Iron Deficiency In Cannabis Plants

The most obvious sign of an iron deficiency in cannabis is the yellowing of the leaves, most noticeable in new shoots. If the deficiency is not solved, the leaves will begin to discolor completely.

An iron deficiency is often accompanied by other deficiencies. Because of the most common causes are pH errors, high root salinity, or low temperatures.

Iron Deficiency Colors And Symptoms

cannabis-iron-deficiency-color-chartThe biggest problem of iron deficiency is that it can be tricky to identify. Growers often confuse the lack of iron to other deficiencies such as magnesium, zinc, or nitrogen.

Yet iron deficiency signs and symptoms are quite unique.

Early Signs

First, upper leaves will begin to lose color starting at the base of the leaf.

Then, newborn leaves will turn completely yellow or almost white due to the lack of chlorophyll.

Late Signs

On a late stage new shoots will be damaged and growth will slow down.

Iron deficiency spreads slow. So you have plenty of time to make the right diagnosis and provide the remedy.

But, if you let this continue leaves will turn completely white and shoots will be so damaged that they will no longer grow.

Now the plant’s development is seriously compromised. You must solve the iron deficiency as soon as possible.

Leaf ColorSymptoms
Strong yellow while veins stay greenGrowth and development slows down
Discoloration to the point of turning whiteNew small or stunted shoots
Tips stay greenLeaves develop necrosis spots and die

How To Differentiate From Other Nutrient Deficiencies

Growers sometimes confuse signs of iron to other deficiencies, most commonly with magnesium, nitrogen and zinc.

To differentiate early signs of iron deficiency ask yourself which part of the leaf turns yellow first. As iron deficiency starts at the base, and slowly moves towards the tips.

When facing a mild deficiency you may see that it ceases after a few days, and leaves that were completely yellow start getting their green back. This does not happen with most deficiencies, so this symptom confirms that you were facing a lack of iron.

If these don't look like your plant, check my cannabis leaf symptoms pictures to find the correct one and solve it as quickly as possible.

The Importance Of Iron In Cannabis

Iron fits the micronutrient category, as it's required by plants in lesser amounts than other nutrients. Yet, iron is very important to the health and growth of cannabis.

The role of iron in most plants is to help carry important elements through the circulatory system. It also helps with the development of younger tissues.

And it's an important component during photosynthesis, as without iron a plant can’t produce chlorophyll, so it can’t get oxygen and can’t be green.

How To Solve Iron Deficiency

It is very rare that a cannabis plant is lacking iron. Most likely there's a reason your plant cannot take iron from the soil.

⚠ Your Plants Have Iron Already — But Can't Take It!

Pipe water comes loaded with iron thus it’s not a commonly missing element. In most cases, when there is a lack of iron, there is usually another problem.

To solve a deficiency, you must know your plants very well, as well as what your soil or substrate. The water you use to water the plants, and your watering habits.

Also, you should know what fertilizers you have added and make sure to check them for insects and pests.

Let's explore the possible causes.

PH Error And Excess Salts

The most common reason for a lack of iron. Since iron becomes available to the plant when the pH of the soil stays below 7.  

You may have a blockage on the roots due to excess salts. If you have been fertilizing your plant, especially if you have used strong fertilizers such as guano or chemical fertilizers.

Hard pipe water may also generate a variation in pH and cause a blockage of iron and other elements.

  • SOIL, iron is best absorbed by the roots in the 6.0 – 6.5 pH range.
  • COCO or HYDRO, iron is best absorbed in the 5.5 – 6.5 pH range.
Soil pH tester
Soil pH tester

If you have high pH you need to lower it to about 6.5 to enable iron absorption and fix the deficiency.

One way to fix this is to flush the roots with abundant water. Which you have previously left to stand uncovered for 24 hours.

Make sure your pots drain well and keep adding water until the run off comes out clean. The abundant water will remove excess salts and fertilizers, returning the pH back to normal.

I recommend you use this guide to find methods of varying the pH of different growing mediums.

pH Meter for water and hydroponics
pH Meter for water and hydroponics

Excess Phosphorus (P)

An excess of phosphorus is also a common cause of an iron deficiency. If you are fertilizing your plants, especially with strong fertilizers this problem may arise.

Too much phosphorus blocks the absorption of various nutrients including iron, calcium, magnesium, zinc.

In this case, you can flush the roots as I mentioned in the pH error section. By flushing with abundant water you wash the nutrients and salts away.


One potential cause of iron deficiency is if your plant has suffered from stress.

There are several ways that one can cause enough stress:

  • Too many hours of darkness,
  • transplants,
  • excessive or poorly done pruning,
  • and extreme temperatures.

If this is the case, continue caring for your plant, and the deficiency should back of on its own after a few days.


Temperature changes are not favorable for marijuana plants. Maintaining a stable temperature both in the growing environment and at the roots is essential.

If the temperature gets below 68ºF (20ºC), the leaves will begin to show problems, especially if it prolongs over time.

If you grow indoors, you can try a heater (check my guide on grow tent heaters) or a carbon dioxide (CO2) generator to increase the temperature. Another option is to use electric blankets, but be careful to use them with a timer to avoid the roots to get overheated.

If you grow outdoors you should consider raising a greenhouse around your plants, or cover them with a tarp at night. If you have pots, bring them into the house when the temperature drops.

If you’d like to know more, check this guide from Alchimiaweb about how cold affects cannabis plants.

Small heater for plants
Small heater for grow tents

Over Watering And Bad Drainage

Over watering or poor drainage can cause various root problems and can even lead to fungal growth. And this may be causing a blockage that does not allow for the absorption of iron.

Cannabis is a hardy plant, but it does have limits. Make sure your growing medium has good drainage. If you have containers, they should have enough holes at the bottom, and little water should draining out every time you water.

I leave you this Sensi Seeds' watering guide if you think this may be the cause.

Reverse Osmosis, Distilled Or Soft Water Lacking Minerals

Iron, calcium and magnesium are abundant in natural water as dissolved minerals. In usual growing environments, plants should get enough of these nutrients from the water itself.

Soft water has a lower pH and fewer minerals in it. Reverse osmosis and distillation are processes that remove minerals from your water.

If you happen to have this kind of water, you may have to look for Cal-Mag supplements. with good amounts of Iron in them.

Adding Iron To Your Medium

If you find you need to give iron to your plants you can simply buy fertilizer.

Cal-Mag Fertilizer

Calcium and magnesium supplements that come with good amounts of iron are ideal. Cal-Mag products are suitable for Hydro, Coco Coir and Soil.

This may be a great choice because these calcium and magnesium deficiencies often happen alongside an iron deficiency. Contains formulation of 2-0-0 NPK.

Cal-Mag fertilizer
Cal-Mag & Iron fertilzier

Other Soil Additives

If for whatever reason you can’t have Cal-Mag, there are many organic and non-organic products that are excellent sources of iron.

I advice you to go up to your gardening store and ask for a soft fertilizer with good amounts of iron. Or if you’re more of an online person you can search Amazon or eBay.

Remember to make sure you don’t have a root blockage first, or it’ll make things worse.

🕓 Wait For Recovery

As you can see, most of these causes are because of the plant roots not having the best conditions to thrive. It is the newer roots that absorb iron better, so it is important that you keep the root system healthy and in good condition.

After carrying out any of these solutions in less than two weeks you will begin to see the leaves improve and recover some of their color. And the growing of shoots should resume its original rhythm.

It is possible that some very stunted shoots can’t recover. In this case the best thing to do is to cut them, since iron is an immobile element and those sheets will have no solution.

But you will see that newer growths will come out green and alive.

I hope you can solve your iron deficiency without major problems. I will be publishing more guides on nutrient deficiencies and other steps of growing marijuana.

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Hey there, I'm the person behind this website. I started growing cannabis in 2014 right after marijuana was legalized in my home country. Today, on, I address growers need for knowledge and show you how to grow cannabis plants and enjoy amazing buds.

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