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St. Augustine Grass Turning Yellow? 6 Possible Causes

St. Augustine Grass Turning Yellow? 6 Possible Causes

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Paul Brown

St. Augustine Grass is a popular lawn grass due to its versatility in soil type and tolerance to high temperatures. It’s a sturdy plant that can thrive in several conditions, which would typically damage its counterparts. However, it can also be sensitive to some situations that can turn its leaves yellow.

Some possible causes for St. Augustine Grass to turn yellow include iron deficiency due to high pH and phosphorus levels, fungal diseases that cause yellow patches or root rot, excessive nitrogen, nitrogen deficiency, over-watering or heavy rain, and high temperature.

This article will discuss the various reasons that may cause your St. Augustine Grass yellow and share some tips on how to fix it. If you want to know more, read on!

1. Iron Deficiency Due to High Phosphorus and High pH Levels

Iron deficiency is one of the most common causes of the yellowing of St. Augustine Grass. It may be due to high phosphorus content in the soil or high pH levels (source).

Turfgrass like St. Augustine needs iron for photosynthesis, chlorophyll formation, and nitrogen breakdown. Ideally, the lawn must contain 30-100 parts per million (ppm) of iron (source).

Excessive amounts of phosphorus in your soil can significantly reduce your plants’ ability to absorb iron. St. Augustine grass needs high iron levels but has shallow root systems, preventing it from reaching the nutrient deep into the soil (source). Therefore, when applying fertilizer, select products that have low phosphorus or avoid using the nutrient altogether.

In ordinary situations, St. Augustine Grass may thrive in slightly acidic soil with pH ranging from 6.0 to 6.5. As the pH level increases, the amount of iron that the plant can absorb from the soil decreases. The decrease in iron absorption can result in your grass turning yellow—a condition called chlorosis.

See How To Identify And Manage Alkaline Soil.

Here’s a video showing an easy way to check your soil pH at home: 

Simple Soil pH Testing

One quick way to see if the yellowing is due to iron deficiency is by observing the affected leaves’ pattern and age. Chlorosis caused by iron deficiency usually shows by forming mottled patterns on the turf. In addition, it usually affects young leaves or new growths.

You may also test the soil to see if there’s enough iron for the plant. You can send some soil samples to a nearby lab for comprehensive testing that includes the measurement of pH and concentrations of various nutrients. It may also help you determine what kinds of fertilizers you need and how frequently you must apply them (source).

In addition to pH and phosphorus levels, iron deficiency in the soil can also result from over-watering or too much rain. Excessive water can wash away the iron. If the amount of iron is less than 30 ppm, it’s best to apply an iron supplement to the spots in your grass that show a distinct yellow appearance. 

You may check out the Southern Ag Chelated Liquid Iron (link to Amazon). It’s easy to apply and works well on various types of lawn grass. Be sure to follow the instructions carefully when using the product on your lawn.

Lowering the soil pH can also help fix the problem as it will facilitate easier iron absorption. You may apply 5 lbs (2.3 kg) of sulfur pellets for every 1,000 square feet (93 sq. m) of turf. However, the results may take time to appear.

2. Fungal Disease

A large patch sometimes referred to as a brown patch, is a fungal infection that causes St. Augustine grass to turn yellow or brown. It results from a fungal infection brought about by Rhizoctonia solani. It usually occurs in warm, humid seasons, especially in patches under moist, shaded areas. 

All turfgrasses are susceptible to this kind of disease. Infected St. Augustine grass shows a doughnut-shaped pattern in spring, with green inner and brown outer layers. This pattern indicates that the grass is recovering, and new leaves are growing at the center. 

To diagnose the disease, check for rotten leaf sheaths and if the browned blades easily pull free from a light tug. You can prevent the infection by reducing nitrogen supplements in the early spring and late fall because the nutrient encourages the growth of green blades usually attacked by fungi. 

Prevention also includes applying fungicide in late fall before the grass becomes dormant and early spring before it starts to green. However, if the infection begins showing in spring, you can treat it by applying fungicide (source).

Another fungal disease common in St. Augustine grass is the Take All Root Rot caused by the fungi Gaeumannomyces graminis var. graminis. Unlike the disease caused by Rhizoctonia, it doesn’t form circular patterns (source). Instead, it starts with the yellowing of the leaves to brown and black as the grass dies.

Sufficient nitrogen concentration in the soil can prevent the Take All Root Rot disease from occurring. However, if you cut back on nitrogen fertilizers in late fall and there is heavy rain in winter, the condition will be conducive for the fungi to prosper.

Since increasing or decreasing the nitrogen fertilizer in the soil can cause either fungus to attack the St. Augustine grass, most people would instead use a fungicide that can more effectively prevent or treat both issues. 

Check out Scotts DiseaseEX Lawn Fungicide (link to Amazon). It’s easy to use and has excellent reviews from people with St. Augustine Grass. When dealing with fungicides, always make sure to follow the application instructions.

Mowing can spread the disease to other parts of your lawn. Applying fungicide to the entire turf may be a good idea. However, it may be costly. Some people use fungicide only on infected patches. To prevent the spread of infection through mowing, mow the infected area last and quickly remove infected clippings.

3. Excessive Nitrogen

When dogs habitually pee at certain spots on your lawn grass, you’ll notice the formation of round, yellow patches in that area.

The chlorosis or yellowing of the blades is due to high levels of nitrogen in the urine that build up in the area over time. While nitrogen is necessary for healthy foliage production in plants, excessive amounts of it can be counterproductive as it can burn the roots and dry up the leaves.

Watering the area where the dogs urinate frequently can help dilute the nitrogen concentration in the soil and protect your grass. Other people also use dog or cat repellent spray to prevent these animals from doing their business on the lawn.

More harmful effects of too much nitrogen for your turfgrass include a higher risk for diseases, lower tolerance to high or low-temperature stress, and increased thatch buildup. Thatch is formed from the pileup of fresh or dried grass and roots that can block water and aeration (source). 

See How To Clean Up After Dethatching: A Practical Guide.

Using compost and fertilizer with high nitrogen content may cause more damage than harm. (See our guide on compost for lawn use). When you confirm that the yellowing is caused by excessive nitrogen, you must avoid applying fertilizer that contains the nutrient. 

For instance, you may look for a fertilizer that says 0-0-3 on the label. The numbers indicate the proportions of nitrogen, phosphorus, and potassium, respectively. Since excessive amounts of nitrogen and high levels of phosphorus can be harmful to your grass, it’s best to use a high-potassium fertilizer.

It’ll also help to set a schedule for nitrogen application. Summer months may vary depending on your location. Since St. Augustine is a warm-season grass, it is best to apply slow-release nitrogen fertilizers only twice or three times in the summer at 3–4 week intervals. Cut back on it in late fall to early spring.

See our complete St. Augustine Lawn Care Fertilizer Schedule (Includes Free Printable)

4. Nitrogen Deficiency

While excessive amounts of nitrogen can cause harm to your grass, so does its deficiency. Nitrogen is available or applied to the soil as either nitrate (NO3) or ammonium (NH4+). It helps boost leaf and root growth, lushness, and color. Lack of nitrogen can cause the grass to turn yellow.

If you had your soil sample tested and results show low nitrogen levels, you may use quick-release nitrogen fertilizers. They usually come in the form of ammonia or urea. You must use this type of fertilizer only in warm and dry seasons to prevent rapid leaching.

One of the most common causes of nitrogen deficiency in the soil is leaching. It’s more likely to occur in sandy soil and during long periods of heavy rainfall. The water will cause the nitrogen to leach deep into the ground beyond the reach of the grass’ short root system. 

If your area has plenty of rain, you may use slow-release or water-insoluble nitrogen fertilizer to prevent the nutrient from leaching. Runoff can also result from too much rain. Instead of seeping below the ground, runoff can bring the nitrogen out into the surface and run downslope into the sewers.

Even if your soil has plenty of nitrogen, your St. Augustine grass may still have a reduced ability to absorb or utilize it due to the issues mentioned above, leading to a deficiency. Pay attention to the weather patterns in your area to determine what kind of fertilizer to use and when to apply it.

5. Over-watering or Too Much Rain

St. Augustine grass has a low tolerance to drought and requires moist soil. However, as discussed, too much water can cause several problems with your St. Augustine grass. These problems can cause the blades of your grass to turn yellow.

To summarize, it can flush out or leach nutrients deep into the ground, beyond the reach of the roots, and can leave the soil moist enough for fungi to thrive. Heavy rain or over-watering can also spread infections and diseases to other parts of your lawn more quickly. 

To prevent the damage that may be caused by excessive water, be sure to set a proper schedule or timing for fertilizer application. Ideally, you must apply the nutrients during warm months. You must also select the type of fertilizer based on the weather.

See our review on Sunday Lawn Care’s customized lawn fertilizer program that matches a fertilizer plan to your soil’s specific needs.

You must also consider the type of soil you have when dealing with this issue. St. Augustine grass naturally grows along sandy coastlines, making sandy soil the most suitable substrate for producing it on your lawn. Sand-rich soil also has good drainage properties.

However, good drainage also means a higher tendency for leaching. Water-insoluble and slow-release fertilizers can resist the effects of leaching. Small amounts of quick-release fertilizers and proper timing can also do the trick for optimum utilization of the nutrients and prevent them from contaminating ground or drinking water.

6. High Temperature

Although St. Augustine grass is a warm-climate turf grass that can tolerate temperatures ranging from 80–100°F (26.7–37.8°C), it has low drought tolerance. As such, a sufficient water supply is necessary.

St. Augustine grass thrives best under full sun, with more than eight hours of direct exposure to sunlight, making it a nice warm-climate grass. However, very high temperatures and lack of rain in summer may cause the soil to dry up more quickly. Lack of moisture in the soil can cause the blades to turn yellow. 

In addition, high temperatures may prevent the grass from absorbing water-soluble nutrients from the soil, causing further yellowing of the leaves. Providing enough water to the lawn can help revive the grass. However, extended periods of drought can make it difficult to revive the plant.

When mowing the lawn in extremely hot and dry summer, keep your grass 3–5 inches tall (7.6–12.7 cm) using a mulching-type mower and leave the clippings on the ground to create mulch.

Mulch can help with moisture retention and provide nutrients back into the soil as it decomposes. It’s especially helpful because St. Augustine grass’ shallow root system prevents it from reaching water or moisture deep into the ground.

Final Thoughts

St. Augustine grass provides a beautiful, green turf on your lawn. However, to keep it lush and green, you must meet its nutrient, pH, and water requirements and understand the seasonal changes in your area that can affect its overall growth and health. Regularly mowing the lawn is also necessary for your grass.

Maintaining your turfgrass may be tedious, but once it becomes part of your routine, you can enjoy a healthy, green lawn.

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