Consistently great harvests depend on plants receiving the right amount of water, with the right contents, at the right time, throughout their lifecycle from clone to harvest. Accomplishing this is no easy task, especially in larger facilities where thousands of plants need daily tending. For craft results, however, some measure of individual attention is still necessary. Even if the irrigation system is automated, growers should develop mastery of hand-watering in order to give the plants exactly what they want when they want it.
Like most veteran growers, I learned water management the hard way: by doing everything wrong until I unraveled all of the underlying dynamics of the process, found the right equipment and supplies, did a zillion experiments, and finally dialed it all in. I still sometimes make mistakes when I try different, unfamiliar types of media, since each one has its own unique water-holding tendencies, but now that I understand all of the factors, I can get everything balanced much more quickly.
The bulk of this article is about balancing water quantity with oxygen levels for the sake of happy roots, because happy roots equals happy plants. But determining the right amount of water to apply will be elusive if the water doesn’t also contain the proper components, so let’s start by discussing water quality and contents.
These days, more indoor growers are watering either with reverse osmosis water or recaptured condensate from the HVAC system (equivalent to non-sterile distilled water). I use AC/Dehu condensate in my grow and I like to start with nothing in my water, rather than compensating for what’s naturally in tap or well water. Tap and well sources can vary from season to season and sometimes contain high levels of sodium or micronutrients. Some regions, however, have perfectly good tap water that primarily contains calcium and magnesium, with only tiny bits of other minerals or contaminants.
In either case, it’s important to know what is or isn’t in the water to begin with, so that you can add the appropriate quantity of minerals and buffers to keep the plants in good condition. For this reason, when one of my clients is starting with tap or well water, I ask that they obtain a mineral analysis from a lab, so we’ll know how to formulate their nutrient recipes to complement the natural contents of the water. We might also find from the lab report that certain minerals are too high to start with, which means a reverse osmosis system will need to be installed.
On site, growers generally don’t have advanced measuring equipment for mineral analysis of water or plants, but most of us do use EC (electrical conductivity) and pH meters to provide a general idea of how much total “stuff” is in the water and at what pH. I’m a stickler for pH control, because I see pH mismanagement as a very common error I encounter in my site evaluations, with fairly harsh and immediate negative consequences for the plants. This goes for growing in an organic medium just as much as it does for coco or rockwool. The pH of the root zone must be between 5.7–6.7 in order for a nutrient recipe to work properly. Outside that range, on the high side of pH, macronutrients will be excessively available and micronutrients deficient, where on the low side, micronutrients will be excessive in the plants and macros will be deficient.
I like keeping the pH in rockwool or coco between 5.8–6.3 and in soil between 6.0–6.6. Soil does better at a higher average pH because, at a lower pH, iron and manganese become toxic more quickly because the total quantity of available iron and manganese is not naturally limited the way it is in hydroponic media.
Some pH and EC meters are much better than others, but what’s critical—no matter which brand is used—is that pH probes must stay clean, wet, and calibrated at all times in order to provide the information needed. I like Bluelab meters the best. They hold their calibration better than other brands I’ve tried. About 40% of the sites I visit have pH meters but the growers don’t manage the probes properly, which results in inaccurate data,causing all sorts of confusion down the line as we attempt to solve plant health problems and improve yields.
EC meters aren’t nearly as sensitive as pH meters and need barely any calibration. If they are kept clean, they’ll provide the ballpark information needed—which is a rough estimate of the total mineral content of the water. EC measures the total quantity of conductive salts in a solution. Some growers know this as PPM, but PPM is a less accurate term since some meter companies arrive at PPM by multiplying ECx500 and others multiply ECx700. I grew up using PPMs at the x700 calculation, but I’m increasingly switching to EC in order to avoid translation errors with clients when I teach them to mix and apply their own nutrient formulas. EC has been the standard term used in agriculture for decades, so it makes sense that cannabis growers would follow suit now that we are integrating with mainstream society.
Developing Good Water Sense with Your Crop
Plants do best when they have just the right amount of water applied. Because the “right amount” isn’t some kind of mathematically derived quantity, but rather depends on how several plant-related and environmental factors come together, less experienced growers often mishandle watering, leading to lackluster results. In fact, overwatering is one of the most common problems I encounter in my site advisory work. Underwatering is a less common pattern, probably because growers are more likely to apply too much of everything—nutrients, light and water—in an effort to give the plants everything they might need, not realizing when they cross the threshold into overdose territory.
There’s no such thing as overwatering, per se. Under-oxygenating is actually what happens when too much water is applied too frequently. Cannabis roots love oxygen, which is an essential nutrient for plants that can hardly be over-applied; more oxygen is usually always better for plant health. Out of the tap, water usually has around 7–8 ppm of oxygen, and that amount of oxygen can be gobbled up by roots and/or microbes in the media in about 12 hours. If most of the water isn’t also consumed in that time frame, the roots are left with a lot of de-oxygenated water around them, which puts the plants into an asphyxiated state and gets in the way of nutrient uptake and harms the crop’s potential.
To balance the oxygen depletion with the water uptake, a few different approaches can be taken. The two most common approaches are to reduce the quantity of media or to increase the porosity of the media. The third, less common, approach is to use some kind of oxygen infusion technology that increases the total dissolved oxygen levels in the water. This method is up and coming and has lots of promise (See our article on page 16 to learn more about oxygen supplementation), but currently, the vast majority of commercial cultivators use the first two methods, so we will break those down further in the following paragraphs.
Reducing the Quantity of Media
You’ve probably either toured facilities or seen pictures of operations where relatively large plants are planted in quite small rockwool cubes or pots of soil. The two main advantages of using this method are 1) reduced supply costs for media and 2) quick dry downtime for plants, which means overwatering is virtually impossible and roots get plenty of oxygen. Another minor advantage is that a smaller amount of media is easier to flush water through, allowing for somewhat easier management of mineral nutrients in the media.
The downsides of the “Minimal Media” approach are 1) the plant can dry out and die quickly (within hours) if something goes wrong with the irrigation system and 2) the limited root space can negatively affect flower size and plant structure. I’ve done many tests over the years comparing one cultivar started in several small containers vs. the same one in a bigger containers, and they grow up differently. The plants in smaller pots become tall and skinny, while the plants in bigger pots become wider, with bigger leaves and flowers. I’ve also had my share of mechanical failures and power outages, so relying on the irrigation system to always work when plants are just hours away from death in the event of a glitch, is something I’ve had a hard time embracing. I’d personally rather have a full 24 hours after a pot gets saturated to the time it completely dries out, which means using more media and bigger containers (or cubes).
Increasing Aeration of the Media
Rockwool holds the amount of water it holds by design, so nothing can be done to increase its porosity; but any soil-like medium, whether peat, coco, or dirt, can always have perlite or rice hulls added to allow for more drainage, and more drainage means more oxygen in the root zone. Up to 40–50% perlite can be added to most media types to increase aeration, and this mitigates the risks of overwatering. It also causes the plants to dry out faster, which means more frequent watering is necessary; but it’s a good compromise when you want more root space and some moderate insurance against overwatering.
Learning to Judge Water Status Based on Weight
Adding water to a rockwool slab, cube, or container obviously adds weight, and this is something growers should both measure and get a feel for if they want to develop good water sense. A dry cube or container of media can be weighed before and after saturating with water, and the difference indicates the range of weights, from one extreme to the other. The ideal amount of moisture will be somewhere between the two extremes. For example, if a 6” x 6” rockwool cube weighs a half pound when new and 4.5 lb when completely drenched, then the ideal range for mid-sized plants in those cubes will be somewhere around 2–3.5 lb.
Weighing plants to determine how much water they need is certainly not an exact science. It is convoluted by the variables of root mass, stems, and leaf biomass, which can vary widely depending on the age and pruning status of the plant. This is why a scale alone won’t do the job; it must be used in tandem with getting a “feel” for the weight, which is a sense that can be developed by weighing the trays, cubes, or containers, then watering, then feeling the weight and observing over time to see how quickly different sizes of plants dry out.
When growers develop a feel for their plants’ water status, they’ll learn, for example, to casually lift up the end of a clone tray and know if it needs water or not. They’ll also know to give bigger plants more water sooner and smaller plants less water, less frequently. A scale can always be nearby to confirm the assessment since some growers are naturally good at guessing at water weights, while others easily lose their judgment if they are inexperienced, tired, or distracted.
In the areas of a facility where plants are usually hand-watered, such as the mother plants or clone trays, I always recommend that each day begin with someone doing a quick weight assessment on all of the trays and containers, and either marking or pulling out the ones that feel light, so they can be watered right away, before any wilting or damage occurs.
Watering Quantity as it Relates to Plant Size and Health
When small plants are freshly transplanted into bigger cubes or containers, they must be watered lightly until the roots spread out to the sides and bottom of the new container. This can take anywhere from 4 to 10 days. During this time, the cube or container must be kept moist, but not be anywhere near saturated. Saturating the parts of the media where no roots yet live will lead to deoxygenation. When that occurs, roots won’t grow into those areas and the plant will be stunted. If you can lightly moisten the media instead of saturating it, enough oxygen will remain to create an inviting environment for newly-forming roots.
As an example, let’s say you are planting a newly rooted clone into a 6” x 6” rockwool cube. A rockwool cube this size will hold a half gallon of water when fully saturated, but at first, it’s better to give it about 16 oz, then follow up with 2–3 oz per day until it starts to grow and thrive in its new setting. When it’s clearly established and starting to get bigger, or when roots are coming out the bottom, you can fully saturate the rockwool cube without worrying about deoxygenation. This is because the plant and roots are developed enough to consume a larger amount of water in a shorter amount of time—which concurrently regenerates the oxygen levels in the root zone. See the diagram below for a visual representation.
What If The Plants Are Unhealthy?
Plants are probably imperfect more often than they are perfect for most growers. This can be because of clones taken from unhealthy mother plants, a poorly executed cloning process, too much light, the wrong kind of light, extreme temperatures/humidity, or some kind of insect or microbial infection. If the plants are not dark green and perky, then the amount of water they will consume will generally be much lower, regardless of their size. Root growth will also be slower. In this case, they must be babied in order to get back to a normal rate of growth.
When I was running facilities in Northern California back in the early 2000s, we often would pull sickly plants from tables to make room for new plants, and they’d get set on the floor to dry out before being discarded, which could take a few days. To my surprise, being taken out of bright light and starved of water often brought them back to life. This was completely counterintuitive to growers who had the natural instinct to put the sicker smaller plants in brighter light with more food and water. We learned through that process that sick plants need rest; they need a break from the photosynthetic demands of the bright light and the wet conditions they’ve been in. Now whenever I need to rejuvenate plants, my basic approach is minimal watering and low light until they start to bounce back. Then I can increase the light, flush and drench with fresh nutrient solution, and get the growth cycle back on track.
There’s more to say about water uptake as it relates to plant health, so please come back for next month’s feature article on plant health troubleshooting, where I will describe the steps involved with deducing what could have happened and how to fix it when your plants take a turn for the worse.