How to Treat Iron Deficiency in Cannabis?

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Added 24 January 2023

Ironically, your plant does not need a lot of iron, but it can suffer from serious consequences if it doesn’t get enough of it. Pun intended. 

To be honest, it’s not a problem you’d joke about, especially if your plants have experienced it before. In the early stages, iron deficiency affects the plant slowly and does not look like a serious threat.

But give it enough time and it will slow down your plant’s growth, reduce its yield, and in some cases, even kill it. 

Fortunately, iron deficiency is easy to fix (although it is a little tricky to identify). Learn all about fixing iron deficiency in cannabis in this guide.

How Does Iron Deficiency Affect Cannabis Plants?

How Does Iron Deficiency Affect Cannabis Plants?

Image Credit - DonWild

Iron is crucial for the cannabis plant’s good health and yield, and the plant typically absorbs it in the form of ferric ions (Fe++) and sometimes organically. But since it is a micronutrient, only a small amount of it is required by the cannabis plant.

For instance, an acre of land would only need 1.5 pounds of iron, whereas the same area would need up to 200 pounds of nitrogen. 

One of the primary roles of iron is in the formation of chlorophyll — the component that converts sunlight into energy for the plant. Iron is also helpful for chlorophyll’s structure and chloroplast maintenance. Apart from this, iron manages various other functions like aiding in enzyme production, nitrogen uptake, etc.

This is why iron is crucial for the cannabis plant — without it, your cannabis plant won’t be able to produce and store energy, and it will fail to absorb essential macronutrients like nitrogen. Eventually, your plant will stop growing, become weak and riddled with diseases, and grow poor buds. In severe cases, the plant will die. 

How to Identify Iron Deficiency?

How to Identify Iron Deficiency?

Perhaps the biggest challenge most growers face when their plant is suffering from iron deficiency is spotting the problem. The symptoms of iron deficiency in cannabis are vague and can often be confused with other nutrient problems. More on this later, but first, let’s look at the common signs of iron deficiency.

1. Early Stage of Iron Deficiency in Cannabis Plants

Iron deficiency is a gradual problem that takes a little time to show itself on the plant. Generally, the first sign of this problem is chlorosis development in the leaves, where they start turning pale yellow while the veins stay green. 

If the problem is not fixed at this time, the chlorosis keeps growing, eventually turning the entire leaf yellow, including the underside and the veins. 

However, since iron is a non-mobile nutrient — it does not move around the plant — the chlorosis symptoms typically show on young leaves that are situated at the top of the foliage. This is one symptom that sets it apart from many other nutrient problems, whose symptoms tend to appear on the lower tiers of the foliage.

2. Late Stage of Iron Deficiency in Cannabis Plants

If left untreated, iron deficiency starts wreaking havoc on other parts of cannabis, too. The leaves will start turning bright yellow, which will inhibit the leaves’ ability to produce energy. 

Later, the leaves will start dropping and the plant’s growth will slow down. Eventually, many leaves will shed and the plant may even stop growing entirely. 

But even if it does not die and only a few leaves turn yellow, don’t expect a good yield because the plant is unable to produce enough energy to support bud development. This is why it is crucial to spot and treat this problem as soon as possible.

How to Differentiate Iron Deficiency from Other Nutrient Problems?

How to Differentiate Iron Deficiency from Other Nutrient Problems?

As mentioned earlier, it is usually quite difficult to differentiate iron deficiency from other nutrient problems because the symptoms are often so similar, especially for a novice grower. 

Most new growers often confuse iron deficiency with other nutrient deficiencies related to nitrogen, manganese, zinc, or calcium. This is because all these problems also exhibit yellowing leaves. 

If you want to be sure about iron deficiency, you need to look a lot closer at your plant and observe how the symptoms are developing. With iron deficiency, the symptoms always start at the top of the foliage in young leaves, and gradually move down. 

At the same time, you need to know the subtle differences in symptoms of different conditions. Here are some common differentiating symptoms that appear on leaves with different nutrient problems:

  • The leaves turn yellow but appear burnt on the edges in magnesium deficiency
  • The leaves exhibit brown or rusty spots in calcium or zinc deficiencies
  • The buds turn brown and the leaves underside turn purple, orange, or red in sulfur deficiency
  • The leaves start browning and the tips start curling in boron deficiency 

It is crucial that you learn about other common nutrient problems you face so you can easily distinguish one from another and act accordingly. 

What Causes Iron Deficiency in Cannabis Plants?

What Causes Iron Deficiency in Cannabis Plants?

The first step in fixing or preventing iron deficiency is knowing what may have caused it. So, here are a few common causes of iron deficiency in cannabis plants.

1. Poor Soil Drainage

One of the primary causes of iron deficiency in cannabis plants is bad soil drainage, which happens when the soil is of poor quality, has too much water retention, or is compacted. Sometimes, it can also happen if the drain holes are plugged shut by debris. 

When the soil has poor drainage, the roots get choked and don’t get enough air to breathe, which inhibits their ability to absorb various nutrients properly, including iron. This is why it is crucial to use fluffy soil and don’t use clay-based soil. 

2. Growing Medium Like Coco Coir

Coco coir is most prone to iron deficiency. This medium naturally lacks iron and is rich in salt, which further impacts iron absorption. So, when you use coco coir as the primary growing medium, your plant may suffer iron deficiency if the nutrient solution does not compensate for the lack of iron in the medium. This is why it’s important to purchase thoroughly-washed peat that doesn’t contain excess salts. 

3. Excessive Nutrients in the Solution

Similarly, your plant may also face iron deficiency because of excessive nutrients in the growing medium. This can happen in two ways — you have mixed the nutrients in the wrong ratio or there is a mineral buildup in the growing medium.

When there is an excess of nutrients, especially manganese, calcium, copper, zinc, and phosphorus, the roots may fail to absorb iron efficiently even if there is enough iron already there, leading to iron deficiency. 

4. pH Imbalance in the Substrate

Another cause that inhibits the uptake of iron, among other nutrients, is a pH imbalance in the growing medium. This can also happen due to strong fertilizers, low-quality soil or water, or the wrong ratio of nutrients. 

Ideally, the pH for a cannabis plant should hover within the range of 6.5 to 6.7 for efficient iron uptake, but if the pH stays out of the recommended range, the roots may fail to absorb iron. 

5. Stress

Cannabis is a resilient plant — it can grow in the deserts of Afghanistan — but some strains are more susceptible to stress than others and may face iron deficiency due to the same. 

Some common stress factors that can cause iron deficiency in cannabis plants include improper transplanting methods, too much darkness, shock, extreme temperatures, or poor maintenance and care. 

6. Overwatering 

Overwatering is a common cause of iron deficiency, especially for hydroponic plants. Yes, it can also occur in soil-based plants but since soil already has iron, it is not so common. 

Essentially, when you overwater your plant, you wash out any nutrients from the soil, including iron. And in some cases, overwatering can cause pooling in the growing medium, which brings us back to the first cause — poor drainage. 

7. Cold Temperatures

Sometimes, iron deficiency can also occur if the temperatures drop far below 68°F or 20°C. This problem is more common in some strains that have low cold resistance. But it can happen with any strain if the cold is accompanied by stress or nutrition problems.

How to Fix Iron Deficiency in Cannabis Plants?

How to Fix Iron Deficiency in Cannabis Plants?

Knowing what may cause iron deficiency is crucial, and once you have a clear idea of why your plant may be suffering from this problem, you need to act quickly and treat the problem before it starts affecting bud development. Here are the most effective treatments for iron deficiency in cannabis plants.

1. Flush the Plant

There are a few ways to fix iron deficiency, but it is best to start with the most basic one, which is flushing the plant and correcting the pH, followed by other more complex methods. 

So, what is flushing? It is a technique where you essentially rinse the plant with clean water to wash away all the nutrients from the growing medium and the roots. Flushing the plant clears out any excess nutrients that may be inhibiting iron absorption, allowing you to start with a clean slate.

Here's how you can do that:

  1. Instead of feeding the nutrient solution, feed your plant clean water with a 6.0 pH reading
  2. Follow this by feeding your plant a nutrient solution that is pH corrected (more on this below)

These two steps will balance your soil’s pH and remove excess nutrients or salt buildup from the medium.

  • Improve the Nutrient Solution

When giving your plant nutrients again, you need to ensure that, this time, the solution is ideal for your cannabis plant — the nutrient ratios should be as recommended by the manufacturer and the pH should also be corrected.

If the nutrient solution was already ideal but still created a nutrient lockout in the substrate, consider making it more dilute or switching brands. The latter may be expensive, but in the long run, it is totally worth it. And always ensure you use cannabis-specific nutrients.

  • Correct the pH of the Nutrient Solution

It is wise to invest in a pH meter so you can test the pH of the solution before and after you feed it. And you can use pH up/down solutions to balance the pH. 

If you are using a soil medium, you should stick to a pH ranging from 6.0 to 6.5 for the nutrient solution — this is the range where iron gets absorbed best in the soil. For hydroponic plants, the pH should be between 5.5 to 6.5.

Measure the pH before feeding your plant and measure the same runoff water after feeding — the pH readings should be in the same ballpark figure. 

2. Check the Drainage and Watering Routine

Overwatering flushes out the nutrients from the growing medium, so you need to ensure you are not overwatering. At the same time, you need to ensure the container has good drainage so the water does not pool up. Both practices go hand in hand.

So, when you water your plant, ensure you water at the right time, i.e., early in the morning or when you turn on the lights. And only water the plant when the top inch of the soil is dry, otherwise, you risk overwatering the plant.

While watering, pay attention to the runoff water. Ideally, there should be around 20% runoff from the drain holes. If the runoff isn’t adequate or the water drains too slowly, check the drain holes to see if they are clogged.

If they are not clogged, you likely have poor-quality soil that is clay-based or the medium lacks enough aeration. To improve your soil, you can add peat moss, vermiculite, or perlite, which will improve aeration and drainage and prevent the symptoms of overwatering. 

3. Correct the Temperatures

Next, you need to correct the environment in which your cannabis is growing, especially the temperature. Ensure your grow room’s temperature is around 68°F or 20°C, which is where iron gets best absorbed by the roots. 

If you are growing your cannabis outdoors and the nights tend to be cold, you can cover the plant with a tarp at night — doing so will trap some heat within and keep the temperatures in check. 

Or better, you can move your outdoor plants into your house at night, given that they are growing in containers. 

4. Supplement Iron Chelates to Your Plant

Sometimes, you may also need to give your plant an iron boost while you fix the main cause of the iron deficiency. For this, you should use iron chelates.

Iron chelates are molecules that are electrically charged and hold iron, and their solubility is not affected by other minerals. This makes iron chelates particularly helpful if your cannabis is experiencing an iron deficiency due to nutrient lockout. 

Iron chelates are available in the form of foliar sprays as well as power, so you can either spray them directly on the plant’s foliage or add it to the soil; they are quite effective even in alkaline soils.

Do note that if you apply iron chelates as a foliar spray, it only works as a booster and does not fix the problem in the long term. It’s only a quick fix that will help your plant get iron while you fix the main problem. 

When using it as a foliar spray, only spray it on the leaves during the early morning as hot temperatures can cause burning on the leaves. 

So, if you want a long-term solution, add iron chelates directly into the soil. However, the type of iron chelate you have to use depends on the acidity of your soil. Here are the three common types of iron chelates:

Fe-EDTA: This is stable in soils with a pH below 7.0 and is ideal for calcareous soils as it’s not affected by calcium. 

Fe-DTPA: This type is stable in soils with a pH of around 6.0 but when the pH increases by half a point, the solubility of iron goes down by 50%.

Fe-EDDHA: The most stable and expensive type of iron chelate, which works best for alkaline soil with a pH of 9.0 or above. 

You can choose one depending on the soil you are using.

5. Or Use Other Soil Additives

Iron chelates are not for everyone, but if you want to give your plant an iron boost, you have many other options. Of course, most growers don’t need to add any soil additives once the main problem is fixed — soil is already rich in iron and your plant doesn’t need a lot of it anyway. Even tap water contains more than enough iron for your plant. 

But if your plant’s iron deficiency is caused due to actual lack of iron in the roots, especially if you are using RO water, you can add iron soil additives such as the following.

  • Compost

Compost is a terrific soil stabilizer that contains iron and various other essential nutrients. The best part is that compost is often free and quite healthy for your cannabis plant. 

To apply compost to your garden, start by adding up to four inches of compost to the soil and gradually keep increasing it up to 12 inches. Bear in mind that compost can decrease the overall pH of the soil by up to 0.2 units. 

  • CalMag Solutions

CalMag solutions are supplements that provide your plant with calcium and magnesium in the right ratios, but many of these solutions also include some iron. So, CalMag solutions are a terrific way to give your plant a boost of not only iron but also calcium and magnesium. 

  • Other Options

You can also try out other supplements that are rich in iron, like specific fertilizers that contain ammonium sulfate or urea, bone meal, or cannabis-specific iron supplements. Try these as per your preferences and always follow the manufacturer’s recommendations. 

How to Prevent Iron Deficiency in Cannabis?

How to Prevent Iron Deficiency in Cannabis?

Congrats, you have treated iron deficiency in your cannabis plant, but now, you have to ensure it does not return to haunt your plant. Follow these tips to prevent iron deficiency in your cannabis plants:

  • Add six inches of elemental sulfur to the soil a year before planting cannabis for a long-lasting preventative measure 
  • Instead of using RO water, use tap water as it is rich in various minerals, including iron
  • Take good care of your roots by watering at the right time and letting enough water drain out of the containers 
  • Ensure the pH of the substrate is always within the recommended range and the nutrient solution is well balanced 
  • Pick a good grow medium that has enough aeration to prevent waterlogging the roots 

It’s simple steps like these that can help you prevent iron deficiency in cannabis. It’s all about taking good care of your plant.

Summary: How to Treat Iron Deficiency in Cannabis?

If you notice chlorosis or yellowing of the leaves on your cannabis plant, awaken your inner Sherlock Holmes and start figuring out what is causing the problem.

If the symptoms appear on the top — yellow leaves but green veins — it is likely due to iron deficiency. And start fixing it immediately. While iron deficiency is a slow and gradual disease, it can still significantly affect your plant’s growth and yield. 

Use the methods listed above to figure out what is causing the problem and fix it ASAP. And in the meantime, you can also give your plant an iron boost using iron chelates as a foliar spray.

Once the problem is fixed, keep a close eye on your plant to ensure it does not develop any symptoms again. However, note that most yellow leaves may never recover or regain their lush color. 

Lastly, use the tips mentioned above to prevent iron deficiency from occurring again in your cannabis garden. It’s easier than fixing it, and did we say it already? Fixing iron deficiency is also quite easy. 

And if you want to know more about growing cannabis while dealing with common problems your plant may face along the way, stay tuned. We post regular guides on growing cannabis like a pro.


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