Houston usually has plenty of rainfall so sprinklers have been unnecessary. With that in mind withholding water is your main control for weeds. Seed sprouting require frequent watering whether they are grass or weeds. Since you recently had a new sprinkler system installed, I'm going to predict that the installer set it up to water for 10-30 minutes every day. They love to do that, but that is exactly what weed seeds need. A better routine is to water an inch (however long that takes for your system). In the hottest heat of summer the frequency for watering should be about 5-7 days. This time of year you can stretch that out to monthly watering. By watering deeply the roots can get water for many (many) days from the moisture in the deeper soil. By letting the surface of the soil dry out between watering, the seeds do not stay moist enough for long enough to sprout.

A second approach to fixing weeds is to allow the grass to become as tall and dense as possible. For St Augustine if you allow it to grow up as high as possible, it will cast the most shade on the soil and help prevent weed seedlings from taking root if they do happen to sprout. You can raise your mower's height all the way and weld it in place. That height is about 4 inches. Allow the grass to get to about 5-6 inches and mow it back to 4.

I just bought a new house in a desert area. I'm trying an experiment by letting much of the grass grow unmowed. Some of it is knee high right now and blows in the wind. Although it looks unkempt, the color is excellent. I have another area where I have mowed it down to 4 inches. The mowed area has burned tips and looks much less healthy than the unmowed grass. Next part of the experiment is to withhold water in the tall grass area. Fortunately I don't have neighbors nearby to complain about the yard. But another side benefit is there are no weeds in the tall grass areas.

Yeah, mine needs help too, but for a different reason...which, what is the reason?
I watered it regularly, friend fertilized in the summer, it got v. green. Great. Now it is browning on the tops of the blades. Does it need fertilizer or is it just because it's Fall? I haven't watered in a while. I'm a lawn greenhorn. Do I continue to water even tho it's now cold out (hasn't been raining in a couple weeks)? Gets up to 60 in the day (yeah, Calif is a crazy place!)

ekaw, you should start a new thread with your concerns. People looking here want to keep up with the originator's issue. Your issue is different so you should start a new topic.

Here are some general elements of lawn care but first it would help out to know where you live (I assume you live in the zip code you posted in your profile), what kind of grass you have, what kind of soil you have (sandy, clayey, etc.), how often you water and for how long, how high/low you mow and how often, and what products have you used on your lawn in the past 3-6 months.

Some grasses get brown tips that turn to brown grass as winter causes it to become dormant. Some other grasses are expected to remain green all year.

Generally in the cool months you can back way off on watering. I see you live on the dry side of the hills, so you might have extremely low humidity. Very low humidity might require more frequent watering. This time of year in South Texas, if my grass has gotten rain sometime in the past 30 days, I'm not watering it. When I do water, I will water it for hours and hours until I get a full inch of water on it.

Summer is not a good time to fertilize UNLESS you are using organic fertilizer. Then you can fertilize any day of the year. Chemical fertilizers in the summer heat can burn the grass or roots. Most grass needs most of its nitrogen in the fall. If you know how much it needs, apply 2/3 to 3/4 of that amount in two apps in the fall (Labor Day and Thanksgiving are easy dates to remember). Apply the rest on Memorial Day. With organic fertilizer you can apply any day, or every day, all year long. It works completely differently from chemical fertilizers and does not burn the grass.