Have your stunning trees and shrubs faded to an unsightly yellow? Your woody plants may be suffering from an iron deficiency known as iron chlorosis. With patience and determination, you can green up your landscape in no time. Learn what causes iron chlorosis in your trees and shrubs and how to treat it.
Here’s what we’ll cover in the article:
- What is iron chlorosis?
- What causes iron chlorosis in trees and shrubs?
- How to identify iron chlorosis in trees and shrubs
- How to treat iron chlorosis in trees
- How to treat iron chlorosis in shrubs
- FAQ about iron chlorosis
What is iron chlorosis?
Plants experience iron chlorosis when they’re not receiving enough iron from the soil.
What causes iron chlorosis in trees and shrubs?
Iron chlorosis occurs in trees and shrubs when there’s an iron deficiency in the soil.
Another cause for iron chlorosis is when high soil pH makes the iron unavailable for plants to absorb. Some treatment methods call for a sulfur application to help lower the soil pH, making the iron more soluble and available for uptake.
How to identify iron chlorosis in trees and shrubs
Iron chlorosis causes leaf tissue to appear yellowish while the veins remain dark green. That’s because plants can’t produce chlorophyll without iron (you remember chlorophyll from science class –– the pigment that gives plants their green color). The lack of chlorophyll eventually leads to reduced plant growth and vigor.
In severe cases of iron chlorosis, the foliage can fade from yellow to white to brown, and the plant eventually dies.
Iron chlorosis symptoms in your trees and shrubs can mimic the same symptoms as nitrogen deficiency. Here are some tips to help tell them apart:
| Iron chlorosis | Nitrogen deficiency |
| Leaf tissue typically appears yellow coupled with dark green veins. | The whole leaf appears yellow, including the veins. |
| Leaf yellowing first appears on new growth. | Leaf yellowing first appears on old growth. |
Severe iron chlorosis can look nearly identical to nitrogen deficiency. For example, severe iron deficiency can cause the whole leaf to yellow (including the leaf veins), and the yellowing can progress to old growth. Once all the foliage turns yellow, it can be difficult to determine whether it’s an iron or nitrogen deficiency based on looks alone.
The best way to confirm whether your plants are suffering from an iron or nitrogen deficiency is to send your soil to a local laboratory for soil testing. Laboratory tests offer more detailed results than DIY soil tests, often listing details about soil pH, nutrient levels, and required amendments.
How to treat iron chlorosis in trees
Are your trees showing signs of iron chlorosis? You might notice symptoms on only one branch, the whole tree, or portions of the tree.
Tree species most affected by iron chlorosis include:
- Apple
- Birch
- Cherry
- Dogwood
- Magnolia
- Pin oak
- Red maple
- River birch
- Silver maple
- Sweetgum
- White oak
- White pine
So how can you treat a tree showing signs of iron chlorosis? Your first step is to perform a soil test to confirm whether or not it’s iron chlorosis. If it is, choose between the following treatment options:
Foliar application
If you need a quick treatment for iron chlorosis, spray the affected tree’s foliage with an iron sulfate or iron chelate solution.
The Kansas State University Agricultural Experiment Station and Cooperative Extension Service recommends applying a solution containing 2.5 ounces of iron sulfate, 3 gallons of water, and a few drops of detergent (the detergent helps the solution adhere to the foliage).
Here are a few things to keep in mind about foliar treatment:
- It’s not a long-term solution. The iron won’t go beyond the foliage and into the tree, which means new growth will be chlorotic.
- Reapplications may be necessary.
- The foliar spray can be difficult to apply to large trees.
- Apply the solution during the tree’s growing season.
- Spray in the evening or during cool weather conditions.
Soil treatment
For lasting results, create a mixture of equal parts iron sulfate and elemental sulfur. Remember to read the product labels to ensure the appropriate concentrations and application method.
The Utah State University Forestry Extension recommends making holes in the ground 1 to 2 inches in diameter and 12 to 18 inches deep. Space the holes 18 to 24 inches apart and keep them within the tree’s drip line (the drip line is the crown’s outer edge). Remember to call 811 to confirm there are no underground utilities where you plan to dig.
Next, pour the iron sulfate-elemental sulfur mixture into each hole within 4 inches of the soil surface.
Pro Tip: When making the holes, use a soil probe that removes the soil from the ground. A probe that doesn’t remove the soil will squish and compact the surrounding soil.
The Utah State University Forestry Extension recommends creating the following number of holes and pounds of product to apply depending on the tree’s size:
| Tree trunk diameter (inches) | Number of holes made in soil | Pounds of iron sulfate and elemental sulfur combined |
| 1 | 4 | 0.5 – 1.0 |
| 2 | 6 | 1 – 2 |
| 4 | 8 | 3 – 6 |
| 6 | 12 | 8 – 12 |
| 8 | 16 – 24 | 12 – 16 |
| 10 | 25 – 30 | 16 – 20 |
| 15 | 30 – 40 | 20 – 30 |
| 20 | 40 – 50 | 30 – 40 |
Trunk injection
Injecting the tree’s trunk with ferric ammonium citrate (iron citrate) or ferrous sulfate (iron) can be effective for two or more years. The best place to drill the injection holes is in the root flares near the soil surface. Use a sharp brad-point drill bit to minimize tree damage.
Always follow the label directions of the products you’re using to ensure proper hole sizes and injection amounts. If you’re concerned about damaging the tree trunk, hire a certified arborist for the job.
Pro Tip: Ensure the tree is well-watered before treatment and several weeks after treatment.
How to treat iron chlorosis in shrubs
Do your once beautiful azaleas and rhododendron have yellow leaves? Iron chlorosis can affect several shrubs in the landscape, including:
- Azaleas
- Blackberry bush
- Boxwoods
- Camellias
- Grape
- Holly
- Hydrangea
- Lilac
- Raspberry bush
- Rhododendron
- Strawberry bush
Soil treatments are the best long-term solution for shrubs, while foliar applications provide short-term relief.
Soil treatment
Depending on the results of a soil test, you can remedy the soil with ferrous sulfate (iron) or elemental sulfur.
Keep in mind that a soil test may recommend a different combination of these amendments. For example, if the soil test reveals the pH is too high, it may recommend applying only elemental sulfur to lower the pH. On the other hand, if the pH is normal but the soil needs iron, it may recommend using only iron.
There are several ways you can work these amendments into the soil:
- The Utah State University Forestry Extension recommends creating a 4-inch deep trench 12 to 24 inches away from the shrub’s base. Apply 1 inch of equal parts ferrous sulfate (iron) and elemental sulfur to the bottom of the trench and fill the trench with soil.
- Create holes in the ground within the shrub’s drip line as you would for the tree treatment mentioned above. Pour the ferrous sulfate-elemental sulfur mixture into each hole within 4 inches of the soil surface.
- Work the ferrous sulfate-elemental sulfur mixture into the soil around the shrub’s drip line. Check out how Laura from Garden Answer performs this application in her garden:
Foliar application
Another treatment method is to apply an iron sulfate solution to the shrub’s foliage. Keep in mind that this treatment is only a temporary remedy and will not repair new growth. You can apply the same foliar solution we mention above for trees. However, always refer to the product label for the best ingredient ratio.
FAQ about iron chlorosis
The best way to prevent iron chlorosis in your trees and shrubs is to perform routine soil tests, preferably laboratory tests. Soil conditions significantly affect plant health, so the sooner you can spot a change in soil health, the better.
In some cases, aluminum sulfate can be a good substitute for elemental sulfur. Aluminum sulfate lowers the soil pH much faster than elemental sulfur, making iron readily available for plants.
However, you may want to be cautious about applying aluminum sulfate around your trees and shrubs. The Purdue University Extension does not recommend using aluminum sulfate for your trees and shrubs because applying too much may damage the plants.
Iron chlorosis occurs in turfgrass for the same reasons as your trees and shrubs, including iron deficiency in the soil, high pH level, and compaction.
Protect your lawn from iron chlorosis
Curing your trees and shrubs of iron chlorosis is no small feat, especially if your lawn is suffering from the deficiency, too. Don’t let chlorotic grass ruin your home’s curb appeal. Hire a local lawn care professional to give your landscape the utmost attention and care while you enjoy the beauty of your healthy, green lawn.
Main photo credit: Scot Nelson | Flicker | Public domain
