and now for another opinion:Humidity, Vapor Pressure Deficit and the Transpiration Stream
by Dr. Lynette Morgan
2011-01-01
PrintEmailWith indoor gardens our focus for optimizing growth is often light, warmth and just the right mix of nutrients, but humidity, or more specifically vapor pressure deficit (VPD), is sometimes overlooked. While relative humidity is fairly easy to measure with sensors or meters in the plant canopy, it's difficult to know exactly what to aim for and how to adjust levels�and what exactly does it all mean for the plants? High humidity gets the blame for all sorts of scourges and nasty disease outbreaks, while low humidity may mistakenly be held accountable for anything that looks like burning, drying, shriveling or bleaching. However, the issues of humidity and VPD in the growing environment are a little more complex, both in terms of plant growth and disease or growth disorders. To complicate matters further, different levels of humidity and VPD are appropriate for different plant species�from dry atmosphere cactus to wet, steaming tropicals.
What is relative humidity (RH) and vapor pressure deficit (VPD)?
Relative humidity is the most commonly used measure of how much water vapor is held in the air and it's something most of us are familiar with, as we all know how uncomfortable hot, steamy air can be. 100 per cent relative humidity is extremely humid, while a humidity reading of only 50 per cent represents a dry environment, but what these values don't tell us is how much water is actually being held in the air, unless the temperature is also given. This is because cold air holds much less water vapor than hot air. For example, air at 50oF can hold 9.4 grams of water vapor per 35 cubic feet, while air at 86oF can hold three times as much, up to a maximum of 30.4 grams per 35 cubic feet. This has some implications for growers as it's not just a simple case of aiming for one `ideal' relative humidity value, since the effect of humidity on plants also depends on temperature. As a rough guide, the table below is used by greenhouse growers of many fruiting and vegetable crops and shows the relationship between temperature and humidity levels.
Temperature °F Minimum ideal RH
(fog or wet down) Ideal RH Maximum RH
(to prevent disease)
59 - 50 per cent 73 per cent
68 46 per cent 64 per cent 80 per cent
77 60 per cent 73 per cent 86 per cent
86 70 per cent 80 per cent 89 per cent
The problem with using relative humidity is that it's hard to set one optimum RH value, particularly when temperatures tend to vary between day and night and often throughout a 24 hour period. For this reason many growers prefer to use vapor pressure deficit (VPD) as a more accurate measure of the water vapor content of the air and how this affects plant growth.
VPD is the difference (or deficit) between the amount of moisture in the air at the current time and how much moisture the air can hold when it is saturated. Saturated air will condense out to form dew or condensation and leaf wetness, which in turn can lead to rot and a higher occurrence of certain disease pathogens. So while plants don't want an overly dry atmosphere (high VPD), which sucks the moisture from the foliage, they also don't want a wet environment (low VPD), which slows transpiration and can lead to an increase in disease outbreaks. VPD is typically expressed in the units kPa (kilopascals) with the range for most plants being 0.45 kPa to 1.25 kPa, with the optimum being around 0.85 kPa. Most indoor gardens are best run at 0.8 to 0.95 kPa for healthy mature plants, with cuttings needing a more humid environment in the lower VPD ranges. What is important is that unlike relative humidity, the VPD range for optimal growth already takes current temperature into account–so the one ideal value irrespective of temperature is around 0.85 kPa. Growers who come to grips with the concept of VPD can accurately measure and adjust their growing environment to stay within the recommended range and give themselves some considerable advantages both in terms of maximizing growth and the ability to control some rather persistent nasties such as Botrytis (grey mold) disease.
Technically, VPD more accurately describes what the plant experiences in relation to the effects of temperature and humidity on growth and transpiration. It combines the effects of both humidity and temperature into one value, so it's easier to use when setting environmental controls.
Why is humidity or VPD so important for plant growth?
"A dry environment with low humidity typically results in smaller, more compact leaves in tomato crops."
We know that light level and quality and CO2 affect photosynthesis and a number of other plant processes, and that temperature determines the levels of many biological processes within plant cells, but the effects of humidity are a little more indirect. VPD directly affects the rate of transpiration within the plant. Transpiration not only cools the plant, but the transpiration stream from root to leaf surface carries essential minerals up the plant to where they are needed for tissue development. Leaves exposed to the sun or overhead lamps would soon become dangerously hot if they were not cooled by water evaporating from the leaf surface–the process actually works in a similar way to sweat. This evaporated water needs to be replaced from the transpiration stream, which moves in the xylem vessels of the plant. If the water flow from roots to shoots in the xylem vessels is not fast enough, the plant will start to wilt and tissue damage will occur. The plant will shut its stomata in an attempt to retain turgor pressure and prevent wilting if too much water is being lost via transpiration. When the stomata shut to prevent water loss, photosynthesis cannot occur as CO2 can't be taken in from the surrounding air, so plant growth and yield will be slowed if this occurs too often. Low humidity (high VPD) can cause large volumes of water from the transpiration stream to be lost to the air and force the plant to shut down its stomata to prevent desiccation, and this ultimately reduces growth and yields.
High humidity (low VPD), on the other hand, creates a different problem. When the air already contains a lot of water vapor and may even be close to saturation, it cannot absorb much more water from the plant surface and transpiration slows or even stops. If high humidity conditions exist at the same time as high temperatures, the plant has a major problem as it can't evaporate enough water from its foliage to cool its tissue and overheating will then occur. Cell damage, wilting and reduced growth will result where hot plants can't effectively cool themselves via transpiration due to high relative humidity, and in these cases some humidity control is essential.
The transpiration stream moving through the xylem vessels from roots to shoots driven by VPD and humidity is essential for plant functioning. Not only does the water carried in the transpiration stream maintain plant turgor and support, it also carries with it mineral elements and other compounds taken up by the roots. So without a good rate of transpiration drawing the flow of water and minerals up through the xylem tissues from the roots, plant foliage can't obtain sufficient levels of nutrients for growth and development. One of the most important aspects of transpiration is the flow of calcium. If transpiration is restricted in any way, the lack of calcium flow out to the leaf tips and new cells in developing fruits will cause problems such as tip burn and blossom end rot, which are conditions common under warm and humid growing conditions.
How RH and VPD influence disease outbreaks
There are a wide range of fungal and bacterial diseases that will attack even healthy plants under high humidity (low VPD) conditions, because fungal spores in particular are carried on air currents and so tend to be around much of the time just waiting for the right conditions to launch an attack. Spores themselves need to absorb water from the environment to germinate and get inside plant tissue, and having free water available such as that from condensation on plant surfaces is perfect for fungal diseases to develop. However, not all fungal disease spores need water on the plant surface, and many will attack when the air humidity is high. For this reason the safe maximum humidity value is often considered to be around 85 per cent at 77o F, or in other words a VPD value of more than 0.35 kPa is recommended at all times in order to prevent fungal diseases, which are common under humid conditions. While many of the commonly encountered plant diseases such as Botrytis (grey mold) thrive under humid conditions, and a good degree of control can be achieved by running optimal RH or VPD levels, there is always an exception. Powdery mildew species have spores that contain a lot of water themselves, so they don't need high humidity or water for germination, and they can also obtain all the water they need for growth from the leaves that they infect. This means that powdery mildew can develop at humidity levels as low as 30 per cent and that its appearance is not necessarily a sign that humidity has been running high.
How to control RH and VPD
Low humidity (high VPD) is fairly easy to adjust upwards to high RH, as putting water vapor back into the air can be easily achieved with some light fogging, misting or damping down in the growing area. An open pan of water should provide enough evaporation to increase the humidity if you're only dealing with a small area. Evaporative coolers also tend to increase the humidity of the air fairly effectively under warm growing conditions. However, having high humidity is a more common problem, as large surface areas of foliage tend to lose surprising volumes of water through transpiration and this adds to the humidity of the surrounding air. This humid air, referred to as the boundary layer, needs to be removed from directly around the foliage or further transpiration could be restricted. The best way of doing this is with a continual stream of drier fresh air, which not only lowers the humidity directly surrounding the leaf surface but also replenishes CO2 for photosynthesis. The amount of airflow required to continually remove excess water vapor and bring in sufficient CO2 is higher than many growers realize–there should be sufficient airflow to keep the leaves gently moving most of the time. The warmer it is and the faster the plants are growing, the more the air needs to be shifted over the leaf surface for these processes to occur at optimal rates. For very humid climates, sometimes the only option is a dehumidifier. If the outside air being brought in to cool and dehumidify an indoor garden is naturally very humid it can't absorb much more moisture from transpiration, and using a dehumidifier is often useful for smaller areas under these conditions.
Checking humidity or monitoring vapor pressure deficit in the growing area is just as important as maintaining temperature, light and nutrition levels when it comes to optimizing plant growth and yields. Having some understanding of the relationship between RH and temperature and VPD and how to influence and optimize these factors is an essential tool for the serious grower, and also for those gardeners who just want to prevent annoying outbreaks of fungal disease.
But they are looking so much healthier. The big one in the center im gonna take out and put it under 4 4-foot t5 with a mirror reflector and im wondering if its time to start using the HPS bulb? Since ive been running the MH non stop 24-7 I was wondering if I should go gradual with the HPS 18/6 hours a day? Please give me your thoughts on if its time for the HPS or wait and shoudl I shock thwem with 12/12 of work my way down.
