St Augustine Grass, sometimes referred to buffalo turf or buffalo grass, is a commonly used lawn grass in warm areas.
It is popular because of its adaptability, general drought tolerance, and for most yards you are unlikely to have issues with St Augustine grass.
While it is easy to grow and relatively low maintenance, there are some reasons why your St Augustine grass may turn yellow.
If you are seeing yellowing, or browning, of your St Augustine grass, we identify probable causes and management for a healthy lawn.
Why your St Augustine grass is turning yellow
One thing you absolutely don’t want out of St Augustine grass, especially if you live in Florida, is yellowing lawn. Yellow blades in St Augustine grass, and other lawns, mean stress.
If you’re trying to figure out the situation with your grass, here are some common reasons.
1. Fungal disease
It could be a number of common warm-season fungal diseases that are the root of your discolored lawn.
Gray Leaf Spot
There’s a fungus called Pyricularia grisea (or Magnaporthe grisea, if you wanna be fancy) that causes a fungal lawn disease that thrives in wet weather and high humidity in late summer and autumn.
Some cultivars of St Augustine grass are resistant to it, while others aren’t.
It will cause your grass blades to discolor, slow down growth, and make the lawn thin out and bald as it dies.
Take-all Root Rot (TARR)
Gaeumannomyces graminis var. Graminis is a fungus that commonly lives on the roots of St. Augustine grass.
Most of the time, your yard and this fungus will co-exist just fine if it is moist and humid, or your St. Augustine grass is functioning under lawn stress.
This allows the fungus to spread through the roots and grow upwards through the blades.
You can typically notice above-ground signs of the disease when the roots are highly infected and the disease is difficult to control.
Infected St. Augustine grass will show yellowish or light green patches that can range from several inches to several feet in circumference.
Pythium Root Rot
If the soil has water mold, it will infect and damage the grass roots and blades when a lawn stays wet for a few weeks.
Like TARR, Pythium Root Rot starts out at the roots, as evidenced in discolored, yellowing grass.
The positive side of this is it is only an infection caused by fungi and, although unsightly, it usually will not kill the lawn as the roots do not die.
- If you’re looking to minimize disease, you can try these few things.
- Improving drainage and watering when you observe wilt symptoms, as well as dry soil.
- Avoid excessive watering; you do not want your soil saturated all the time.
- Mowing frequently may be the answer to cutting the grass at the right height (best to remove 1/3 of the leaf blade).
- Using the correct fertilizer (use at and with an equal amount of nitrogen, and potassium).
Treating fungal diseases in St Augustine grass
Get on top of these fungal infections as soon as you see the signs, and spot treat with a fungicide of the infected area of your turf.
Do not step on or mow the infected area anymore. This will spread the disease across your lawn.
You can make your St Augustine grass more disease-resistant by eliminating other stresses on the turf, such as herbicides from weeds or by limiting the duration of watering and leaving the blades wet for long periods of time.
You can also improve the aeration and drainage the best you can.
If the roots are diseased, you can foliar feed the turf to keep it fed and strengthen the grass for recovery.
2. Iron deficiency
If your St Augustine grass is yellow, I recommend looking into micronutrient deficiencies right away.
Iron chloride will cause grass blades to turn pale green, yellow, or even white.
That’s because St Augustine grass requires the micronutrient iron for the formation of the clorophyll, the plant cell organelle that does all the work in photosynthesis.
Iron is also required to break down nitrogen, so it’s easy to see how the deficiency of iron will stress and deteriorate your lawn.
Your grass must access adequate levels of iron from its soil, a disease-free lawn requires as much as 100 parts per million of iron.
If nematodes or fungi eat the roots, the grass’s ability to access iron will be diminished, but most of the time it’s several different soil conditions limiting this availability.
Examples of conditions that limit availability of iron to your soil include:
- Alkaline soil pH: iron chlorosis commonly occurs in soils that have a pH between 7.2 and 8.3
- High rates of supplemental organic matter
- High soil moisture levels
- Soil salinity
- High soil concentrations of phosphorus
- Too high or too low average soil temperatures
Manage St Augustine grass iron deficiency
If you suspect the grass may have iron deficiency, look into the possible causes and address that first.
You can consider soil pH testing, and if require lowering nitrogen or sulfur levels to reduce pH. When fertilizing the lawn, omit phosphorus to increase availability of iron.
You may also add iron as a supplement to the affected soil areas.
3. Too much water
St Augustine grass is a warm season grass and it will be thirsty for moisture in its soil, but too much rain or watering creates a tipping point where problems exist for this grass and yellowing occurs and takes hold.
Keep in mind that St Augustine grass is a coastal grass species that does belong in sandy fast draining soils.
Once you start to get excessive water into the soil of your lawn, it creates several problems:
- Letting fungi and diseases flourish and transmit
- Causing changes in the levels of nutrients, washing them away or leaching them deeper into the soils, beyond the reach of grass roots
- Then starving the roots of oxygen and nutrients, putting them at risk of rotting
Solve these issues by being strategic with the water and nutrients you apply to your lawn.
Make a plan for watering, and never leave your sprinklers on for too long if you want to avoid too much moisture on the blades and roots of your grass.
Using slow-release fertilizers may help reduce leaching. If a lawn becomes waterlogged, one method to boost drainage is to aerate the lawn on a regular basis, and/or install a drainage system.
You may also want to consider relaying the entire lawn, and use sand with the soil before you replace the turf.
4. Drought
Okay, folks, even though St Augustine grass gets planted a lot in warmer states, it’s actually not as drought tolerant as you may think, and your lawn may turn yellow due to distress in the summer heat.
Let’s face it: If you’re managing the turf during the upper end of its temperature range of 80 to 100°F (26 to 37.8°C), you should know that the grass will decline quickly without an adequate supply of water.
With hot days, the topsoil can dry, crack, and pull away from the grass, limiting the absorption of essential micronutrients, creating yellowing of the leaf blades.
When you’ve had a period of extended hot and dry weather, you certainly need a dedicated approach to protecting and maintaining your lawn.
Here are some great strategies you can try:
- Try to water your lawn six or more times a week between 4am – 6am. Increase the length of the watering period your lawn needs if it’s still thirsty.
- Don’t water your lawn in the heat of the day because the grass water will evaporate before the grass can drink. Don’t water your lawn the night before because it will cause an outbreak of fungal disease.
- Allow your grass to grow taller than normal. This will allow deeper rooting and will allow your lawn to hold moisture.
- When you mow, make sure your blade is really sharp so the grass can be cut clean and the grass will lose less water.
- Leave your grass clipping on the lawn as mulch to help the lawn retain water.
- Do not fertilize because it makes the grass consume water.
5. Nitrogen balance
Either too much or too little nitrogen can make your St Augustine turn yellow.
If you look closely, you’ve probably noticed that if a dog pees on a lawn, it yellows or kills the grass. This happens because dog pee is rich in ammonia (nitrogen) and it can actually burn it.
Excessive nitrogen from using Nitrogen feed can have the same effect.
Too much nitrogen in the soil makes the lawn more susceptible to diseases and builds up thatch which inhibits aeration and lawn irrigation.
On the contrary – having too little nitrogen can be just as bad for your St Augustine grass. Along with yellow leaves you will get a thin growth of grass.
Heavy rains and irrigation become an issue because they leach nitrogen and other important nutrients from the soil leaving the lawn short.
This can be solved by using a slow release or water-insoluble nitrogen fertilizer that will not leach out of the soil.
Rounding up
Well, there’s a bunch of reasons why your St Augustine grass is yellowing and most of the main culprits are simple to eliminate.
Your turf grass type is not a delicate turf grass, but it can still use some love!
If you are vigilant with your lawn care it can help you get ahead of the issues that can cause yellowing, and in the event of it happening can help halt the growth of yellowing, until you get the grass back on track to grow.
Sure there are better things to do with your summer, but try a little extra effort, or the labor of a mulching robot mower, to keep your St Augustine grass gracnefully growing!