This post is a simplified interpretation with respect to Cannabis of the information presented in the technical article linked below “Response of plants to water stress” in the plant physiology publication called Frontiers in Plant Science:
Everyone knows, plants need water. And as so many over-attentive household gardeners know, over-watering is a great way to kill a plant. But the threshold of harm for under-watering or water stress is also not real clear, because water stress can cause real problems before any actual wilting (as seen in the picture to the left) is apparent.
The primary mechanism of water stress is fairly straight forward. When the plants water intake from the roots drops below a certain threshold, the plant cannot maintain proper fluid pressure, and to preserve it from wilting a chemical called Abscisic Acid (ABA) is created that signals a number of responses, most important of which is it signals the closing of the stomata (small pores in the leaf) to prevent further water transpiration. The upside is the plant doesn’t wilt or fall over. The downside is the plant stops taking in CO2 which enters the plant through those stomata. With the closing of the stomata, the plant effectively stops growing. A similar effect happens with high temperature stress, but that is a more complex function and can be avoided by increasing CO2 levels. But I digress.
There are, however, some advantages to letting a plant get quite dry, even close to the point of shutting down. First, as the root system dries, the absence of water triggers the roots to grow out to seek additional water. The dry soil also inherently increases the air in the soil and consequently the oxygen levels, which also are necessary for the increased root growth. So letting the soil get relatively dry, is beneficial and stimulating to the roots. Also, the increase in air and oxygen in the root zone retards the growth of detrimental anaerobic (non-oxygen) bacteria and fungus’s. Most plants that die from over-watering actually die from the overgrowth of these pathogens. All of the healthy root bacteria and fungus’s like Bacillus Subtilis and Mycorrhiza, need oxygen to live. So there are benefits to allowing a plant to dry out some, but exactly how far can be tricky.
Cannabis during flower, has special issues. Allowing the plant to get close to wilt conditions will not only cause it to shut down growing, but can also reduce the pressure in the plant enough and SNAP! the branches with their heavy resin laden flowers start to break. This risk can be partially mediated through SCROG techniques, but even the slightest reduction in growth at key points in the flower cycle can be very costly in yield and quality. Nutrient supplementation with Silica can also make a plant less susceptible to water and temperature stress and wilt in general.
Another issue associated with water stress is Excessive Light stress (EL). When plants are starved of water, through chemical actions to complex to describe here, they become more sensitive to light stress. The damage from these stresses can vary greatly, but with Cannabis, there is one particular idiosyncrasy that can make any kind of stress extremely problematic.
Cannabis plants have a nasty talent for changing sex pretty much anytime. The unfertilized female or sensimilla, is a highly undesirable condition in nature. So when a female plant is well into the flower cycle and still unfertilized, and also under stress, it CAN change it’s sex to male, becoming what is called a hermaphrodite (hermi), thus fertilizing it’s OWN seeds. Nature prefers a plant to be fertilized from another plant to increase vigor and variation, but in a pinch, when fearing the harsh conditions might kill the female before it successfully seeds, it can go hermi and pollinate itself. All Cannabis strains can do this, but some are more inclined than others. While hermi’s are always bad, they are especially bad in the middle of a flower cycle, and the main causes of Cannabis plants turning hermi in the middle of a flower cycle? Excessive heat, light or water stress. All these conditions can trigger a hermi, and it only takes ONE she-male plant to ruin a crop.
So the bottom line is water stress in Cannabis is a very bad thing, more so than in virtually any other type of commercial crop. And the remedy? Careful monitoring of your plants individually. Even in true hydroponics, a partially clogged nozzle can cause water stress, and associated problems. But for all soiless approaches (Pete, Coco, etc.), the dryness of each plant has to be carefully observed. In pots, fabric or plastic, I have found that the weight is the best indicator, NOT dryness on the top. With rock wool or other media it can be more complicated. But the bottom line is, don’t let you plants get too dry!!! EVER! Even ONCE!
And on that note, there is another issue with water stress specifically with mother plants used for cloning. Water stress can actually turn on and off plants genes in response, and these changes will be carried down to the clones. So while that plant may make alterations to itself to make it more drought tolerant, the result could be slower growth and lower yields for the cloned plants. So water stress events are ESPECIALLY problematic for cloning mothers.
One of the main reasons I am aggressively testing and slowly transitioning from HPS lights to LED’s is for this very reason. I have had whole crops lost to hermaphrodites, presumably caused by heat, water, or EL stress.) HID lights have high levels of infrared which heats the leaf surface to much higher levels than the ambient air temperature, and that creates much greater temperature differentials from the top to the bottom of the plant. So the potential for temperature or EL stress (often connected to water stress) and resultant hermaphrodites is much higher with HID lights than with LED’s, especially in grow rooms with lower ceilings.
So growers beware, don’t let your plants get too hot or dry, especially if you are using HID lights!!!!
To those who grow, we SALUTE YOU!!!