We’ve all had it happen way too many times—showing up to work to find that your plants have taken a turn for the worse. Now what?
After spending almost half my life managing some kind of indoor cannabis grow on a daily basis, my method of identifying plant health problems has gotten fairly systematic, so I’ve decided to share it with Pacific Northwest growers in this month’s issue.
This article follows on the coattails of last month’s article on the art and science of watering, where we explored the many dynamics of how to gauge the moisture needs of the plants and apply just the right amount of water at the right time.
Before I dig into it, though, I need to take a quick detour to address one main assumption, one big caveat, and one major requirement of operations. These things are baked into the following set of recommendations for sleuthing out plant problems.
My main assumption is that your HVAC system and dehumidification are kept clean and in good working order and can maintain fairly narrow and controllable ranges of temperature and humidity. If you have spikes or system failures with any regularity with climate controls, don’t expect the plants to be happy and healthy, even if you are doing most other things right. Faulty environmental controls are a foundational problem and must be repaired before it’s worth putting energy into the complexities of plant health maintenance.
One big caveat regards insect and microbial infections. There’s no doubt that these are common afflictions at cultivation facilities, but they vary quite a bit from site to site, making the identification and treatment approach unique for each grower. Also, I’m not a qualified plant pathologist, so I want to proceed into this troubleshooting article with the caveat that pest management is not included, even though I know it’s relevant.
Before I drop the topic completely, however, there are some universal approaches I’ve found to help minimize the damage if you have pest problems (insect or microbial). The main one is to water lightly. Soil-based insects and microbial infections generally can’t reproduce as fast under dry conditions, so easing up on the water quantity is usually helpful. Other mitigating methods for pests, no matter what the infection or infestation, are to increase air circulation, lower ambient temperatures (to around 70°F), and keep plants well pruned. For a specific infection, if you need help, an IPM specialist or plant pathologist should be able to provide treatment recommendations.
The one major requirement of operations, and I can’t emphasize this enough, is that the site be kept clean. I must have toured 200 sites over the past 20 years, and if there’s one thing that distinguishes the winners from the losers, it’s site cleanliness. To compete in an oversupplied market, there is simply no choice but to clean the floors every day, monitor air quality, clean surfaces at least monthly, and keep all fans and HVAC components wiped down and free of dust and mold. I spend as much time cleaning my grow space as I do tending plants. It’s no fun, but it’s necessary.
Ok, back to the meat and potatoes of the article: Your environmental conditions are clean and stable and, if you have pest problems, they are controlled. What else could be causing plant health problems and getting in the way of killer yields and frosty buds?
First, a list of common causes of problems, and then we’ll break them down one by one.
Under- or Over-fertilization
Faulty nutrient formula
When plants are looking sad, rule out these common root causes one by one. In order to do that most effectively, track settings and treatments used in some kind of handwritten log or plant management software. This is especially true if multiple growers are handling the same tasks from day to day. For example, it will be impossible to tell whether light shock is the problem if there’s no record of when the lights were last turned up and by how much.
Topics to include in a tracking/treatment log are pH/EC of applied mixture and runoff, nutrient recipes and application rates of each watering, quantity of water applied on what date, sprays applied (application rate and quantity), changes in light intensity, and environmental settings including data logging of temperature and humidity.
No matter what the cause of a plant health problem, an important first step to solving it is to turn down the lights. Whether or not overly bright light it is the root cause of the plant stress, dial the lights down while assessing the various possibilities. If the plant stress is modest, reduce the light intensity by about 30%. If it is severe, reduce it by half. This will take the photosynthetic demand off of the plants while you determine the problem/s and give them a chance to recover. Once they start to improve, slowly increase the light intensity again, one week at a time.
Clones need only very modest amounts of light, around 60 PPFD, to stay alive while developing roots. Many growers inadvertently try to push unrooted clones harder than that, causing light shock at the very beginning of the growth cycle. This is the most common phase of growth for lighting to be incorrectly adjusted.
As plants go into veg and flower, lights are turned up, but they must be turned up slowly in order to avoid shocking the plants. In my experience, it can take up to 5 days to know if an increased level of light intensity is being well received by the plants. By that time, if it turns out to be too much, the plants will become stunted and take a few weeks to bounce back, if they ever do. Therefore, it’s safe and well advised to increase light intensity in weekly increments of no more than 100-200 PPFD per week.
Overwatering was covered in last month’s article, but just to circle back for those readers who are just now tuning in, overwatering is a very common mistake made by growers, especially when plants are small and young, and it essentially drowns the roots due to deoxygenation. Damaged roots can’t take up nutrients properly, and the plants get stunted. Once this happens, a full recovery will take a few weeks. It’s common for me to see facilities with an unnecessarily long clone and veg times because of either too much light, too much water, or both.
Out of Range pH
Besides light and water imbalances, the next most common cause of unhealthy plants I encounter is pH mismanagement. Most growers have pH meters, but they aren’t always calibrated and kept in good condition and if they are, measurements are not usually taken frequently enough. I use a preemptive method of pH control by measuring my run-off every time I water. The plants might look great, but if I see the pH moving toward an extreme, I’ll know to modify the input measurements of my next watering to compensate for it.
I have worked with just about every type of cultivator, and I’ve found that soil growers are the least likely to manage pH properly. It seems there’s an assumption among organic growers that pH manages itself when the soil is amended properly, but this has not been my experience. Usually, soil pH drops over the course of the crop’s life, which leads to simultaneous mineral toxicities and deficiencies. It shows in the form of yellowing of the leaves from the outside edges inward. Soil performs best when balanced between 6.3-6.6 pH. When it drops below 6.0, iron and manganese toxicities will quickly develop. When it goes much above 6.7, you’ll see iron deficiency alongside excesses of calcium, magnesium, and potassium.
For growers using mineral fertilizers, pH drop is more commonly correlated with overfeeding. Most, but not all, mineral fertilizers are acidic and drop the pH when overapplied. Fortunately, PH Up and PH Down products used in the irrigation water work very well to keep the root zone within the ideal range of 5.7–6.4. It’s okay to apply irrigation water at settings of anywhere from 4.5–6.8 pH if that’s what is needed to balance the root zone conditions. For example, if the runoff one day is 5.3, and you want it to be 6.0, adjust the pH of the next watering to 6.7, assuming your EC is in range (more on EC in the next section). The 6.8 pH water will blend with the 5.3 pH root zone and balance out near your ideal. Monitor the runoff while applying the counterbalancing mixture so you’ll know when you are in the ballpark of the target pH.
The more you learn about pH dynamics, the crazier it gets. The nutrient ingredients are only one piece of the equation. PH is also heavily affected by plant uptake processes, the media, and microbial activity. In next month’s issue, we will be exploring the many dynamics of pH behavior in the root zone. If you aren’t that geeky, just monitor the pH of the runoff and adjust the water input to keep the root zone in range.
We all want fat and happy plants, and most growers show their love for their plants by giving them too much to eat. In commercial agriculture, the term for this is “High EC.” EC is an abbreviation for electro-conductivity and it is a crude measurement of the total amount of food in the irrigation water.
Below we will discuss how the EC breaks down into the 13 essential nutritive elements; but no matter what the EC is comprised of, plants can only handle so much exposure to soluble minerals. Above a certain threshold, the concentration of soluble substances in the water will cause health problems in the plant, much like drinking salt water would do to humans. It varies from one cultivar to the next, but most plants prefer an EC around 1–1.3 when they are young and 2–2.3 when they are older.
Over-fertilization is usually addressed alongside pH repair. The pH and EC of the runoff can be measured with each watering and both parameters adjusted for the next watering to improve the root zone conditions and get the plants back to a full growth rate. It is easily possible to ruin a crop by over-fertilizing it. A combination of too much total salt, along with the damage caused by individual mineral toxicities, can burn the plants, and they might never fully recover—so be very diligent with fertilizer measurement and application to ensure that this doesn’t happen.
It’s less common to encounter, but some growers underfeed their plants. They might have gotten bad advice on ideal EC ranges or think the media already has food in it. Again, this can be determined by measuring the EC of the runoff. Plants will appear generally pale when they are under-fertilized, and thankfully, they will recover quickly when the problem is corrected, within 2-3 days.
Faulty Nutrient Formula
If all of the parameters in the above sections of this article are in range, and your plants still aren’t booming, the nutrient formula is the next place to look for problems. The EC measurement of the feed you are using will tell you the total amount of conductive dissolved solids in the water, but not how that breaks down into the constituent minerals.
You might think that all cannabis nutrient formulas must have been designed to work reasonably well, but once you look under the hood, it turns out they can be pretty far from ideal. Whether using mineral salts or organics, the formula must contain the right quantities, ratios, and bioavailability (in the case of organics) of 13 different elements in order to thrive.
To find out what the EC is comprised of, have an agricultural testing lab analyze the mix. This usually costs around $100. Before testing the nutrient water, though, I recommend testing the leaves of the plants for mineral content. If the pH and EC of the root zone are in range, a leaf tissue test will accurately reflect the balance of minerals in the nutrient solution. In my experience, it’s rare for a branded formula to be perfectly matched to the crop. Leaf tissue lab reports usually show that some essential minerals are too high in the leaves and, at the same time, others are too low. In the case of finding low minerals, a formula can be supplemented to correct deficiencies. In the case of some minerals testing in range and others testing too high—it’s time to get a new formula.
Some cultivars are just genetically finicky, making them appear unhealthy when nothing is actually wrong. Why do we love Blue Dream? Because it’s vigorous and will pump out large quantities of stinky solid bud even when conditions are not ideal. On the other hand, some of the Cookies cultivars are great to smoke but difficult to grow. You might be able to play around with different light levels, pH and EC settings to dial them in, but I have rarely found that to be worth the time it takes.
For most growers who have more than plenty to manage throughout the day, dealing with sensitive cultivars is a real pain. They need to be monitored and repaired too much and don’t always deliver at harvest time. Fortunately, we have tons of varieties to choose from these days, with new ones coming out all the time. There’s no need to waste much time troubleshooting the problems exhibited by picky plants; it’s a better use of time to just move on to something that’s easier to work with.
Summing It Up
Growing commercial cannabis isn’t easy! There are so many environmental and root zone conditions that need to stay in balance to get the results you need. The main mantras to keep in mind are: keep your facility clean, go easy on light and water, keep your pH and EC in range, use a good nutrient formula and grow only the most vigorous varieties.
I love getting emails and feedback! If you have any questions or comments, please drop me a line at Jennifer@CannabisCultivationConsulting.com