and understand how plants work, our inherent tendency to personify the natural world is inescapable. Growers often like to draw parallels between humans and plants, after all, theres no doubt that plants are marvellous, highly specialized and well-adapted organisms. You might even go as far to say they are intelligent. But lets be honest here. Plants are totally different from us, especially in the way they react and respond to their environment. However, if we can get our heads around the world from a plants perspective, we become what is commonly referred to as green-fingered. We become
better growers.[/FONT]
[FONT="]Have you ever wondered how plants feel humidity? An understanding of what humidity is, what it means to plants, and how you can manage it in your indoor garden will help you and your plants stay happy all year round.
The humidity of the air is basically the amount of water in the air. Water can only truly stay in the air when it is the invisible gas water vapour. Small droplets of water in air, such as fog or mist, are not water vapor; they are simply larger particles of water temporarily suspended in the air that are ready to be turned into water vapour by evaporation.[/FONT]
[FONT="]Temperature plays an important role when it comes to humidity. The warmer the air, the more water vapour it can hold. This means the maximum amount of water that air can hold is directly related to the temperature of the air. As the amount of water air can hold constantly changes with temperature it is difficult to pin an absolute or fixed amount of water that can be held by air. So whats the best way to quantify humidity if the goal posts are changing all the time? The answer is something called Relative Humidity (RH) this is a measure in terms of percentage, of the water vapor in the air compared to the total amount of water vapor that the air could potentially hold at a given temperature.[/FONT]
[FONT="]Why is RH so important?[/FONT]
[FONT="]As growers we measure the RH of our gardens using digital or analogue hygrometers. These readings are very important because RH has a direct effect on the plants ability to transpire and therefore grow. Generally, plants do not like to lose lots of water through transpiration. Plants have some degree of control of their rate of transpiration through management of their stomata but the general rule is the drier the air, the more plants will transpire.
Now lets move on to the idea of pressure this is an important concept to grasp when it comes to understanding a plants response to humidity. All gasses in the air exert a pressure. The more water vapor in the air the greater the vapor pressure. This means that in high RH conditions there is a greater vapor pressure being exerted on plants than in low RH conditions. High vapor pressure can be thought of as a force in the air pushing on the plants from all directions. This pressure is exerted onto the leaves by the high concentration of water vapor in the air making it harder for the plant to push back by losing water into the air by transpiration. This is why with high RH plants transpire less. Conversely, in environments with low RH, only a small amount of pressure is exerted on the plants leaves, making it easy for them to lose water into the air.[/FONT]
[FONT="]What is Vapor Pressure Deficit (VPD)?[/FONT]
[FONT="]VPD can be defined as the difference (or deficit) between the pressure exerted by water vapor that could be held in saturated air (100% RH) and the pressure exerted by the water vapor that is actually held in the air being measured.
The VPD is currently regarded of how plants really feel and react to the humidity in the growing environment. From a plants perspective the VPD is the difference between the vapor pressure inside the leaf compared to the vapor pressure of the air. If we look at it with an RH hat on; the water in the leaf and the water and air mixture leaving the stomata is (more often than not) completely saturated -100% RH. If the air outside the leaf is less than 100% RH there is potential for water vapor to enter the air because gasses and liquids like to move from areas of high concentration (in this example the leaf) into areas of lower concentration (the air). So, in terms of growing plants, the VPD can be thought of as the shortage of vapor pressure in the air compared to within the leaf itself.[/FONT]
[FONT="]Another way of thinking about VPD is the atmospheric demand for water or the drying power of the air. VPD is usually measured in pressure units, most commonly millibars or kilopascals, and is essentially a combination of temperature and relative humidity in a single value. VPD values run in the opposite way to RH vales, so when RH is high VPD is low. The higher the VPD value, the greater the potential the air has for sucking moisture out of the plant.
As mentioned above, VPD provides a more accurate picture of how plants feel their environment in relation to temperature and humidity which gives us growers a better platform for environmental control. The only problem with VPD is its difficult to determine accurately because you need to know the leaf temperature. This is quite a complex issue as leaf temperature can vary from leaf to leaf depending on many factors such as if a leaf is in direct light, partial shade or full shade. The most practical approach that most environmental control companies use to assess VPD is to take measurements of air temperature within the crop canopy. For humidity control purposes its not necessary to measure the actual leaf VPD to within strict guidelines, what we want is to gain insight into is how the current temperature and humidity surrounding the crop is affecting the plants. A well-positioned sensor measuring the air temperature and humidity close to, or just below, the crop canopy is adequate for providing a good indication of actual leaf conditions.[/FONT]
[FONT="]Managing Humidity[/FONT]
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[FONT="]Managing the humidity in your indoor garden is essential to keep plants happy and transpiring at a healthy rate. Transpiration is very important for healthy plant growth because the evaporation of water vapor from the leaf into the air actively cools the leaf tissue. The temperature of a healthy transpiring leaf can be up to 2-6°C lower than a non-transpiring leaf, this may seem like a big temperature difference but to put it into perspective around 90% of a healthy plants water uptake is transpired while only around 10% is used for growth. This shows just how important it is to try and control your plants environment to encourage healthy transpiration and therefore healthy growth.
So what should you aim to keep your humidity at? Many growers say a RH of 70% is good for vegetative growth and 50% is good for generative (fruiting /flowering) growth. This advice can be followed with some degree of success but its not the whole story as it fails to take into account the air temperature.[/FONT]
[FONT="]Humidification systems to increase RH.[/FONT]
[FONT="]Table 1 shows the VPD in millibars at various air temperatures and relative humidity. Most cultivated plants grow well at VPDs between 8 and 10, so this is the green shaded area. Please note that the ideal VPD range varies for different types of plants and the stage of growth. The blue shaded are on the right indicates humidification is needed where the red shaded area on the left indicates dehumidification is needed.[/FONT]
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[FONT="]By looking at this example we can see that at 70% RH the temperate should be between 72-79°F (22-26°C) to maintain healthy VPDs. If your growing environment runs on the warm side during summer, like many indoor growers, a RH of 75% should be maintained for temperatures between 79-84°F (26-29°C.)[/FONT]
[FONT="]The problem with running a high relative humidity when growing indoors it that fungal diseases can become an issue and carbon filters become less effective. It is commonly stated that above 60% RH the absorption efficiency drops and above 85% most carbon filters will stop working altogether. For this reason it is good practice to run your RH between 60-70% with the upper temperature limit depending on your crops ideal VPD range, in the example it would be 64-79°F (18-26°C.)[/FONT]
[FONT="]The table also shows that if your temperature is above 72°F (22°C), 50% RH becomes critically low and should generally be avoided to minimize plant stress.
Please understand that by presenting this information we do not want you to go to your indoor gardens and run your growing environment to within strict VPD values. Whats important to take from this is that VPD can help you provide a better indication of how much moisture the air wants to pull from your plants than RH can. If you want to work out for yourself the VPD of your plants leaves you can follow the steps below:[/FONT]
- [FONT="]Measure the leaf temperature and look up the vapor pressure at 100% RH on table 2 below.[/FONT]
- [FONT="]Measure the air temperature and relative humidity and look up the nearest vapor pressure figure on table 2.[/FONT]
- [FONT="]Subtract the air vapor pressure from the leaf vapor pressure[/FONT]
[FONT="]Example:
Leaf Temperature = 24°C (100% RH) Leaf VP: 29.8
Air Temperature = 25°C @ 60% RH Air VP: 19.0
VPD= 10.8[/FONT]
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[FONT="]Humiditys Effect on Plants[/FONT]
[FONT="]Plants cope with changing humidity by adjusting the stomata on the leaves. Stomata open wider as VPD decreases (high RH) and they begin to close as VPD increases (low RH). Stomata begin to close in response to low RH to prevent excessive water loss and eventually wilting but this closure also affects the rate of photosynthesis because CO2 is absorbed through the stomata openings. Consistently low RH will often cause very slow growth or even stunting. Humidity therefore indirectly affects the rate of photosynthesis so at higher humidity levels the stomata are open allowing co2 to be absorbed.[/FONT]
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[FONT="]Leaf roll on Thai basil- Localized humidity stress causes by the lights being too close.[/FONT]
[FONT="]When humidity gets too low plants will really struggle to grow. In response to high VPD plants will try to stop the excessive water loss from their leaves by trying to avoid light hitting the surface of the leaf. They do this by rolling the leaf inwards from the margins to form tube like structures in an attempt to expose less of the leaf surface to the light, as shown in the photo.[/FONT]
[FONT="]For most plants, growth tends to be improved at high RH but excessive humidity can also encourage some unfavourable growth attributes. Low VPD causes low transpiration which limits the transport of minerals, particularly calcium as it moves in the transpiration stream of the plant the xylem. If VPD is very low (95-100% RH) and the plants are unable to transpire any water into the air, pressure within the plant starts to build up. When this is coupled with a wet root zone, which creates high root pressure, it combines to create excessive pressure within the plant which can lead to water being forced out of leaves at their edges in a process called guttation. Some plants have modified stomata at their leaf edges called hydathodes which are specially adapted to allow guttation to occur. Guttation can be spotted when the edges of leaves have small water droplets on, most evident in early morning or just after the lights have come on. If you see leaves that appear burnt at the edges or have white crystalline circular deposits at the edges it could be evidence that guttation has occurred.[/FONT]
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[FONT="]Guttation on tomato plants caused by high RH and wet coco coir.[/FONT]
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[FONT="]Powdery Mildew from poor humidity control.[/FONT]
[FONT="]Most growers are well aware that with high humidity comes and increased risk of fungal diseases. Water droplets can form on leaves when water vapor condenses out of the air as temperature drops, providing the perfect breeding ground for diseases like botrytis and powdery mildew. If humidity remains high it further promotes the growth of fungal diseases. The water droplet exuded through guttation also creates the perfect environment for fungal spores to germinate inviting disease to take hold.[/FONT]
[FONT="]Quick reference chart:[/FONT]
[FONT="]Low VPD / High RH[/FONT]
[FONT="]High VPD / Low RH[/FONT]
[FONT="]Mineral deficiencies[/FONT]
[FONT="]Wilting[/FONT]
[FONT="]Guttation[/FONT]
[FONT="]Leaf roll[/FONT]
[FONT="]Disease[/FONT]
[FONT="]Stunted plants[/FONT]
[FONT="]Soft growth[/FONT]
[FONT="]Leathery/crispy leaves[/FONT]
[FONT="]So hopefully now you are not just thinking like a plant youre feeling it too![/FONT]
[FONT="]Next time, part two of Plantworks will be looking at foliar spraying and how plants absorb nutrients into their leaves.[/FONT]
[FONT="]http://www.uoguelph.ca/research/apps/news/pub/article.cfm?id=90[/FONT]
[FONT="] [/FONT]
[FONT="]Higher dissolved oxygen great for productivity, health and vigor[/FONT]
[FONT="]Research[/FONT][FONT="] > [/FONT]
[FONT="]Learn About Research[/FONT][FONT="] > [/FONT]
[FONT="]News[/FONT][FONT="] > Higher dissolved oxygen great for productivity, health and vigor [/FONT]
[FONT="]By Robert Fieldhouse
(Guelph, October 13, 2005)[/FONT][FONT="][/FONT]
[FONT="]Dissolving more oxygen into hydroponic solutions could boost greenhouse productivity and provide a whole host of other benefits too, say University of Guelph researchers.[/FONT]
[FONT="]Prof. Mike Dixon and Dr. Youbin Zheng, Department of Environmental Biology, are investigating the positive aspects of using an oxygen diffuser to increase oxygen levels in greenhouse hydroponic solutions used to grow roses, tomatoes, cucumbers and peppers. [/FONT]
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[FONT="]Dr. Youbin Zheng, Department of Environmental Biology, is studying if oxygen levels can be boosted in hydroponic solutions to help growers ward off harmful microbes and boost productivity. [/FONT][FONT="][/FONT]
[FONT="]
Photo credit: Olivia Brown [/FONT]
[FONT="]Preliminary results suggest a higher dissolved oxygen level increase productivity, health and root vigor in greenhouse plants, and helps keep harmful microbes in check.[/FONT]
[FONT="]These findings are really beneficial to the industry, says Zheng. If we can use oxygen to boost plant health, making them stronger and more resistant to disease, we've discovered a very helpful tool.[/FONT]
[FONT="]Oxygen isn't as prevalent in warm water as in cool water, so oxygen levels tend to be low -- about two to four parts per million (ppm) -- at high greenhouse temperatures, compared to eight to nine ppm in cool water. Under hot weather in the greenhouse, the root zone is especially short on oxygen, says Zheng, because root respiration depletes oxygen in hydroponic solutions. Excessive watering can further depress oxygen levels because it makes growth media, such as rockwool or coconut fibre, less porous, blocking air. These factors all weaken plant disease defense systems, making them more susceptible to disease-causing microbes such as
Fusarium and
Pythium which cause root decay.[/FONT]
[FONT="]To prevent this problem, greenhouse growers typically bubble air into hydroponic solutions to bring oxygen levels up to about nine ppm. But sometimes this still isn't enough.[/FONT]
[FONT="]Two years ago, the BC Greenhouse Growers' Association asked Dixon to investigate using even higher oxygen levels in hydroponic solutions. His literature review revealed that very little work had been done in this area suggesting the problem was largely ignored until now.[/FONT]
[FONT="]Dixon and Zheng are using an oxygen diffuser recently developed and manufactured by Seair Diffusion Systems Inc., an Edmonton-based company with an interest in the greenhouse sector. The diffuser concentrates atmospheric oxygen, and dissolves it into hydroponic solutions. With this technology, oxygen levels can reach as high as 60 ppm in hydroponic solutions.[/FONT]
[FONT="]The research team is currently studying the effects of different oxygen levels, ranging from about nine ppm to 40 ppm.[/FONT]
[FONT="]So far, preliminary results are promising. But creating optimal supersaturated oxygen solutions requires extreme precision. Oxygen can be damaging at very high levels, says Dixon , so it's important to establish application methods for using this technology for different crops.[/FONT]
[FONT="]But if the methods can be worked out, Dixon says the oxygen diffusers are inexpensive and stand to emerge as an economical, environmentally friendly solution for growers looking to enhance their crops.[/FONT]
[FONT="]Greenhouse growers are voracious technical consumers they'll try anything, says Dixon . But by the same token, they're also very shrewd business people, and they won't waste money unnecessarily.[/FONT]
[FONT="]Dixon and Zheng will continue their research and will further investigate oxygen's effect on plant growth, physiology and disease. For example, they will inoculate greenhouse plants with specific microbes to see how the plants cope with this challenge under different oxygen levels.[/FONT]
[FONT="]Other researchers involved in this project include technician Linping Wang, graduate student Johanna Valentine and undergraduate student Mark Mallany, Department of Environmental Biology.[/FONT]
[FONT="]This research is being conducted at greenhouses in Guelph and Leamington , Ontario . It is sponsored by Seair Diffusion Systems Inc., Flowers Canada Ontario and the Fred Miller Rose Research Fund. [/FONT]
