St. Augustine Grass Turning Yellow? 10 Causes & Remedies

St. Augustine is a common turfgrass here in Alabama and most southern states in the USA for several good reasons.

For starters, St. Augustine lawns offer excellent drought tolerance and require less amounts of water per week compared to other turfgrasses. In other good news, they also require a moderate level of maintenance for healthy grass growth. 

However, just like most residential grass lawns, St. Augustine grass can develop issues over time, most commonly yellow spots in the summer or other times of the year including cold weather conditions, yellow leaf blades, leaf scorch, and much more. 

There are myriad different reasons for St. Augustine grass turning yellow like root rot, insect, fungus, and pest damage, and excess moisture from over-watering.

That said, here are 10 causes for yellowing St. Austine grass and their fixes:

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10 Causes and Fixes for St. Augustine Grass Turning Yellow

Although there are more than 10 causes for St. Augustine grass turning yellow, I’ve listed the most common reasons for the issue and fixes. 

St. Augustine Grass Turning Yellow

1. Iron Deficiency 

St. Augustine grass requires a boost of iron supplements to grow green and strong. Iron deficiency is usually caused by high phosphorous and high pH levels. 

Iron chlorosis is caused when the soil is iron-deficient or the soil conditions make the iron unavailable. When this happens, St. Augustine grass will fade to yellow because iron is needed to make chlorophyll – the pigment that gives grass its green color. 

Identifying iron chlorosis in St. Augustine lawns can be tricky because the symptoms can mimic those of other lawn ailments, particularly nitrogen deficiency. 

There are several telltale signs of iron chlorosis and nitrogen deficiency, identification may be difficult if either deficiency is severe. 

How to Fix Iron Deficiency in St. Augustine Lawns? 

Before treating your lawn for iron deficiency, I recommend testing your soil. A soil test will let you know whether your soil is indeed deficient in iron or whether you need to add amendments to lower the soil pH. Here’s why soil testing is important.

Adding to this, a soil test will also reveal the appropriate amount of product to add per 1,000 square feet, depending on your soil type and turf type. 

You can buy a soil test kit online or at your local garden center, but the downside to both these options is they do not provide the best amendments for your soil type and turf type. 

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And it’s worth mentioning that adding nitrogen when not required can worsen an iron deficiency so sending soil samples to the lab is an accurate way to gauge the condition of your soil. 

If the results of the soil pH test are too high, your soil is deemed alkaline soil, for which adding elemental sulfur will bring the numbers under control. When the soil is acidic, that is the soil pH is less than 6.5, adding lime will increase the surface soil pH and maintain the current soil pH status. 

However, if your soil is iron deficient, you can acidify it as the #1 cause of iron deficiency is high soil pH, apply liquid iron to your lawn, and/or apply iron fertilizer to the soil. Learn about how much nitrogen is needed for lawns.

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The best time to apply iron amendments is after core aeration because this lawn care process exposes the soil, relieves compaction, and allows the roots to better absorb the iron. 

2. Lack of Water

Not enough water or excessive water can turn your St. Augustine grass yellow or straw-colored. Adding too many gallons of water can drown the grassroots and rob the soil of oxygen and that vibrant green color. 

Less water will lead to drought symptoms that turn your St. Augustine turf yellow or some shade of brown. 

How to Fix Lack of Water in St. Augustine Grass? 

Sticking to a watering schedule ensures that your St. Augustine grass lawn thrives in the summer months including mid-summer, late summer, and in moderately harsh conditions. 

The good news is that St. Augustine grass requires just an inch of water per week either via irrigation or rain including heavy rain. 

However, you may need to provide more water depending on your lawn’s condition such as high wind areas, which require more water due to evaporation, hot weather, and periods of drought. As a general rule of thumb, water your St. Augustine grass lawn 3 to 4 days per week during the summer. 

St. Augustine Grass

3. Too Much Fertilizer 

It’s easy to go overboard with a fertilizer application but as the saying goes “Too much of anything is good for nothing”.

Applying too much fertilizer can lead to issues such as fertilizer burn, which is applying more fertilizer than your grass can process. Fertilizer burn can make your grass turn yellow, wilt, and eventually die.  

How to Fix Too Much Fertilizer in St. Augustine Lawns? 

You should start fertilizing St. Augustine grass roughly 3 weeks after the grass turns green and when there are slim chances of a late frost. 

About quantity, spread 1 pound of soluble nitrogen per 1,000 square feet of lawn every 8 weeks, or 11/2 pounds of slow-release nitrogen every 10 weeks. A 3-2-1 ratio is a good fertilizer number for St. Augustine grass. 

The Simple Lawn Solutions Growth Booster Lawn Fertilizer is rated as the best fertilizer for St. Augustine grass and contains nitrogen, iron, and other essential nutrients for healthy grass growth. 

4. Pests 

Lawn pests such as grubs eat at the roots of your lawn, causing yellow patches, discolored patches, and/or dead patches in your St. Augustine turf. 

Even though proper watering, mowing, and fertilization practices can prevent your lawn from turning bright yellow from dark green, pests and diseases can otherwise overpower healthy lawns. 

Chinch bugs are the leading cause of yellowing grass patches. These bugs may be small in size but can be massive damage to your lawn sucking the sap from the nodes and crown of grass. 

Leading publications claim that St. Augustine grass is highly susceptible and the type of grass most likely to be damaged by chinch bugs and other pests. 

Basic Steps to Eliminate Pests and Insect Infestation from St. Augustine Lawn? 

The first thing to do is identify the pests and insects in your lawn that are causing yellow-infected patches, brown patches, and dead grass because each requires a different approach for extermination. 

To safeguard your St. Augustine grass, natural methods to get rid of pests are a good option depending on the extent of the problem. 

These methods include nematodes, introducing milky spore disease, applying neem oil, and encouraging birds on your lawn. 

Pest Infestation in St. Augustine Grass

5. Mowing with Dull Mower Blades 

It may come as a surprise but mowing with dull mower blades can cause yellowing St. Augustine grass. Dull and blunt lawnmower blades tear the grass and fray its edges, which dries out the tips of the grass, causing it to turn yellow. 

Speaking of which, if you own a gas-powered lawnmower, spilling gas can cause your St. Augustine grass to turn yellow. 

How to Fix Dull Mower Blades? 

There are several ways to sharpen dull mower blades. The easiest and fastest way to get the job done is with a bench grinder. If you don’t have a bench grinder, you can use a flat file to sharpen a dull mower blade but note that this is a long process that does require a fair bit of elbow grease. 

In the event you spill gas on your grass, clean the spillage as quickly as possible, and soak the areas with more than an ounce of water or whatever it takes to dilute the fuel. Next, add an absorbent product to collect the fuel and dispose of it as instructed. 

6. Pet Urine 

Most dog parents and pet parents (I’m guilty of this one) let our pets use the backyard as a personal toilet, especially in the winter. But this practice can have dire consequences after the snow melts such as a lawn speckled with unsightly yellow spots and patches. 

Dog urine for example is rich in a variety of nitrogen compounds but isn’t nitrogen content good for your lawn? Yes, it is, but excess nitrogen will burn the grass and create yellow patches. 

The concentration of nitrogen in dog urine depends on several factors such as the type of dog, its sex, and the dog’s diet.

Female dogs tend to cause more damage to St. Augustine grass than male dogs because they squat and urinate in one concentrated area compared to male dogs that spray their urine over a larger area. 

How to Fix Dog Urine Spots in St. Augustine Grass? 

Preventing the appearance of these unsightly yellow patches, brown spots, and dead spots resulting from excessive nitrogen in dog urine in your yard is easy and can be done in two ways. 

The first way is to change your dog’s diet to include low-protein content, which results in less nitrogen in dog urine.

Additionally, you can try diluting your pet’s urine by watering down the food. You can also give your dog a product such as PAWFECTCHEW, which not only works to lower nitrogen levels in dog urine but promotes healthy UT and bladder function. 

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How To Fix Yellow Grass - St Augustine Grass Turning Yellow

7. Mowing Lawn Too Short 

St. Augustine grass may turn yellow if you mow it too short and here’s why! When you cut the grass too short, it fails to absorb enough moisture, eventually turning green grass blades into yellow grass blades. This typically happens right after you mow the lawn, so avoid cutting more than 1/3 of the grass blade.

What is the Right Mowing Height for St. Augustine Grass? 

The ideal height to cut St. Augustine grass is between 2½ and 4 inches. These numbers depend on your lawn’s growing conditions, where St. Augustine lawns growing in shady areas perform better when mowed between 3 and 4 inches high. 

If you allow your St. Augustine turf to grow past 4 inches, you risk the possibility of mowing too short into the stems and cutting off the nutrient supply. 

8. Root Rot 

Root rot, aka root burn, is a disease that attacks the roots of grass growing in wet and damp soil. If left untreated, root rot will first turn St. Augustine grass yellow and brown and eventually kill the grass. 

Root rot can be caused by fungi and/or poorly drained or overwatered soils. These soggy conditions prevent the roots from absorbing essential nutrients and oxygen, eventually weakening and killing them. 

And weakened roots are more susceptible to fungi such as disease, and pests. The symptoms of root rot mirror the signs of most pest infestations and include wilted, yellowed, or browned grass leaves, stunted or poor growth, and small, pale grass blades. 

Identifying can be a bit challenging because take-all-root rot and brown patch disease are often confused with each other but there are a few ways to tell them apart. Brown patch fungus causes the blades of grass to die off and commonly occurs during the fall. 

Root rot makes the grass blades appear discolored before they die off. Further, take a close look at the roots to see if they’re turning dark brown or shiny black. 

How to Fix Root Rot in St. Augustine Lawn? 

The best way to fix root rot is by prevention but other steps you can take are aerating your turf, applying low rates of nitrogen, and applying high-quality fungicides that are designed to tackle root rot. 

9. High Temperature 

St. Augustine grass has a high tolerance for shade and heat, but a poor tolerance for cold temperatures. However, if it hasn’t rained for weeks, and the weather reaches extreme heat, then it’s just a matter of time before yellow patches will appear on your lawn. 

How to Fix High Temperature in St. Augustine Grass? 

High temperatures are bound to happen. Pay attention to the weather and set your sprinklers accordingly or maintain a good watering schedule. 

10. Poor Soil Quality and Soil Compaction 

This one’s sort of obvious – poor soil does not have the right nutrients for green grass growth but this is a repairable condition once you perform a soil test to see what nutrients are lacking in the soil. 

Once you receive the results from the soil test, spread the right fertilizer or organic material (compost or peat moss) at the appropriate application rates. It may take a couple of weeks to solve the issue of patches of yellow St. Augustine grass. 

St. Augustine Grass Turning Yellow? 10 Causes & Remedies

FAQs

Q. What Causes St. Augustine Grass to Turn Light Green? 

Lack of nitrogen is a leading cause of light green St. Augustine grass. Perform a soil test and replenish the soil with the right nutrients. 

Q. What is the Best Fertilizer for Yellow Grass? 

A. Milorganite organic nitrogen fertilizer is perhaps the best fertilizer for yellow grass. Adding to this, it is also a great option for slow-growing grass or grass that is susceptible to weeds. 

Q. Should You Water St. Augustine Grass Every Day? 

A. Watering St. Augustine grass every day will result in overwatering and root rot. St. Augustine grass requires an inch of water per week either via irrigation or rainfall. 


Final Thoughts 

St. Augustine grass is easy to grow warm-season grass but does require regular maintenance to stay healthy. There are many causes for St. Augustine’s grass blades turning yellow.

But there’s no need to break a sweat just yet because most of the issues can be resolved. Start by sending soil samples to the lab to see what nutrients if any are missing from the soil, and add appropriate fertilizer to replenish the deficiency. 

If that doesn’t and you can’t identify the causes of the problem, my advice is to call professional help. Lawn care companies know to diagnose the issue and the tools to fix the yellowing grass blade issue.

Keep in mind that the reason for yellowing grass blades could be an underlying problem and not correcting the issue (s) can make things a whole lot worse. 

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