Watering Marijuana & Measuring Root Zone Moisture Content
Unless you grow in pure hydroponics such as deep water culture or aeroponics, your marijuana roots are in a solid root zone material generically referred to as a “substrate.”
Substrate materials include soil, soilless mix, coco coir, rockwool and others. And each substrate has to be correctly managed to ensure your roots have just the right amount of moisture and nutrients, as well as the correct pH.
If you don’t manage your substrate properly, you can injure or even kill your plants. You lose maximal growth rate, harvest weight, and bud value. You might drown your roots or create conditions ideal for root pathogens. Many of us make mistakes in substrate management. The important questions marijuana growers want to answer are:
- How much water should I use when I water?
- How often should I water?
- How can I tell when my root zone substrate is too wet or too dry?
- How can I avoid overfeeding or underfeeding?
If you water too often or use too much water when you irrigate, you risk overwatering your root zone which drowns your roots so they can’t absorb enough oxygen and they eventually shut down and rot.
Also if you water too much, you could be overdosing your root zone with nutrients, creating a variety of pH and nutrients-related problems that can harm or kill your plants. Overwatering often produces a characteristic leaf-droop that looks a little like wilting, but the leaves retain their shape. Experienced growers can tell the difference between waterlogged drooping leaves and wilting leaves.
If you don’t put enough water into your substrate, your plants wilt–and a wilt that lasts more than a few minutes permanently damages your plants and roots and can eventually kill the plants. Further, underwatering may deprive your plants of an adequate supply of nutrients.
The topic we’re discussing here is formally called “substrate management.” Growers using pure hydroponics systems have a far easier time, because their plants’ roots dangle in nutrients water, or in open space periodically sprayed with nutrients water. As long as the nutrients water is properly oxygenated, has the correct nutrients dose, and is the right temperature, you won’t have the problems growers have when they use substrate marijuana growing.
When you’re using solid root zone substrate, the challenge in knowing how much water to use starts with the fact that each type of marijuana root zone substrate has unique characteristics that affect your substrate management tactics.
Substrates are supposed to be crafted by combining components such as sphagnum peat moss, perlite or pumice type materials, coir, bark or wood products, vermiculite and other materials to produce physical properties that provide the plant with the best possible air porosity and water holding capacity.
In many cases, whoever is making the substrate isn’t doing a very good job of combining components, and the substrate may have the wrong air/water-holding traits for marijuana roots.
One very important substrate characteristic is water holding capacity (WHC). This is also called saturation capacity and refers to how much total water the material can hold when fully saturated. In almost all cases, you never want your marijuana roots to be in fully saturated substrate.
Another substrate trait to pay attention to is air porosity, the amount of air a substrate holds after it has been saturated and drained. Air porosity is best created by having coarse particles in the growing medium, such as coarse perlite, that create large pores that maintain air space and facilitate drainage.
Fine particles, such as coir dust, create smaller pores that hold water but don’t provide enough space for air. The relative amount of large and small pores characterizes the growing medium’s total pore space. The combination of components and their particle size determine air porosity, total porosity and water holding capacity in your substrate.
Total porosity is the measurement of the growing medium’s total volume of these pore spaces. Air porosity refers to the measurement of the volume of pore space in a growing medium occupied by air after it is saturated and allowed to drain. Air porosity is calculated as a percentage of the volume of the growing medium and is inversely related to the bulk density of a growing medium.
Another substrate trait is referred to as “field capacity.” This is the amount of water in the substrate after it has fully drained.
Note that this does not mean the same thing as “fully dry.” Instead, it means that after you’ve poured water into a substrate to fully saturate it, a percentage of the water may drain out of the substrate, so total saturation is no longer present.
Field capacity also does not refer to the level of water in the substrate after water has evaporated from the substrate or been taken in by plants.
By far the most important substrate trait in this watering discussion is how much water should be in the substrate after watering and drainage to facilitate optimal root health and root absorption of oxygen, moisture and nutrients.
This is a tricky thing to manage and many growers use amateurish methods. The most professional method is as follows:
- First, contact the substrate manufacturer. Ask them what “volume water content” (VWC) percentage is best for their substrate. Volume water content is not saturation content or field capacity. VWC is a numerical measure of soil moisture–the ratio of water volume to substrate volume. In some ways, VWC is similar to relative humidity, which measures how much water is in the air, compared to how much water the air could maximally hold.
- How do you measure VWC? The only totally reliable way is by using a professional soil moisture meter. Problem is, most moisture meters that cost less than $150 are worthless. I recommend the Extech MO750 meter, but call the manufacturer first to ask if their meter will give accurate readings in the substrate you grow in. Extech told me their meter gives accurate readings in soil, soilless mix, coco coir and Grodan rockwool. If you have an integrated, automated grow op controller system, you could have the option of using a plug-in sensor such as this one.
No matter what the substrate manufacturer tells you about VWC, it’s useful to test it for yourself. Here’s how you do it.
- Fully saturate your substrate with reverse osmosis water and take a soil moisture reading before the substrate drains to determine saturation VWC.
- Take a VWC reading after the substrate drains, to determine field capacity VWC.
- Take a VWC reading after substrate has completely dried out due to evaporation to determine fully-dry VWC. Alternatively, you could do a substrate reading as soon as you remove the dry substrate from its manufacturer packaging, before you’ve ever added any water to it at all.
If you don’t have a soil moisture meter, and you’re using individual pots to grow in, you can do the following (although it’s kind of messy):
- Get a waterproof weighing scale big enough to put one of your pots on. Fully saturate the substrate, and note the weight.
- Let the substrate fully drain, and note the weight.
- Let the substrate fully dry out, and note the weight. Alternatively, you could fill a pot and weigh it when you’ve just taken your substrate out of whatever container you purchased it in. Most substrates are shipped dry.
- Note that some growers believe they can determine how dry or moist a substrate is by picking up plant pots and using intuition. I’ve seen this method fail more often than succeed.
- By doing percentage comparisons, you approximately determine saturation weight, field capacity weight, and dry weight.
Now that you have your moisture meter VWC readings or your pot weights, you can begin to ascertain the right amount of water for your plants.
You never want your plants’ roots to sit long term in fully saturated substrate. When the substrate has drained to field capacity, that’s a useful benchmark, but field capacity (the VWC of your substrate after saturation drainage), is still too much moisture for your roots.
On the other hand, you never want your plants in fully-dry substrate. Some growers combine the above-mentioned techniques with an observational strategy in which they watch their plants carefully to see when they first start wilting. As soon as cannabis leaves show the slightest signs of wilting, the grower measures VWC using a meter, or by weighing the pots.
This gives a useful indication of what VWC or pot weight is associated with wilting. Depending on the type of substrate, the ideal VWC percentage will be in a middle range between the field capacity VWC and the dry (wilting) VWC. Please note: growers should be careful to only use this method once per plant phase, and to not let plants fully wilt, which can cause permanent damage.
As soon as leaves start to wilt, but long before they’re fully wilted, you need to do your measurements, and then quickly add water.
Also note: as plants grow, the pot weight changes due to the weight of the plant. This has to be considered when weighing.
Let’s examine a couple of case studies to see how substrate management works…
Case Study A: The marijuana grower is using coco coir. Coco coir is known to have favorable water-holding and aeration properties. But like peat, it can break down or become compacted during use, leading to decreased porosity and unfavorable traits.
Using the Extech meter, the grower discovers that fully saturated coco coir reads at 73% VWC. Field capacity VWC is 57%, and fully dry VWC is 11% or less. The grower consults with the manufacturer, who tells her the ideal VWC is between 32-46%. She adjusts her irrigation frequency and volume so her coco coir substrate is in the ideal VWC range almost all the time. Her plants have maximal growth, root health, and harvest weight, compared to when she was overwatering her coco coir.
Case Study B: The marijuana grower is using peat-based soilless mix. The manufacturer doesn’t know what the VWC percentage should be, so the grower has to figure it out on his own. He does this by first watching his plants’ leaves droop from overwatering, indicating fully saturated substrate, and weighs the pots to determine saturation weight. When they’ve fully drained after saturation watering, he weighs them to get the field capacity weight.
He’s also watching to see when leaves perk up as the overwatered condition recedes. When the leaves have fully returned to normal and aren’t drooping anymore he weighs the pots, and logs this as the upper range of ideal pot weight.
You might be confused by the appearance of cannabis leaves when plants are overwatered versus when they’re wilting. In both cases, the leaves are no longer parallel to the ground, horizontal, or even angling slightly upward. Instead, the leaves are hanging off their stems.
In overwatering, the leaves are drooping, but they’re still turgid, fully formed, and green. In wilting, leaves are also drooping, but they’re wrinkling, more limp, and may start turning from green to brown.
The growers continues to weigh the pots and monitor the leaves as the substrate dries out. His final weighing is when the leaves have just begun to wilt. These measurements give him ideal pot weight data across the range of acceptable VWC. If he was using VWC metering, his substrate management data would be much easier and more precise.
These measurements are the foundation of optimal fertigation strategy. Growers want to ensure that their substrate isn’t consistently oversaturated or undersaturated. Such conditions damage plants and the substrate.
Growers using automated drip irrigation or similar automated systems have an easier substrate management situation than growers who hand water. Regardless of how you provide water to your plants, the goal is to keep the substrate in the ideal VWC or weight range, balanced precisely in the mid-range between saturation and totally dry.
Beyond that, take a look at other substrate management details:
- Growers should apply water as early in the light cycle or sunlight day as possible, depending of course on when the substrate first needs water during that period.
- Watering should be at sufficient volume to generate 8-15% runoff of excess water out the bottom of the pots, slabs or other containers. This purges old nutrients salts from the substrate, and also moves fresh oxygen into it.
- Vapor pressure deficit, light hours and intensity, plant growth phase, and other factors influence how quickly plants intake water, and by extension, how substrate VWC changes over time.
- Do not water in the last two hours of sunlight or grow lights-on unless the plants are wilting.
- Especially in bloom phase, do a flush with pure reverse osmosis water to replace nutrients watering every third watering. For example, if you water three times a week, use nutrients the first two times, and only reverse osmosis water the third time. This purges excess salts from substrate, creating a healthier root zone.
- Measuring runoff pH and parts per million is very smart. If runoff pH is more than + or – .4 pH compared to incoming pH, or if runoff parts per million is more than + or – 15% different than incoming parts per million, it could indicate problems. For example, if runoff parts per million is 30% higher than incoming parts per million, that tells me the root zone is overfed, and I need to do a flush at next watering.
- As long as my runoff ppm is significantly higher than my incoming ppm, I’ll use lower than the manufacturer’s recommended dose, and do intermittent flushing, to remove excess nutrients salts from the root zone.
- Growers should ask substrate manufacturers about their substrate’s cation exchange capacity (CEC). This is the total capacity of a substrate to hold exchangeable cations. In simple terms, it refers to the substrate’s ability to facilitate nutrients elements transfer into roots. Substrates such as clay, organic materials, and vermiculite have negative electrical charges that attract and hold positively charged nutrient ions, known as cations. CEC varies widely, but it’s a valuable substrate trait to understand, given that your marijuana plants need positive cation elements such as calcium (Ca2+), magnesium (Mg2+), potassium (K+), and ammonium (NH4+) and other elements–or else they won’t grow. Your marijuana plants also require negatively charged anions such as nitrate (NO3-), chloride (Cl-), sulphate (S04-), and phosphate (HPO4).
Anybody giving you advice about managing your marijuana root zone substrate has to acknowledge that every substrate and grow op is different. You’ve got to work with the information in this article, and carefully monitor your plants, runoff pH and ppm, grow room environmental factors such as vapor pressure deficit, and substrate condition, to customize substrate management in your particular situation.
For example, if you’re growing in a rockwool slab recirculating system, it’s hard to measure runoff parts per million and/or pH, because the water that leaves your rockwool after every watering is immediately gravity-channeled back into your reservoir.
It could be worth it to isolate at least one slab tray so you collect runoff water before it is dumped back into the nutrients reservoir.
In some cases, frequent watering with small amounts of water is better than one big watering event every couple of days.
Another thing to know is that of all the substrates marijuana growers use, fertile soil is the hardest one to manage and can be almost impossible to flush if you have a nutrients overload.
When I use fertile soil, it’s either supersoil (which doesn’t need added nutrients), or I’ve amended the soil so it contains about 10-20% coarse perlite. This adds porosity, promotes drainage, makes it easier for roots to penetrate the soil, oxygenates the substrate, and makes it easier to flush.
I tend to use veganics if I’m running a soil substrate grow op. Veganics fertigation doesn’t create root zone nutrients salts excess, but hydroponics fertilizers frequently do cause damaging, excessive amounts of nutrients elements in your root zone.
Many marijuana growing problems that growers blame on nutrients, root zone pathogens, defective substrate, or cannabis genetics are actually caused by improper substrate management, with overwatering the main culprit.
Fortunately, you now have this article and its embedded videos (be sure to watch the videos) so you do more professional substrate management, and you’re sure to see healthier roots, faster growth, and bigger harvests from your marijuana plants.Here's how to properly manage the water content of your marijuana root zone so you have healthy roots and big, fat harvests. ]]>