Why Are My Cannabis Leaves Turning Yellow?

You've taken great care of your cannabis plants; you water them, feed them nutrient-rich foods, provide the ideal light cycle for their different developmental phases, and prune them just enough to promote light exposure and optimum growth in a marijuana grow tent. You've done everything right and yet your marijuana plants just don't look as lively as they should. Specifically, your cannabis plants are showing signs of stress through yellow leaves. Despite all the love and attention you've given them, they just aren't growing into the bountiful beauties you had hoped.

Fear not fellow cannabis cultivators! Just because your cannabis leaves are turning yellow does not mean they are done-for. In fact, there are steps you can take right now to correct yellow cannabis leaves and prevent it from occurring again. But first, let's discuss why cannabis leaves turn yellow in the first place

Yellow Marijuana Leaves? How to Obtain an Accurate Diagnosis

In scientific terms, the yellowing of leaves is called chlorosis. This is a process wherein plant leaves lose their chlorophyll (the compound that keeps them green).

Chlorosis may be the result of a specific issue (environmental problems, sunlight deficiency, etc), or it may simply be due to senescence – the process of change due to biological aging.

Natural senescence is nothing to worry about. In fact, at the end of each growing season, many plants in a crop will start to turn yellow and drop their leaves (think of deciduous trees during autumn). Some growers “flush” their crops with pure water or a flushing solution at this late stage in cultivation. This helps to get rid of any excess nutrients, and will speed up the natural process of senescence.

Of course, other factors may be responsible for the yellowing of your marijuana crop. Rarely, however, is chlorosis a cause for panic. Often times it’s something simple that is preventing photosynthesis. If you act quickly, you can generally help your plants recover without issue.

Let’s explore several diagnostic tools that you can consider to help figure out what’s causing the leaves on your cannabis plant to turn yellow.

Improper Watering Habits

When you over-water your cannabis plants you prevent the roots from getting oxygen. This will prevent your plants from metabolizing nutrients and cause yellow leaves.

The leaves of an over watered plant will look droopy because they are full of water.

Under watering your cannabis plants will cause the leaves to wilt and turn yellow over time.

The solution is simple: adjust your watering habits. If you see over-watering symptoms, water your plants less frequently and vice-versa.

The rule of thumb is to water your cannabis plants once the top inch of soil is dry. Cannabis plants like soil that dries out a little between watering. Finding the right balance will depend on the size of the plant, the size of the planter, and temperature.

“Water your cannabis plants once the top inch of soil is dry.”

The Fix: Stop watering over-watered plants and increase watering for under-watered ones. Sounds easy, but it’s one of the most common mistakes beginner growers make. Lift your containers if you can to get an idea of what they feel like when soaked and how much less they weigh when dry.

pH is Too High or Low

Another common issue is pH imbalance. If the pH of the water you feed your plants is off, they will have a hard time absorbing nutrients through their roots. pH is the measurement of acidity or alkalinity of a liquid, it’s determined by the concentration of hydrogen ions in the water.

In most cases, your tap water will have a pH around 7.0, but this will vary from location to location. Adding nutrients to your water can change the pH. Keep an eye on the pH of the water you use to feed your plants. You will get the best results from your cannabis grown in soil by maintaining a pH around 5.8 or 6. It makes sense when you realize the pH of rainwater is slightly acidic with a pH of around 6.0.

For hydroponics solutions you will want a slightly more acidic pH of around 5.5

By using a pH level measuring pen or kit you can monitor the acidity of what your feeding your plants. If your levels aren’t in the optimal range, add a base (pH up) or acid (pH down) to either raise or lower the pH.

Incorrect pH is the usual culprit for most cannabis related growing problems.

The Fix: Use a pH meter to measure the level of acidity or alkalinity of your soil and nutrient solution. Adjust using pH up or down accordingly. Bear in mind that these solutions come in concentrated form, so add them sparingly to raise or lower pH incrementally.

Light Burn

Are the tops of your plants turning yellow, crispy, and starting to curl? One reason could be light burn.

The leaves closet to the light source will be showing the most severe symptoms. The heat from your lights is frying these leaves damaging them.

Typically, the more powerful, high-intensity lights will result in light burn. LEDs usually give off the least amount of heat.

You will need to raise the lights a few inches to fix the problem. The optimal range is usually one or two feet away.

If your hand is uncomfortably hot where the light hits the leaves, then your leaves are also feeling the burn. The temperature at the canopy (uppermost leaves) shouldn’t go over 30°C (86°F).

The Fix: Increase the amount of light the plant is getting. This could mean lowering an existing grow light to the proper level above your plants’ canopy or investing in a stronger lighting unit. I highly recommend using HID (High Intensity Discharge) lighting, such as MH (Metal Halide) or HPS (High-Pressure Sodium) lighting for growing pot plants indoors. LED (Light Emitting Diodes) such as full spectrum LED grow light is best for plant growth and Compact Fluorescents are a decent, if not perfect, alternative if heat or power usage is an issue.

Cold Shock or Heat Stress

Next on the list is temperature. Normally, heat is going to be the enemy here, either from the lights or poor air circulation. If you’re growing outside or in your basement, a quick cold spell in the night could harm your plants as well.

For heat stress, the leaves will turn yellow and dry. At the other end, cold shocks, the plant will droop and turn yellow. Either extreme could kill your plants very quickly.

Keep a thermometer (or two) near your plants to check the temp. The ideal range is between 18°C and 28°C. Proper air circulation will keep the temperature balanced throughout the room and help the exchange of CO2 and oxygen.

The Fix:

Make sure the light source is the right distance away from your crop. You will need to monitor it and change it regularly as the plants grow. You should also invest in a thermometer. If it’s too hot, then add air conditioning or inline duct fans.

Plants that have experienced heat stress may need to be moved away from the light. Keep them at a further distance until they begin to recover.

Deficient Nutrients

If the pH is spot on and the plant is being watered correctly, then the most likely problem is nutrients (or the lack thereof). For chlorosis, the usual culprits are iron, nitrogen, magnesium, phosphorus, potassium, sulfur, or zinc.

Take note of where the yellowing occurs on the plant and the leaves. These are your clues to finding out what nutrients are deficient.

Using a fertilizer designed for cannabis is a good first start. Finding one that contains all the essential macro and micro nutrients cannabis needs is the easiest way to avoid deficiencies.

Nutritional needs may vary from strain to strain, but having those base line nutrients will save you a lot of trouble.

This is a reason why many growers love organic living soil. Soil contains all sorts of nutrients that are broken down by roots and microbes within the soil itself. With nutrient rich living soil you may even need to use less fertilizer.

The Fix: Water with a nutrient solution high in nitrogen. Plant food bottles typically display NPK ratio on the labels. N is for nitrogen, P for phosphorus and K for potassium. Choose the nutrient with a higher number at the beginning. They’re labels will say “Grow” or “Vegetative” as opposed to “Bloom” or “Flowering.” If you decide you have a lack of iron, foliar feed with chelated iron. You should see your leaves greening up within a few days.

The plant in the photo is showing signs of an iron deficiency (and a few others as well). It was caused by a pH that was too high which locked out several nutrients.

Using too much fertilizer can also cause problems that would result in yellow leaves with burnt looking tips. Fixing over fertilized cannabis plants is simple. Flushing the soil with water will remove the build up of nutrients in the soil.

Final Thoughts on Yellow Cannabis Leaves

Whenever you see yellow leaves on your marijuana plants, the first step is not to panic. Yellow leaves are not always a death sentence. Sometimes, the leaves turn yellow for no particular reason, even though there’s no harm to your plant.

In other circumstances, you may need to act to save your crop from complete ruin. If you spot any leaves that are severely wilted and frail, it’s time to defoliate (remove the affected leaves). Make sure you identify the underlying issue so you can address it.

Hopefully, this guide has provided you with some tips to help you save your beloved cannabis plants. Do you have any more tricks to rescue yellowing plants? Drop them in the comments below!