Yellow cannabis leaves: symptoms, problems, and fixes
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Yellow cannabis leaves aren't pretty. Aside from reducing the aesthetics of a plant, they're usually a sign that something is off. It's almost like a form of communication between plant and cultivator. Use this information to learn the causes of yellow leaves, and how to fix it.
Yellow leaves are a sight that often induces panic in cannabis growers. The culprit behind the discolouration often varies. It could be a nutritional deficiency, a sign of burning, or merely a symptom of the flushing process. In this article, we'll discuss what causes yellow leaves and how to remedy the issue.
Do yellow leaves mean your plant is unwell? Maybe it's dying? Sure, something isn't right; but don't worry, your plants are merely trying to tell you something. Yellow leaves can have multiple causes. Growers can often fix the underlying issues with a few tweaks of their setup. Rather than seeing yellow leaves as a threat, view them as your plant's method of communication—a botanical language of sorts.
Before we delve into this, let's quickly run over some cannabis anatomy to make things a bit more clear.
THE TWO TYPES OF CANNABIS LEAVES
During the vegetative phase of the growing cycle, cannabis plants only produce one type of leaf, unless you count the two cotyledon leaves at the start of the seedling stage. The green foliage put out by plants during this stage are the fan leaves, which feature up to 11 fingers depending on where they are located. Indica-leaning cultivars produce broad-fingered leaves, whereas the fingers of sativa-dominant strains are much more slender.
Fan leaves are essentially biological solar panels. Their green colour is the result of the pigment chlorophyll—a molecule that absorbs light. Together with carbon dioxide, water, and light, energy is harnessed to create glucose and oxygen.
Fan leaves don't feature many cannabinoids—the active constituents in cannabis flowers. However, many growers put them to use by making juices or teas. They also make an excellent addition to the compost pile.
A second type of leaf occurs during the flowering phase of the growing cycle. These smaller leaves grow out of the flowers themselves. They present as spiky, tapered structures that protrude from the buds. Unlike fan leaves, sugar leaves boast a good amount of cannabinoids. Upon inspection, you'll notice a layer of sparkling trichomes on the surface of these structures. Cultivators often use them to make hash or cannabutter.
Now that we know the distinction between fan leaves and sugar leaves, let's explore why fan leaves sometimes turn from a luscious green to a dull yellow. We'll discuss the most common causes of yellow leaves and how to remedy them.
Overwatering is a prevalent error, especially among novice cultivators. The desire to continually meet every need of a plant can lead to overdoing things. Care turns into smothering, and growers often end up unwillingly drowning their plants.
Overwatering saturates the soil, leading to a reduction of oxygen, which can starve the roots. Eventually, they die and begin to rot away. Plants use their roots to uptake nutrients from the soil. As their roots die off, plants have access to much fewer nutrients, which can lead to deficiencies. Yellow leaves are a common occurrence when plants aren't getting the minerals and elements they need to thrive.
HOW TO FIX IT
The solution to overwatering is simple: water your plants less. Cannabis requires less water than you think. A general rule of thumb is to allow the top inch of soil to dry out before adding any more water to your containers. Verify this by pushing a skewer into the soil, or use your fingers.
Underwatering is another cause of yellow leaves. If you go too far the other way, your plants will suffer the consequences—a lack of water results in yellow leaves via numerous mechanisms. Water potential is used to pull water up into the stem and leaves during the process of transpiration. It's also vital in the creation of energy during photosynthesis. It's no wonder that leaves begin to wilt and turn a sorry shade of yellow when this key molecule is missing.
The tips of leaves will turn downward and develop a claw shape. Eventually, they'll keel over and perish if growers don't intervene.
HOW TO FIX IT
Again, the solution is simple: water your babies! Use the same rule of thumb to avoid overwatering. Allow the top inch of soil to dry out. As soon as this happens, water the soil again.
Cannabis is a hardy plant, but it can only take so much abuse. Light is its source of life, but it can also cause yellow leaves if administered incorrectly. Indoors, lights need to be positioned close enough to provide maximum growth, but far enough so as not to burn the leaves.
When plants grow too close to their light source, they can become subject to excess heat. Eventually, the leaves closest to the light will yellow and wilt. These leaves are essential in the growth of the plant and need to be kept healthy. Growers might also notice these leaves curling up or down, or even folding inward.
HOW TO FIX IT
To avoid heat stress, you need to monitor your crop closely. As plants grow, they'll slowly edge closer and closer to the light source. You'll need to regularly change the position of your light source to maintain the appropriate distance from the canopy. You can also take measures to cool down your grow room during heat waves. Add more fans to the room, or even temporary air conditioning.
If heat stress has already set in, distance your lights away from the tops of your plants. Wait for the situation to normalise before slowly moving the source closer to the canopy again. Do so a few centimetres at a time to avoid reproducing the symptoms.
As you may recall from science class, pH stands for "potential of hydrogen". pH is a logarithmic scale that ranges from 1–14. Measures between 1–6 indicate acidity, 7 indicates neutral, and 8–14 indicate alkalinity. So, what does this have to do with yellow leaves?
Well, cannabis can only uptake nutrients when the soil features a slightly acidic pH of 5.8–6.8. If the growing medium is too acidic or alkaline, plants will begin to starve. Eventually, nutrient deficiencies will take hold and fan leaves will start to become yellow in appearance.
HOW TO FIX IT
If you believe pH is causing your cannabis leaves to turn yellow, it's worth testing your soil. Samples can be sent to a lab to obtain extremely accurate readings. However, you can also conduct reliable tests at home. pH kits are cheap and easy to use. Follow the instructions, and you'll see results in no time.
The downside to home kits is that they don't test for the presence of calcium carbonate—an alkaline substance with a pH of 9.9. To check for this at home, add a soil sample to a cup of vinegar. If the solution fizzes, calcium carbonate is present.
If your test reveals the soil pH to be too high or low, there are steps you can take to normalise it. If it’s too high, add sulphur to the soil at a quantity of 90g per square metre of soil. Sulphur takes a while to change the soil pH, but it will ensure success in future crops. Aluminium sulphate is a quicker option. However, it can lead to toxicity in the soil.
If your test indicates a pH that is too low, you can raise it by adding lime and wood ash to the soil. Of course, there are also pH up/down products specially made to solve these very issues.
LACK OF NUTRIENTS
Most of the mechanisms described above have their roots in nutrient deficiency. Incorrect pH and over/underwatering prevent plants from tapping into nutrients that are already there. However, sometimes nutrient levels can be low, or missing from the soil altogether.
Laboratory analysis is required to obtain an accurate insight into the nutrient levels in your soil, although growers can attempt to identify deficiencies by searching for specific signs and symptoms.
Several nutrient deficiencies produce yellow leaves. Let's explore their characteristics below.
Nitrogen is a vital nutrient for all plants. The mineral element is a significant component of the chlorophyll molecule. Nitrogen also forms amino acids—the building blocks of proteins—and is used to build DNA.
Nitrogen deficiency often manifests as yellowing leaves, especially in mature leaves at the base of plants. Plants tend to move available nitrogen to the new leaves receiving the most light, causing older leaves to turn yellow first during deficiency. Other signs that nitrogen deficiency is present include decreased bud sites and early flowering.
Potassium is another critical macronutrient within plants. The mineral is involved in photosynthesis and regulates the opening and closing of stomata, as well as the synthesis of ATP (the molecular unit of energy).
Yellow leaves are one of the significant symptoms of a potassium deficiency. Other signs include brown spots on fan leaves, orange-brown leaf tips, and the drying and curling of leaves.
In plants, calcium is responsible for holding cell walls together. A deficiency in this mineral is terrible news for the structural integrity of a plant. Symptoms include distorted growth of new structures like root tips and young leaves.
Eventually, the lower leaves will begin to curl. Fan leaves won't turn entirely yellow, but will exhibit brownish-yellow spots.
Magnesium is a micronutrient that plays several essential roles in plants. It's intimately involved in photosynthesis—the mineral is the centrepiece of the chlorophyll molecule. Magnesium also helps metabolise carbohydrates and stabilise cell membranes.
When plants receive less than adequate levels of magnesium, the veins in their fan leaves will become yellow. However, this won't occur until approximately four weeks after the deficiency has set in. Leaves will also start to curl and die off.
Sulphur is a micronutrient within plants; it's only needed in small quantities to form important enzymes and plant proteins. However, if plants don't get access to these low volumes, things can quickly turn ugly. These symptoms are often the result of high soil pH.
Fan leaves will yellow, experience stunted growth, and become dry and delicate.
Zinc makes carbohydrate and protein synthesis possible, as well as the creation of chlorophyll. It's also involved in growth hormone production and internode elongation.
Zinc deficiency manifests as wrinkled and distorted leaves that often turn 90 degrees sideways. Leaf veins will also become yellow in appearance.
Plants need molybdenum to form two key enzymes that convert nitrate into nitrite, and then nitrite into ammonia. Plants then use ammonia to create amino acids.
Molybdenum deficiency causes older leaves to turn yellow, twist up, and eventually drop.
Iron is a major building block of key plant molecules, and plays a role in energy production. Iron is not a constituent of the chlorophyll molecule, but is essential in its synthesis.
Iron deficiency presents a specific yellowing pattern. Young leaves are the first to be affected. The veins at their bases start to yellow, and eventually, the same colouration spreads to the veins of older fan leaves.
Manganese contributes to photosynthesis, respiration, and nitrogen assimilation. The nutrient also plays a part in plant protection. Cannabis uses the mineral to combat root system pathogens.
A lack of manganese results in older leaves turning yellow.
REMEDYING NUTRIENT DEFICIENCIES
Considering that soil pH is within an optimal range, growers can fix yellow leaves caused by nutrient deficiencies by adding the required minerals to the soil. After identifying which deficiency you're dealing with, proceed to add it to the growing medium.
Specific products can be used to add readily available nutrients to the soil. For long-term remediation, add organic matter such as high-quality compost. Adding beneficial microorganisms such as mycorrhizal fungi and bacteria to the soil will help break down this organic matter and make it bioavailable to your cannabis plants.
TO DEFOLIATE OR NOT TO DEFOLIATE?
Defoliation is a term that refers to removing fan leaves. Defoliation is used to manage plants and expose flowers to more light. Removing all of the yellow leaves on your plants won't resolve their issues. It's a sign that your plant is experiencing difficulty, so cultivators must address the underlying issues.
That being said, growers can defoliate yellow leaves to a certain extent. Severely wilted leaves are beyond help—defoliate at will. However, cultivators should save leaves that only exhibit early signs of yellowing.
WHEN NOT TO WORRY ABOUT YELLOW LEAVES
Yellow leaves aren't always a bad sign. Sometimes they can be a natural phenomenon, or intentionally triggered by growers.
The youngest leaves at the bottom of plants close to full maturity will turn yellow and die off—don't worry! Almost all fan leaves will start to become yellow as harvest time approaches. During this period, growers stop feeding their plants in a process known as flushing. Flushing encourages flowers that produce a much smoother smoke. Yellow leaves are a good sign in this context!