St Augustine Grass Turning Yellow? 6 Most Likely Reasons

You pride yourself on your beautiful lawn, and then, disaster strikes. Your lush St Augustine grass is turning yellow, instead of the deep green you’ve come to enjoy.

The most common reasons for St Augustine grass turning yellow include lack of key nutrients, overfertilization and pests, but we will explore all of these and more in this article.

St Augustine Grass 

St Augustine is dark green grass with flat blades. It is dense and creates a lush soft lawn.

The grass grows great in zones 8-10. If you live in a tropical location, it’s a great choice for your lawn. It does well in shade and full sun areas. It does require plenty of water, as well as nitrogen and other nutrients. 

6 Most Common Reasons For St Augustine Grass Turning Yellow

You can determine the cause of yellowing St Augustine grass by the pattern of yellowing, the time of year, and weather conditions. 

The main causes of yellowing include:

  • Iron Chlorosis
  • Lack of nitrogen
  • Fungal infection
  • Summer Chlorosis
  • Overfertilization
  • Chinch bugs

1. Iron Chlorosis 

Yellow Streaks and Spots are typically caused by iron chlorosis. This is essentially an iron deficiency. 

Signs of Iron Chlorosis 

The most common sign of iron chlorosis is yellow grass with green streaks in the leaves. You’ll notice streaks of green, with the majority of the leaf being yellow, due to the limited amount of chlorophyll. 

In severe cases of iron chlorosis, the leaves will develop brown margins or brown spots. This will only occur if the leaves have been yellow for a period of time.  

What Causes Iron Chlorosis

Iron chlorosis is a lack of iron, which leads to chlorophyll deficiency.

To create chlorophyll, a plant must have iron. Chlorophyll is essentially food for the plant. It’s also what makes plants green. 

When the plant is deficient in iron, it also becomes deficient in chlorophyll, which leads to yellowing.

This is most common in early spring and summer when the grass is in its peak growth period. It requires more chlorophyll to sustain its rapid growth during this time. 

There are two potential causes. 

The first, and most straightforward, is a lack of iron in the soil. If there’s not enough iron in the soil, there’s no way for St Augustine grass to get the iron it needs. 

The second cause is highly alkaline soil. In this case, the soil does contain plenty of iron. However, due to the alkalinity of the soil, the plant can’t access iron and other nutrients. 

Treating Iron Chlorosis 

You can treat iron chlorosis by adding iron to the foliage or the soil. The fastest way to get your lawn green again is to use a chelated iron or iron sulfate spray. This form of iron is sprayed directly onto the grass, which allows it to absorb the iron quickly. 

You can also apply iron sulfate, which is applied to the soil. It boosts the iron content of the soil and makes the iron more available because it is acidic. 

You can add the iron fertilizer every 2 to 3 months to treat and prevent iron chlorosis. 

2. Lack of Nitrogen 

If your grass is uniformly yellow, a lack of nitrogen may be the culprit. This typically occurs during periods of heavy rainfall. 

Signs of Low Nitrogen

When low nitrogen levels turn your grass yellow, you will not notice green stripes. Instead, the entire blade of grass will be uniformly yellow. 

Causes of Low Nitrogen 

St Augustine grass loves the rain. However, the rain can leach the nitrogen from the soil, essentially rinsing it away. A waterlogged lawn can also cause nitrogen deficiency. 

Even if there’s plenty of nitrogen in the soil, the grass can’t get the nutrients it needs if the ground is waterlogged. It blocks the plant’s ability to uptake nutrients, including nitrogen, effectively. 

Treating Low Nitrogen 

To treat nitrogen deficiency, you’ll simply need to add a nitrogen-rich fertilizer.  Choose one with a 19-5-9 ratio to ensure your lawn gets enough nitrogen. You can also choose a combination product like Iron Plus to ensure the grass has plenty of nitrogen and iron. 

3. Fungal Infection 

If you notice yellow circles on your St Augustine blades, a fungal infection is likely to blame. 

Signs of Fungal Infection 

When St Augustine grass turns yellow due to a fungal infection, you’ll see green circles on the blades of grass. There may also be both green and yellow leaves in the same area. It will typically be confined to some areas of your lawn, leaving other areas green and healthy. 

You can confirm a fungal infection by checking the roots of your St Augustine grass. A fungal infection will cause the roots to be brown or black. They may also have a foul odor. 

Causes of Fungal Infection 

The most common fungal infection in St Augustine grass is Take All Root Rot or TARR. TARR, and other fungal infections, are typically caused by excess water or fertilization. 

Fungi thrive in wet soil. Overfertilization can also lead to TARR, because it damages the roots, making them more susceptible to fungus. 

Treating Fungal Infection 

If you want to treat a fungal infection naturally, you can use sphagnum peat moss. You’ll simply apply 1 to 2 bales per 1,000 sq. ft of lawn. It’s only necessary to apply it to the affected areas. 

Once the moss is applied, apply water until the moss is wet, thoroughly soaking it. 

You can also treat a fungal infection with a fungicide.  Myclobutanil and Azoxystrobin are great options for TARR. To apply, you’ll mix 1/2 an ounce of fungicide with 4 gallons of water for 1,000 sq. ft of lawn.

Reapply about 28 days after the first application. After that, you can apply it in the spring and fall as a preventative. 

Apply this fungicide mixture to your lawn on a windless day. Two applications 28 days apart would suffice. Spray in spring and fall.

4. Summer Chlorosis 

As mentioned earlier, chlorosis is the lack of adequate chlorophyll in the grass. It can be caused by summer heat.

Signs of Summer Chlorosis

If your grass is turning yellow during the height of summer, summer chlorosis is probably the cause. 

Causes of Summer Chlorosis 

The higher temperatures can stop the grass from absorbing essential nutrients, like iron and nitrogen. In this case, the nutrients are present in the soil, but the St Augustine grass can’t access them properly. 

Treating Summer Chlorosis

You can treat summer chlorosis by making the soil conditions more favorable for the grass. Add modified compost to boost the acidity of the soil, which makes it easier for the grass to absorb nutrients. You can follow this with liquid seaweed with iron. 

5. Over Fertilization 

Overfertilization can turn your St Augustine grass yellow. This is caused by excess nitrogen. 

Signs of Over Fertilization 

The biggest indicator that over-fertilization is causing your St Augustine to turn yellow is that you’ve recently fertilized the lawn. If you’ve recently applied fertilizer, and then the grass turns yellow, you’ve probably over-fertilized. 

The grass will turn brown at the top, while the base will become yellow. This is known as leaf scorch. 

Causes of Over Fertilization 

The high nitrogen content of the fertilizer burns the grass blades, causing leaf scorch. This is only an issue with quick-release fertilizers. Slow-release fertilizers will not cause leaf scorch. 

Treating Over Fertilization 

If caught early, you can easily undo the damage. Just water your lawn thoroughly. This will wash away the excess nitrogen. If the scorch is severe, you will have to re-sod the area. Rake and till the affected areas, and then apply new sod. 

6. Chinch Bugs

Chinch bugs are the most common pest that affects St Augustine grass. They create an easily identifiable pattern of damage. 

Signs of Chinch Bugs

If you have chinch bugs, you’ll notice patches of dead grass. Around these patches will be a circle of yellow grass.

Causes of Chinch Bug Infestation 

Chinch bugs are attracted to decaying plant matter, also known as thatch. They also prefer dense soil. 

Treating a Chinch Bug Infestation 

You can treat chinch bug infestation with Triazicide. You’ll need to inspect the area every 3-5 days to be sure the treatment is effective. 

Final Thoughts on Yellowing St Augustine Grass 

There are several causes of yellowing St Augustine grass. Proper treatment starts with identifying the cause. Once you know the cause, you can easily remedy the problem. 

You’ll be enjoying your green lawn again in no time. 

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About Me

Hi, I'm Joe! I'm the head of SEO and content management at Bloom and Bumble. I'm a huge plant lover and over the years my home has become more like an indoor rainforest. It has taken a lot of trial and error to keep my plants healthy and so I'm here to share my knowledge to the rest of the world.

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