I'm in Bandera, so not that far away from you in drive time, but temperature and rainfall wise, we could be in different states. I used to live in George West, which is a lot closer to your climate than Bandera.

Your picture #2 looks like there is some deadish yellowish spots in the foreground. Today's a bad day because it is so sunny, but maybe as the Sun goes down you could take a very close up picture of that grass. I'm looking for any signs of yellow or brown lesions on the blades. Generally it looks good, very good in fact, so I'm at a loss as to why it is thin in there. The only reason I might suggest is lack of water over a long time. The area under the tree should become very lush quickly with proper watering.
About your tree: you wasted your time raking all that stuff up, but no harm done. Do you have any ball moss in the tree? Ball moss does no harm to the tree, but it does indicate how long it has been since the tree was trimmed and thinned. If you have a lot of ball moss then you likely have a lot of dead limbs inside the canopy. Live Oaks only really thrive at the outside surface of the tree. You can remove all the leafless limbs on the inside and thin out most of the new growth. Doing that will give you more sunlight for the grass to work with.
Are you familiar with deep and infrequent watering from reading this forum? Deep=1 inch. Frequency depends on daily high temperature. If not say so and I can explain it. Or search the forum for deep and infrequent. Do you have any grass out in the full sunlight?

Mowing: Always mow St Augustine at the mower's highest setting. There is never a good reason to scalp it or even lower it one notch.

Fertilizer: If you are going to use organic fertilizer, you can do that now at 10-15 pounds per 1,000 square feet. That's about half the bag rate. I suspect the sellers did nothing for a good long time, so the grass will use the fertilizer wisely. But then in late May (Memorial Day) apply again at the full bag rate. When you run out of Medina, go to the feed store and get either ordinary corn meal or alfalfa pellets. Mumme's in Hondo has it at pretty good prices, but if you're in Uvalde or west, you likely don't have to drive all the way to Hondo. I use these because they work just as well as the commercially bagged organics and cost about 1/5 as much per 1,000 square feet.

Weeds: If you have weeds of any kind, NOW is the time to apply. Wait for the wind to calm down. The product to use is one like this...

The purple label contains atrazine and is only for St Augustine and centipede lawns. Use it one time per year in mid April (NOW). Read the directions, all of them, three times before you use it. I walk backwards so I don't have to get the stuff on my shoes. It takes three full weeks to see that all the weeds have disappeared. Not only that, they won't come back next year. It seems to kill the weed seeds on poa annua and horse herb. If any return next year, it's because you missed a spot this year. Ortho has an atrazine product. I liked the hose end sprayer on this one. The newer ones are much better than the one pictured.

For one thing, St. Aug needs sun, and I see a lot of shadows in the pictures. For another, if these lawn areas are under oak trees, you can assume that the oaks are competing with the lawn for nutrients. I used to wonder about that because, hey, oak tree roots should be VERY deep. But a horticulturalist reminded me that for a regularly irrigated lawn, the tree has lots of roots near the surface, simply because that's where the water is (and fertilizer as well). That is, a tree on a cared-for lawn is built very differently than a tree in the wild. I have pretty bare areas under all my oaks.

Thanks for the reply dchall and dannithedirt I've included some pictures of those same areas with the sun goin down as well as some close ups of the areas of concern. I don't notice in my own observations any brown or yellowing of the blades of grass the color is actually a pretty vibrant green. The trees don't have any ball moss and or observable dead branches though I think I could probably do some thinning of some of the new growth and allow as much sunlight as possible through. Ill include some pictures of the canopy as well. I'll also include a picture of the west facing side of the lawn which gets the most time of direct unobstructed sunlight, The areas of concern are mainly on the East, southeast portion of the lawn. Ive read through a post where you explain the deep infrequent watering and i think I'll start there along with fertilizer. I've just been doing my watering by what felt right which has been setting the irrigation system 20 min per station bout 4 times a week depending on rain the deep infrequent makes more logical sense appreciate the direction on that. What is your thought on beneficial nematodes. I've seen a few Small June bugs. All in all I'm fairly pleased with the lawn just don't want these areas to get outta hand. Thanks again for your insight

Same spot setting sun

Close up

Close up

Most direct sunlight thick n vibrant in color

St Augustine is well known for doing well in pretty deep shade. I lived under a tree like yours for 25 years in San Antonio. You get more afternoon light than I ever got. You should be fine. The organic fertilizer will help as will deep watering and mowing high.

Regarding your mower, check it to be sure it is turning the right direction. After about 15 years I decided to check my old Black and Decker and found out that the blade had been installed upside down. I had been breaking the grass blades off rather than slicing them. If it seems to be turning toward the slicing edge, you might need to sharpen the blade. If it is turning toward the blunt edge, flip the blade over.

I'll be specific. St. Augustine tolerates some shade, but really needs five hours of direct sun to do well. That is, it'll live with less, but it will be thin and spindly. St. Augustine is actually one of the more shade-tolerant grasses. That doesn't mean it won't do better with lots of sun. The question you need to ask yourself is whether your problem is in the shadiest part of your yard.

I can be specific, too. My front lawn in San Antonio got 0.0 hours of direct sunlight for the 25 years I lived there. It was entirely filtered light and, too often, the trees blocked far too much sunlight. Yet the grass did not look thin and spindly. Even Floratam, a sun loving variety of St Augustine, did fine.

Having said that, Floratam in the full sun is extremely dense. I mean extremely. Now that I have Floratam in full sun, I'm happy to have a self propelled mower. So by comparison it becomes thin in the shade, but to look at it, it looks fine. There are other varieties which are more dense in the shade than Floratam. If you want to mix varieties in your yard, you would likely have to visit San Antonio (Milburgers Nursery).

Take it from Neil.

https://neilsperry.com/notes/2017/02/st-augustine-diagnostics/

https://www.mysanantonio.com/life/article/St-Augustine-grass-grows-best-under-shade-trees-3984943.php

This is consistent with my experience. The title of the latter is misconstrued, and was certainly not written by Sperry. He's saying that St. Augustine is the best grass to grow under shade trees. Not that it grows especially well under shade trees.

He'd probably like to see your setup.

For those of you who are unfamiliar, Neil Sperry is a Texas based garden guru. He has a radio show and is syndicated in many of the big news papers in Texas.

Neil Sperry is the bane of my existence. Whatever he says, I do the opposite. I followed him from about 1992 to 2002 and realized my grass looked worse every year. Then I switched to organic and learned a lot about lawn care on this and other forums. He, apparently gets all his information from sponsors selling products. I could make a living following him around to fix the mistakes he has people making.

Sunlight - Neil says, St. Augustine needs 4 hours of sunlight daily just to survive, and 6 hours of hot, direct sunlight if you hope for it to grow vigorously. Again, my front lawn in SA had zero hours of sunlight per day for the duration of 25 years. It was under the canopy of several very large live oak trees. For 10 years it deteriorated, and for 15 years under organic fertilizer, it looked great. Here is a picture of my grass grown in the most dense shade in that yard.

Looking straight down reveals all the flaws in the turf. What you can see is the grass was growing outward and not upright. When it is in full sun, it grows more upright driven that way by the sheer amount of grass. Here is the same yard from an angle.

The house faced north and this is the front yard. There is a huge tree to the left of the walkway and two large trees in the bed to the right. This yard got no direct sunlight at all. You can see grass invading between the bricks and over the concrete. It is not doing poorly. Neil Sperry would say this is impossible.

Here is a picture of my current house - back yard, south facing slope in full sun. The grass is dense.

You can see the grass needed fertilizer, but besides that...in the foreground is weeds mowed at the mower's lowest setting. You can see the St Augustine runners expanding their territory into the weeds at a rate of about 15 feet per year. You can see the hose and a long handled yard tool sort of buried in the grass. The only reason you can see them is the grass is too dense for the hose and tool to sink farther down. You can tell the depth of the grass on my dog's legs. They are buried to the hock.

Drought - Neil says, St. Augustine needs more water than most of our other lawngrasses (fescue excepted). If you let it get too dry, and especially if you do so repeatedly, St. Augustine will be weak, and it may even die away. This is about the worst thing you can say about St Augustine. The main reason is it is bordering on untrue. It "may" die away. St Augustine is not bermuda. Bermuda will come back from no water for a year. Here in Bandera I had a side yard where the St Aug was golden brown. It had been that way for months before I bought the house. I figured it was dead and neglected it for another year, which was a drought year. But when I watered it, the golden runners came to life. It was solid St Augustine. Had this grass been in full sun, it "may" have died away, but for whatever reasons, it came back after getting no irrigation from June to March.

Here is a picture from my previous house in George West, TX. The grass is 32 inches high and had not been watered for many months.

You can see the top of my dog's tail and her ears (different dog). When you mow St Augustine high, it needs less water. If you don't mow it at all, it needs no water to thrive.

Please do not let the so called drought intolerance deter you from St Augustine.

Fertilizer - Neil says, in part, ... Most clay soils have excessive amounts of phosphorus (middle number of the analysis), so soil tests usually suggest a high-quality, all-nitrogen fertilizer be added. What the hell? Who has clay in Texas??? Okay, now the OP describes his location as between San Antonio and Del Rio. As it turns out, there is a pocket of clay soil in D'Hanis, TX, along that route, so this might actually pertain, TO HIM, but almost noplace else. Clay soil is a red herring. Get the soil tested if you absolutely cannot grow anything. Otherwise, go with organic fertilizer and see the miracle unfold.

Disease - Neil says, Gray leaf spot is a fungal disease that shows up in mid-summer into early fall. You’ll see diamond-shaped, gray-brown lesions on the midribs of the blades and occasionally on the runners themselves. This disease is most prevalent following applications of nitrogen. In fact, the additional symptoms of gray leaf spot are yellowed leaves that look like they need more nitrogen. I get gray leaf spot any time of year. My lawn woke up with it this year in early spring. Caused by too much nitrogen? I get it after months and months of no nitrogen at all. Like now, for instance.

The lawns on his St Augustine Diagnostics page are all mowed way too short. Between that and his comments on drought scaring people into watering too much, it's no wonder people have problems with St Aug.

Oooh, brutal. But this particular advice doesn't pertain to any commercial products. His "Complete Guide to Texas Gardening" is a classic, and widely admired.

Sunlight? Your experience is different from mine and Sperry's. I'll leave it at that.

Drought? I've seen St. Aug die from lack of watering. Nothing left but brick-like soil. Dead as a doornail. But if you have a high water table, it can survive.

Fert? Well, if you don't have clay, that advice isn't even pertinent. But a heavy dose of N is always appreciated.

I don't have any experience with St. Aug disease, so I recuse myself on that point.

Oooh, brutal

It's gotta be said.

I have killed St Aug in direct sunlight by withholding water. What I described was in the shade of another kind of oak. Apparently the shade made a big difference.

As for sunlight, Dan, do you use organic fertilizer? Mine in the shade was pathetic until I changed to organic. Again, pathetic was before I went organic and I was following Sperry's newspaper advice on watering, mowing, and fertilizing. The pictures of mine above are all after going organic.

Interesting reading well as of yesterday I sharpened my blade ( I should have known it was in need of a sharpening) raised my mower deck to the top setting. It was a notch above halfway. Gave the yard a nice even trim. Fertilized with the Growin Green at 13 lbs per 1000 and watered it deep to one inch. Kids had to eat tuna and sardines so I could have me some measuring containers 🤣🤣🤣. So today's my starting at square one with the watering. We had about an inch and a half of rain bout ten days ago probably a lil early on the watering vs the temps here lately but the fertilizer said to water it in after applying. Speaking of gardening gurus with radio shows I'm sure you've heard of Bob Webster from Shades of Green Nursery on KTSA he is all about organic gardening. I really enjoy his show. Never come across the other guy y'all referred to.

Organic schmorganic. Nitrogen is nitrogen. I use ammonium sufate. I can't imagine that organic nitrogen fertilizer is going to change how grass responds to sunlight. Now, organic fertilizer is pretty low intensity stuff. So it may be that you were over-fertilizing and burning the foliage. In this context, note that

"Moderately or heavily shaded areas should not be fertilized as much as areas in full sunlight. Grass growing in shade is more succulent and has a weaker root system than grass growing in full sunlight. Fertilizer tends to make the grass more succulent and increases its susceptibility to disease, drought and other stresses."

https://aggie-horticulture.tamu.edu/extension/ornamentals/lawnfert/lawnfert.html

I use ammonium sulfate as a slug and snail killer, because it is pure salt. It kills them overnight, literally. But once you killed them out, you still have a lifetime supply of ammonium sulfate in the garage.

Dan I suspect you have no interest in learning about soil biology and how organic fertilizers work. But if you did, and you had a little time on your hands from not having to rush off to work, then you might be interested in reading this primer written by Dr Elaine Ingham and published by the USDA back in 2000 or so. To quote from the linked page, "The online Soil Biology Primer is an introduction to the living component of soil and how it contributes to agricultural productivity and air and water quality. The Primer includes chapters describing the soil food web and its relationship to soil health and chapters about soil bacteria, fungi, protozoa, nematodes, arthropods, and earthworms." Seriously, get a cup of coffee and get comfortable. It's about 50 pages and not light reading.

So after reading about soil biology, I learned that the point of organic fertilizer is not simply to feed the plants (nitrogen is nitrogen), but more to improve the health of the soil. Salty ammonium nitrate cannot improve the health of the beneficial fungi, bacteria, arthropods, etc.

Carl, Bob Webster is a local treasure. I used to live about a mile from his store in San Antonio. Now that I am in the country, I am one of his neighbors. And now I understand what he was talking about regarding deer resistant plants.

Who has clay in Texas???

Sperry has been based for decades around McKinney in the heart of the northern end of the Texas Blackland Prairie, so much of his widely quoted advice can have an underlying clay soil/north central Texas climate bias.... just something to be aware of.

dchall, What in your organic program do you think accounts for the St Augustine improvements in sun and shade?

bostedo,

I think I had killed my soil biology with synthetic fertilizers, herbicides, insecticides, and fungicides, as recommended by Sperry and Jerry Parsons from TAMU. Chemical fertilizer had completely stopped working. Once I changed and used corn meal to fertilize, the soil health improved and the grass seemed to explode in both front and back. So I stopped reading Sperry and Parsons and started listening to Bob Webster on the radio. Everything turned around for me with the organic fertilizer, but I still had to unlearn a lot about watering and mowing, which are much more important than fertilizer. Take a look at the Soil Biology Primer linked in my previous msg here. It really explains a lot.

Please. No lectures about organic gardening. I largely do it, and I am quite familiar with soil biology. I just think that organic fertilizers (aside from compost, which is not really an NPK fertilizer) are unnecessary. Compost hugely improves improves the health of the beneficial fungi, bacteria, arthropods, etc. but it is silly to consider it a real NPK fertilizer.

"Salty" ammonium nitrate? Soluble salts are not good for plants in large quantities, but they leach very effectively, and are rarely a problem in garden soils. My geology is limestone, so my soluble salts from that certainly dominates any soluble salts I apply.