Why Is My St. Augustine Grass Turning Yellow? (Read This First)

St. Augustine Grass is generally supposed to be a nice, rich green. It certainly can be, if treated right. But sometimes it may start to turn lighter in color or even change color completely from green to yellow. 

So what does it mean when your St. Augustine Grass starts to change color, and how do you fix it?

When St. Augustine Grass starts turning yellow, it is usually a sign of a lack of iron, improper nitrogen levels, overwatering, or irregular temperatures. If your grass is still green but just turning a lighter shade, it may be a sign of pests, a nutrient deficiency, lack of sun, or even root rot. 

Always try to identify the problem before taking steps to solve it, or you may be unintentionally harming your grass more than helping it. For example, if your grass just needs some sun, you don’t want to add on drought stress by incorrectly assuming the problem is overwatering and subsequently not watering it as much as you should. 

Why Is My St. Augustine Grass Yellowing or Browning?

If you notice that your grass is turning yellow or even brown, consider some of the following potential causes: 

Iron Deficiency

Iron deficiency can turn St. Augustine Grass yellow or light green. Usually, too much phosphorus or a high pH level can cause the roots to be unable to absorb iron properly. Try to find fertilizers low in phosphorus to help out your grass. 

Too Much or Too Little Nitrogen

pouring fertilizer
Both an excess or lack of nitrogen can cause your St. Augustine Grass to turn yellow or brown. You want to apply slow-release nitrogen to prevent too much from being absorbed by your plant all at once. Generally, in the summer growing months, apply fertilizer every three to four weeks. In fall, winter, and spring, application can be even less frequent. 

Too Much Water

St. Augustine Grass can’t handle drought and it needs constantly moist soil. However, it can easily become overwatered. Too much water can lead to root rot which will in turn leach the nutrients out of your soil. 

Make sure you only water to the point that the soil is moist, and not soaked or with water sitting on the surface. The amount you will have to water depends on the type of soil you have and other environmental conditions. 

Irregular Temperatures

St. Augustine Grass is designed to grow best in areas with a high average temperature. Its optimal temperature is between 80 and 100°F (27 to 38°C). Too much warmer or colder than this can kill your grass. 

Why Is My St. Augustine Grass Turning Light Green?

If your St. Augustine Grass is turning a lighter green than you might expect, the problem may be caused by root rot, nematodes, nutrient deficiency, or even a lack of sun. It usually is a precursor to yellow grass and is a sign that your grass isn’t very happy. 

Root Rot

If your St. Augustine Grass stays too wet, or if you overwater it, the chance of root rot developing increases. Root rot can cause your grass to develop a light green or yellow color. Eventually, the roots will die off, turning white and black. 

While root rot may start in a patch or specific area, it can quickly start to spread and kill the rest of your grass. 

Once root rot spreads, it can be hard to get rid of. Fungicides are a good option to help prevent the spread and stop the rot in its tracks, but it doesn’t always work on grass that is already starting to get infected and if the disease has spread beyond the roots. 

To prevent the issue from happening in the future, you need to be aware of how you mow. Generally, you never want to cut more than one-third of your grass height off at a time. This prevents water issues and stress. 


Nematodes are little worms that enjoy attacking and eating grass. They are one of the most abundant animals on earth and can be found in soil, water, vinegar, and even in the earth’s crust. Many of them are beneficial to people and plants, but there are unfortunately some that get a kick out of eating the root systems of grass.

To test if nematodes are the problem, there are places where you can send soil samples so they can check for you. 

It isn’t easy getting rid of nematodes. Generally, the best thing to do is to just make sure your grass is getting what it needs to thrive and be happy. Don’t let your grass get too short, usually no less than 3 or 4 inches (7.5 to 10 cm) in length, and make sure that you are properly fertilizing your grass and providing it with plenty of potassium so that it is healthy. 

Nutrient Deficiency

Buffalo grass
Speaking of nutrient deficiency, that is another reason your grass may turn light green. While St. Augustine Grass needs a variety of nutrients to stay healthy, iron and nitrogen are especially important to maintaining the right shade of green. If the soil is too sandy, or there just isn’t enough nitrogen or iron in the soil, your grass may be lacking in nutrients. 

If you’d prefer not to test for nitrogen levels, you can get away with just using a slow-release fertilizer in the soil to make sure it has the boost it needs. 

However if you think your iron might be low, we do recommend testing it first. If it is low, you can get a ferrous sulfate treatment or you can do it yourself. Generally, 2 ounces of liquid iron with 3 to 5 gallons of water per 1000 square feet of lawn is what you will need to give your plants a boost of iron. 

Lack of Sun

Finally, too little sun can also cause your St. Augustine Grass to turn a light green. If your grass isn’t getting enough sunlight, try to trim trees or bushes in the area to let your grass receive a bit more light exposure. 

If you don’t want to do that or are unable to, it is best to look for different grasses for the shadier areas of your yard, and stick to St. Augustine Grass only for areas where there is a lot of light. St. Augustine Grass does best with at least six to eight hours of sunlight a day, but it can handle as little as four hours as long as it is direct light. 

How Often Should You Water St. Augustine Grass?

St. Augustine Grass does best when watered so that the soil is wet to about 6 inches (15 cm) down. After watering, you should generally wait until your grass shows signs of stress before watering again. 

This is usually about every five to ten days. You want just the earliest signs of drought to show, such as the grass turning more of a blue-ish color, or your footprints staying visible on the grass for longer than normal.

If it is rainy in your area, you may not have to water very often at all. 

Will Yellow St. Augustine Grass Grow Back?

Once the grass turns yellow, it usually means that the grass is in the process of dying. It is hard to bring back grass once it is dead or in the process of dying. However, if you clear out the dead grass, fix the problems that caused it to die in the first place, and take good care of it, you should have new, healthy St. Augustine Grass growing back in no time. 

How Do You Turn St. Augustine Grass Green?

If you want your St. Augustine Grass to stay or turn into a bright, rich green, the key is to give your grass what it wants. Make sure you don’t over-mow your grass, keep it watered according to its particular requirements, and provide it with access to lots of sunlight and the nutrients it needs. If you do this, you will have vibrant St. Augustine Grass in your yard. 

What Is the Best Fertilizer for Yellow St. Augustine Grass?

For St. Augustine Grass, fertilizers that are slow-releasing and high in nitrogen are best. Typically, your nitrogen, phosphorus, and potassium (NPK) ratio for St. Augustine Grass should be about 15-1-5

Are Coffee Grounds Good for St. Augustine Grass?

As long as your St. Augustine Grass isn’t already receiving too much phosphorus, then coffee grounds can be an excellent fertilizer. Coffee grounds, especially used coffee grounds, are a natural slow-release fertilizer filled with nitrogen, phosphorus, and more. 


St. Augustine Grass can be a little finicky to take care of. It doesn’t want too much water, but too little can lead to drought stress. Also, it doesn’t do well with too much – or too little – nitrogen, phosphorus, and iron.

Your St. Augustine Grass may be turning yellow due to an iron or nitrogen deficiency, overwatering, or temperatures higher or below 80 to 100°F (27 to 38°C). It’s important to know that if your grass turns light green, it may be due to root rot, nematodes, or a lack of sunlight.

Sometimes, it may seem almost impossible to keep your St. Augustine Grass healthy, but with enough care and attention, you will find yourself with happy, thriving grass – a beautiful, vivid green blanket for your lawn.