Why are plant leaves turning yellow?

Are some or all of your cannabis leaves turning yellow? Maybe your leaves also have other symptoms like spots, curling, wilting, brown patches, etc. Marijuana plants may get yellowing leaves for several different reasons, so it can be hard to figure out the true root of the problem!

The first thing, however, is to diagnose the problem correctly. Cannabis leaves can become yellow for a wide range of reasons, but you can’t solve the underlying problem until you know the cause.

Here is a run-through of several common reasons why the leaves on your marijuana plants might be yellowing, as well as potential solutions to help you fix it.

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.

Light Deficiency

Lighting can be a complicated matter, especially for new growers. First, you have to establish the correct type of lighting, and then figure out an optimal distance from your plants.

Lights are crucial because your plants need them for photosynthesis. It is especially important for seedlings to get the right amount of light to encourage them to grow. Young cannabis plants with a light deficiency are likely to turn yellow, whereas mature plants may darken if they’re not getting enough lights.

Young cannabis plants with a light deficiency will turn yellow, while more mature plants will darken.

To help determine if a light deficiency may be your problem, check nodal spacing (stretches of stem between the leaves) of the newer/smaller leaves on your plants. If there’s more than one leaf-width of space between leaves, inadequate light may be the culprit (check out a complete beginner’s guide on the proper lighting for cannabis plants).

The Fix

Strong lights, such as HID (high-intensity discharge) bulbs, are better for cannabis seedlings. Invest in metal halide (MH) or high-pressure sodium (HPS) lights for best results. In a smaller grow room where heating becomes an issue, LEDs are a decent substitute. Also, you may consider moving the lights lower so that they sit closer to your plants. You’ll need to move them as the plant grows taller, though.

Watering Issues

Both over and under-watering cannabis plants can lead to chlorosis. Unfortunately, water is a tricky thing to get right. You need to water your crop the perfect amount – and at the right times.

Too much water can reduce the oxygen content of the soil, which starves the root system and leads to death. You will know if you have over-watered your plants because their leaves will be swollen, droopy, and yes – maybe even yellow.

Under-watering is less common, since most cultivators are hyper-aware that their crop will die without proper hydration. However, under-watering does happen, and under-watered plants may appear weak, thin, brittle, and yellow.

The Fix:

The solution here is simple: stop watering plants that are over-watered, and water under-watered crops more! Get to know your plants well, and get to know the environment they thrive in. Get a feel for the weight of the growing medium when it’s dry, as well as when it’s saturated. You can even invest in a humidity meter to view the water content of your soil more accurately.

pH Imbalance

pH stands for potential hydrogen, and measures the acidity of a specific medium. pH works on a scale of 1-14, with 7 being neutral. For cannabis plants, the pH of your soil should be between 6 and 7 (hydroponic growers should opt for 5.5-6.2).

If the pH isn’t right, your plants won’t be able to obtain the correct nutrients, and leaves may begin to yellow. (We cover nutrient deficiencies in more detail later, but generally, you should note that pH imbalance can cause a variety of symptoms). Yellow leaves, burning at the edges, and marks like spots and stripes are all indications of a pH imbalance.

The Fix:

You can buy a pH tester online to check the quality of your growing medium. Home kits, however, don’t always test for calcium carbonate, which can make your soil heavy in alkaline materials. You can check for its presence by adding a soil sample to a cup of vinegar; if it fizzes, there’s calcium carbonate. Alternatively, send a sample off to a lab.

Once you know the problem, you can either re-pot in a new growing medium, or adjust the soil pH. Certain substances are known to change pH. For example, wood ash and lime can increase it. However, it’s probably a lot easier to purchase pH UP and pH DOWN substances from a gardening store.

Heat Stress

Heat stress can cause yellow, burnt leaves if plants are kept too close to a light source. Some leaves may also begin to curl.

For lack of a better explanation, this is because marijuana plants (as is the case with most plants) are susceptible to temperature extremes. And again, this often comes down to a lighting issue. Bulbs give off heat while they provide light; put your plants too close to a bulb(s), and you run the risk of frying them.

A quick way to tell whether heat stress is an issue is to use the hand test. Place your hand by your plants and hold it there for thirty seconds. If it’s too hot for your hand, then it’s too hot for your cannabis!

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 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.

Nutrient Deficiencies

Nutrient deficiencies are a serious concern for all cannabis growers, but the last thing you want is for your plant to die on account of one – after all, it’s something that’s so easy to fix!

Nutrient deficiencies often end up causing yellow leaves. However, finding an accurate profile of your soil content could require sending it off for analysis. To help you figure out what your crop is lacking, here are some common nutrient deficiencies:

Nitrogen

As one of the most essential plant nutrients, nitrogen is vital in the production of chlorophyll. As a result, lack of nitrogen will result in yellow leaves. Typically, yellow coloring will appear in older leaves first. Other symptoms include early flowering and fewer buds.

Potassium

Potassium is another vital nutrient. Lack of it will leave to yellow leaves with brown spots on the fan leaves. Some leaf tips may also turn brown and dry out. Curling is also common with a lack of potassium.

Calcium

Calcium binds the cell walls together in plants, making it an essential element for plant structure. Lack of calcium can lead to stunted growth in new structures, and the root tips and young leaves may also grow distorted.

Eventually, the bottom leaves will curl and develop yellow-brown spots.

Magnesium

Magnesium is another element that helps to form chlorophyll. Less magnesium will cause yellow veins in the fan leaves, which eventually curl up and die.

The troubling thing about lack of magnesium is that symptoms only appear about four weeks after the plant starts to experience a deficiency. Try to keep magnesium levels up throughout growth.

Sulphur

Cannabis plants only need a small amount of sulphur, but a deficiency can still be devastating. High soil pH is often the cause. A sulphur deficiency manifests in yellow fan leaves with stunted growth; they also become frail.

Zinc

This mineral plays a key role in the production of chlorophyll, among other things. You can spot a zinc deficiency due to wrinkling of the leaves. Often, the leaves will also rotate 90 degrees to the side. The yellow coloration will appear in the leave’s veins.

Iron

Although iron is not a molecule within chlorophyll, it is necessary to produce it. An iron deficiency is easy to spot because young leaves will turn yellow. Eventually, this off-coloring will spread to the veins in older leaves.

Manganese

Manganese deficiency can lead to issues within a plant’s root system. You can detect it because the older leaves will turn yellow.

The Fix:

After identifying the nutrient(s) your plant needs, increase levels by adding it to your soil or growth medium. Try to do this slowly to avoid nutrient shock.

Pests and Insects

Finally, yellow leaves on marijuana plants can occur due to bites from an insect infestation. The exact symptoms depend on the specific pests responsible. However, fungus gnats are a common culprit in triggering chlorosis.

These troublesome insects are likely to appear when over-watering your plant. The gnats won’t eat your plant directly, but the larvae feed on the roots, eventually leading to yellowing of the leaves.

The Fix:

Kill off insects naturally using measures such as diatomaceous earth, neem oil, and other natural insecticides.

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!