cannabis nitrogen toxicity

Nitrogen Deficiency vs. Nitrogen Toxicity in Your Weed

Nitrogen Deficiency vs. Nitrogen Toxicity in Your Weed

When it comes to a nutrient problem in your cannabis crop, you can do more harm than good if you don’t correctly identify the problem. Nitrogen is a critical component in the proper growth and function of marijuana. Understanding this macronutrient will equip you to grow high-quality cannabis while saving yourself stress if you encounter a deficiency or toxicity issue.

✅ Understanding Nitrogen’s Role in Cannabis

When I’ve gone to survey grow operations when I worked with a consultant agency, I found that the most common problem in marijuana gardens had to do with nitrogen. I can’t count the number of times that I’ve seen canopies as far as the eye can see that was a deep yellow. This nutrient is responsible for a host of functions within our beloved marijuana crop. Let’s take a look at what nitrogen is responsible for before we outline common problems.

First and foremost, nitrogen is a crucial component in the production of chlorophyll. Some of you may remember this term from biology class, so hopefully, this time you remember! Chlorophyll is the pigment that’s responsible for the plant’s green color. More importantly, it functions as a light-absorbing pigment that enables plants to photosynthesize the Sun’s rays into food.

Without this function, plants would not exist in their current form as they do today. This key adaption is what has driven plants all over the world.

Another function of nitrogen is its role in amino acids. Amino acids are the building blocks of proteins, and without these proteins, plants would not survive. Just like with our own bodies, we require proteins to function correctly, and it’s not different for plants.

All of the required nitrogen is taken up through either the root system or through foliar sprays. The form in which nitrogen is taken by the plant is either nitrate (NO3) or ammonium (NH4). These two forms are essential to remember, but even more important is to understand the dosage.

Remember, nitrogen is the main component for your plant to produce new leaves. Besides the root system, the cannabis canopy is crucial for the survival of your cannabis plant. The leaves are the engine, the light is the fuel, and the chlorophyll is combustion. Nitrogen allows for all of these critical components to happen, so it’s with the utmost importance that you take the time to understand and respect the role of nitrogen fully.

🎯 Feeding Cannabis Nitrogen

If you’ve grown weed, then you’ve seen all the cannabis nutrient products and their display of the macronutrients N-P-K. N stands for nitrogen, the other two are phosphorous and potassium, but we’ll focus on the other two in another series.

As your cannabis plant begins to grow and uptake nutrients, the available supply of nitrogen will decrease. This means that you’ll need to feed your cannabis plant nitrogen continually. Of course, the type of nitrogen you feed your plant is dependant on the method in which you’re growing.

The most straightforward method is to buy nutrients that are considered “safe.” This means that buying a bottle of plant fertilizer that consists of an N-P-K of 10-10-10 is generally a safe standard. The reason for this is because all marijuana strains have different nutrient needs, and a gentle 10-10-10 ratio is best for starting out.

Nitrogen is quickly absorbed through cannabis, and you’ll notice fairly quickly if you gave too little or too much. Let’s take a look at identifying problems related to nitrogen.

👾 Nitrogen Deficiency

Nitrogen deficiency has to be one of the most common problems that growers face. Suddenly your marijuana plant is turning yellow. You don’t understand why the lower leaves are beginning to wither and die off, but you do see it as a problem. Before you go throwing more plant nutrient, stop and identify the problem.

It’s common that growers do more harm than good when they blindly start adding things to their cannabis crop without first identifying the issue at hand.

The biggest giveaway for nitrogen deficiency is the yellowing of lower leaves. The reason this occurs is that the plant has run out of available nitrogen to uptake. This causes higher leaves that are closer to the light to steal the lower leaves’ nitrogen. If left alone, the lower leaves will eventually fall off, and the problem will continue from bottom to top until the plant cannibalizes itself in search for nitrogen.

You’ll know if your plant is recovering if the yellowing doesn’t continue and leaves begin to regain their healthy green shade. Light green is always a sign that a nitrogen deficiency is close to happening.

The worst time for a nitrogen deficiency is at the beginning of the vegetative stage, where the plant is focusing on growing foliage that will help once the flowering stage begins. Alternatively, it’s at the end of flowering that growers tend to intentionally create a nitrogen deficiency to rid the plant of chlorophyll.

Now that you’ve identified the problem, increased the dosage of plant nutrient, you should wait a few days to confirm if your plant is recovering. Remember, patience is half the battle when it comes to growing marijuana.

The Only Time You Want a Nitrogen Deficiency

There is, in fact, a time when you actually do want a nitrogen deficiency. The only time you’ll ever want this as a marijuana grower is during the end of the flowering stage. Depending on the strain, you want your cannabis leaves to turn yellow around week 8-10 of flowering.

But why? This is a situation that is very specific to growing marijuana because it all has to do with the end product. You guessed it- it has to do with the buds that we’ll eventually smoke after the drying and curing process.

Chlorophyll as we learned, is the necessary pigment in absorbing light. Near the end of the flowering process, we’re going to sadly kill our plants to harvest them. The issue with chlorophyll is that this pigment as contains a smell similar to hay or dry grass, which we taste when smoking the finished product

Lack or excess of nitrogen in your marijuana

The easiest way to get rid of this smell and taste is by flushing out as much of the chlorophyll as possible. Growers will usually start only watering their plant with small amounts of nutrient to force a nitrogen deficiency. This is extremely apparent when you look at pictures of growers harvesting their plants. What you’ll see is large buds and most of the leaves a bright yellow color.

This isn’t an essential step because you can also get rid of excessive chlorophyll during the drying process. All you’ll need to do is leave your plants in complete darkness for a few days to speed up the dissipation of chlorophyll from the leaves.

⚠️ Nitrogen Toxicity

The opposite of a nitrogen deficiency is when we see nitrogen toxicity. This is equally as common as nitrogen deficiency because new growers tend to overdose their plants with nitrogen. It’s a fact that new growers kill their cannabis plants by loving them too much.

It’s common that growers cause nitrogen toxicity by following even the recommended dose found on their nutrient product. The recommended dose offered by nutrient companies is just a general amount because some strains will need either more or less. Once again, no strain is created equal, and this means that each strain you grow will more than likely require varying dosages of nutrient.

When encountering a nitrogen toxicity issue, you’ll see a deep shade of green on the leaves of your marijuana plant. The deep shade of green can worsen and even take on a slight deep blue hue in extreme cases. This is a very noticeable symptom and is therefore easy to identify and fix before real damage occurs to your cannabis plant..

Another noticeable symptom of nitrogen toxicity is called the “claw.” The ends of the fan leaves will point down, similar to the way a bird claw looks. This symptom is also present when overwatering cannabis plants, but if you’re not overwatering, then this should lead you to believe that you’re amid a nitrogen toxicity issue.

To add to the list of symptoms is also leaf tip burn. When too much nitrogen is added, the plant tries to get rid of it through the ends of the leaves. This results in the leaf tips looking necrotic and seemingly burned.

Nitrogen: excess vs. lack

If nitrogen toxicity is allowed to continue,then the plant will cease many necessary functions to sustain vigorous growth. You should never let a toxicity issue to gain a foothold on your plants because it will result in a more extended recovery period.

It’s dangerous to suddenly stop giving nutrients when noticing a nitrogen toxicity issue because the plant is still up taking the other necessary nutrients. If you stop giving nutrients, you may find yourself in the exact opposite situation: a nitrogen deficiency!.

Small steps are vital in not creating a bigger problem. Adding or decreasing nutrients will take a few days to show results.

🔥 The Role of pH and Nitrogen Issues

As we all know, there are a host of problems that can go wrong while we’re growing our marijuana crops. A major complicating factor for nitrogen toxicity or deficiency is when it’s accompanied by pH issues.

Depending on the medium that we’re growing in, pH plays a vital role in the health of our plants. This is extremely apparent when growing in hydro or aeroponic systems. These systems don’t have the natural buffer found in soil-grown plants. Cannabis grown in soilless systems rely on their caretakers to set the pH to a correct level that allows the plants to take up plant nutrient effectively.

If you don’t set the pH to a correct level (usually 5-6pH), then you’ll compound your problems as you see deficiencies or toxicities starting to mount up. The root zone doesn’t enjoy the wrong pH and will stop taking up nitrogen, thus creating a nitrogen deficiency.

📖 Conclusion

These are just a few examples of the intricacies of growing marijuana. Most aspects of the plant are interconnected, which leads to one problem affecting another portion of the plant.

For marijuana plants to develop smoothly, the pH and dosage must be continually monitored.

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Nitrogen Toxicity

Problem: Dark green leaves, shiny leaves, clawing, weak stems, and overall slow growth. Marijuana leaves that are nitrogen toxic often get “The Claw” or talon-like leaves that are bent at the ends. They also do an odd curving (or cupping) that is often mistaken for overwatering, but is unique to nitrogen toxicity. You can see a “clawing” leaf in the pictures below (click each picture for a close-up).

Leaves that turn into claws often start turning yellow and dying if the nitrogen toxicity is not treated, much like a nitrogen deficiency, only the leaves will continue to get more and more clawed. Leaves eventually turn yellow or brown and fall off. You can tell if yellowing is caused by too much nitrogen because the rest of the plant will be dark green, and the yellowing leaves will turn into claws first.

The majority of times that growers encounter problems with nitrogen, it’s from giving too much of it to their plants.

Many new growers accidentally give their plants give too much Nitrogen, especially in the flowering stage. This results in dark, shiny, clawing leaves.

Your plant needs a lot of nitrogen in the vegetative stage, and it’s generally hard to give too much as long as you’re not going completely overboard with nutrients. Nitrogen is a big part of what makes leaves green, and is incredibly important to the process of photosynthesis (making energy from light).

But cannabis plants need relatively low levels of Nitrogen in the second half of the flowering/budding stage. While your plants still need N (nitrogen) during flowering, too much N at this stage will prevent your plants from forming buds properly, resulting in lower yields, less potency and possibly inferior buds.

This is why it’s important to avoid any type of “time-release” nutrients or soil (for example, standard Miracle-Gro soil) as they will keep giving your plant a lot of N even after its started flowering.

When it comes to nitrogen, this is what your plant needs:

Vegetative Stage – higher levels of Nitrogen (pretty much any plant food will do)

Most complete plant foods that you get at a gardening store contain high levels of nitrogen (N). These nutrient systems tend to work well in the vegetative stage.

Some examples of cannabis-friendly one-part Vegetative nutrient systems…

Pretty much any complete plant food

Flowering Stage – lower levels of Nitrogen (use “Bloom” or Cactus nutrients)

It’s extra important to find a nutrient system with lower levels of nitrogen for the last part of your plant’s life. Many “Bloom” or “Flowering” style base nutrients are just the ticket.

Some examples of good one-part Flowering nutrient systems…

If you can’t order online and can’t find a good one-part base Bloom formula locally, you do have other choices. Though not an ideal choice, most Cactus plant foods will contain good nutrient ratios for growing cannabis during the budding stage. So in a pinch, you can use the cactus nutrients that can be found at most gardening stores.

Different strains react differently to nitrogen toxicity. Some plants get dark green leaves with no clawing. Some strains will get leaves that do the weird 90 degree bend at the tips, while other strains or individual plants start curling like claws and then turn yellow / brown and fall off like a deficiency. Yet these are all signs of too much nitrogen.

Signs of Nitrogen Toxicity

  • Dark green leaves and foliage
  • Leaf tips may turn down, without signs of overwatering.
  • You may notice yellowing on the affected leaves or other signs of nutrient deficiencies as time goes on
  • Nitrogen toxicity is often but not always accompanied by nutrient burn
  • The Claw often seems random, affecting leaves here and there
  • Heat and pH problems will make the clawing worse, as they stress out the plant and lower her defenses, and cause her to drink more water (and uptake more N)
  • As time goes on, the claw leaves will eventually start turning yellow, getting spots, and dying

This cannabis seedling is dark because it was underwatered in a “hot” soil mix (too much Nitrogen), but after watering the plant as normal for a week or two, the plant started growing vigorously

Solution: Reduce the Nitrogen your plant is getting!

Reduce the amount of nitrogen that is being fed to the plants. If you are feeding extra nutrients, cut down that amount. If you are in the flowering / budding stage, make sure you’re using a formula that’s specifically meant for flowering, or else it could have too much nitrogen.

If you are not feeding extra nutrients, you may have “hot” soil that has been giving your plants extra nutrients. In that case, flush your plants with filtered, pH’ed water to help clear out the extra nitrogen.

Effected leaves likely won’t recover, but you should see the problem halt with no new leaves being affected.

Wait! I’m not sure if it’s Nitrogen toxicity!

Ok, you ruled out overwatering, now what?

When I first got started growing, everyone kept telling me that this particular kind of leaf clawing was caused by under or overwatering my plants, pH problems, or heat problems.

Yet in my case, I knew that it wasn’t over or under watering (I was growing in hydro, where roots grow directly in water and air stones are constantly adding oxygen). I knew it wasn’t pH (my reservoir water had the right pH) and I knew it wasn’t heat since the grow area was slightly cooler than room temperature.

So then what was really causing my claw leaves?

It’s understandable that other growers were mistaken. It is true that many stresses will make any other problem worse.

Plus overwatering can cause a similar kind of leaf clawing (learn more below). And if you do have nitrogen toxicity, than heat or pH problems will make the problem much worse.

Now, you may or may not know that marijuana (or any plant) needs an element known as “Nitrogen” to grow.

In fact, nitrogen is one of the 3 nutrients that are included in almost every kind of plant food.

When looking at plant nutrients, you’ll almost always see 3 numbers listed, like 3-12-6 or 5-10-5. These numbers represent the ratio of Nitrogen (N), Phosphorous (P) and Potassium (K) contained in the bottle. Just about all plant life on Earth needs these 3 elements to grow.

The very first number, “3” in the case of the picture to the right, always displays the proportion of nitrogen in this nutrient bottle compared to the other 2 nutrients (Phosphorus and Potassium respectively).

The reason nitrogen is in all plant nutrient formulations is because it’s vital to plant processes.

For marijuana plants, when they don’t get enough nitrogen, the bottom leaves start turning yellow and dying. Left unchecked, a nitrogen deficiency can cause the whole plant to eventually die.

However, this time we’re the dealing with the opposite problem: nitrogen toxicity, or too much nitrogen.

Why You Should Treat And Prevent Nitrogen Toxicity

  • Marijuana plants that get too much Nitrogen in the vegetative stage don’t grow as vigorously.
  • Too much nitrogen is especially harmful in the flowering stage, because this will cause your plant to produce much smaller buds.
  • If you react quickly and reduce your nitrogen levels at the first sign of toxicity, your plant will quickly recover.

Note: Some strains with the word “Claw” in the name tend to do The Claw more easily than others.

Problems with excess nitrogen are not common in the wild; it’s a lot more common to see nitrogen toxicity on indoor plants, especially when overzealous growers go overboard with nutrients.

Occasionally you’ll come across a strain or particular plant that likes lower levels of nutrients, and when this happens, it’s important to realize the plant is showing signs of toxicity, even if all the other plants in your garden seem fine.

One of the most common signs off too-many-nutrients is “nutrient burn,” or when the tips of your leaf appear brown or burned. Yet there are specific signals your plant will display when she’s getting too much nitrogen…

Recap: How You Know You Have a Nitrogen Toxicity

  • Dark green leaves and foliage
  • Leaf tips turn down, without signs of overwatering.
  • You may notice yellowing on the affected leaves or other signs of nutrient deficiencies as time goes on
  • Nitrogen toxicity is often but not always accompanied by nutrient burn
  • The Claw often seems random, affecting leaves here and there
  • Heat and pH problems will make the clawing worse, as they stress out the plant and lower her defenses, and cause her to drink more water (and uptake more N)
  • As time goes on, the claw leaves will eventually start turning yellow, getting spots, and dying

Light and “The Claw”

  • The distance between the leaves to the lights or irregular light patterns from reflectors often seem to affect the condition, which is why many growers believe that light is somehow causing the problem.
  • You may notice this clawing first appears on dark green leaves that aren’t getting enough light (they aren’t able to use up all their nitrogen and become nitrogen toxic).

The Claw in the Flowering Stage

  • If you use vegetative plant nutrients during the flowering stage, then they’ll deliver too much nitrogen. This is why you need to get special nutrients meant for the blooming / flowering stage. You’ll notice that flowering nutrients always contain a smaller percentage of nitrogen (the first number) compared to nutrients for the vegetative stage. Learn more about marijuana nutrients here.
  • Many growers mistakenly keep raising nutrient levels or adding additional nitrogen when they see yellow leaves in the flowering stage, not realizing that it’s natural for plant leaves to start yellowing as harvest approaches. Adding too much nitrogen in the flowering stage can cause nitrogen toxicity even when you can see yellow lower leaves. Nitrogen toxicity in flowering results in smaller yields and airy cannabis buds, so make sure to watch out!

Note: During the last few weeks before harvest, marijuana plants starts pulling all the remaining nitrogen from her leaves as part of the bud-making process. This causes yellowing leaves starting towards the bottom of the plant. This is part of the natural flowering process and you don’t need to fight it. You may notice that marijuana leaves are yellowing in almost all pictures of marijuana plants with big buds that are close to harvest. You tend to get smaller yields at harvest from nitrogen-toxic plants with dark green leaves.

It’s Normal For Marijuana Leaves To Start Turning Yellow As Harvest Time Approaches, Don’t Keep Adding More Nitrogen!

I know a lot of marijuana plant problems can look similar, but now that you’re armed with the right information, you’ll know exactly what to do if you see Nitrogen Toxicity affecting your marijuana plants.

Plant Symptoms

  • Bronze or brown patches
  • Brown or slimy roots
  • Brown or yellow leaf tips/edges
  • Buds dying
  • Buds look odd
  • Bugs are visible
  • Curling or clawing leaves
  • Dark leaves
  • Drooping plant
  • Holes in leaves
  • Mold or powder
  • Pink or purple on leaves
  • Red stems
  • Shiny or smooth leaves
  • Spots or markings
  • Twisted growth
  • Webbing
  • Wilting leaves
  • Yellow between leaf veins
  • Yellow leaves

This page is part of our Plant Doctor series. You can use our tool to filter by symptom and help diagnose your plant.

Does your plant have "the claw?" The talon-like leaves that are bent at the ends are a sign that your plant may have nitrogen toxicity. Learn how to fix it.