Differing soil or water/drainage conditions.put down some compost on those areas that will increase the organic material and soil conditions. A half inch layer will do nicely. If your house is new it might just be a sandy area. the compost will help even if it is a drainage issue but not as well. You can buy it for pretty cheap.
I had a buddy who had a similar problem. As it turns out, the home builders buried their garbage in the yard. This is quite common: home builders bury their scrap debris in the yard. After my buddy dug up all the trash (soda cans, cigarette packs, scrap metal, cement, wood, etc.) and filled the spot back up with soil (followed by sod) the grass was okay.
It is also common practice for them to scrape up all of the topsoil and sell it off leaving you with nothing but sand. Guys with new construction should be applying compost annually.
St. Augustine loves shade and can thrive in it. The first rule in taking care of St. Aug is mowing higher. the reason your grass is not growing in problem areas is because it is in dormancy. Mow everything higher now. when it comes out of dormancy continue to mow higher. It will save you money in water bills. If after changes the mowing and watering habits the shade shows to be the obvious problem, the only thing to do is to switch to a alternate ground covering. These are only needed in extreme shade circumstances. If the area you are talking about gets any kind of sun, the problem is the mowing habits. It may look weird at first but 3-4 inches is the magic number for this grass.
Yeah in thin layers it will grow right through it. 1/4" to 1/2" You just spread it out right over. It gives you all sorts of wonderful benefits. I just want to add this compost thing is elite-level lawn care. With proper mowing, watering and fertilization you could grow St. Aug in a sandbox. composting annually just makes your lawn soil that much more awesome and drought-heat-stress resistant when you do it for a while. It can also help with bad soil areas.
Yeah, I'm in Sacramento and our lawn's not St. Augustine, but we have a couple of trouble spots, that I think might be due to poor soil. I remember my lawn as a kid in Bellaire was thick St. Augustine. We never watered or fertilized and that stuff was like a jungle.
Casey- do you think it's too hot to throw out some compost. Will it burn in this 100+ heat? Is it best to do spring/fall or is now ok
All of my advice in this thread has dealt with St. Aug so do your own research for your type of grass. The number one thing to check with any grasses trouble spots is watering though. Either poor drainage from the grade of the land or the soil type, or not covered by your watering. Benefits of applying compost come long term. You cannot change your soil quality overnight so I wouldn't do it. Not to mention it is hot to actually do the labor. Compost is very low in nitrogen though so I wouldn't worry about it it burning your yard. All organics are very forgiving in that regard.
