First of all, watering and clean sprinkler heads that rotate are important to the health of your lawn. Makes sure your Hunter heads have a rotating pattern when you turn them on. In many cases underperforming heads are clogged and need to be cleaned out. I pop them up and put a large envelope clip on them before turning the heads for cleaning.
Next, weeds gain only be eradicated by using 41% glyphosate. Anything else will kill your grass.
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The most important thing to remember is St. Augustine can’t be seeded. It’s a runner. You can buy plugs or cut out bad turf and soil and replant. That’s about it. Black Cow soil is great, but it often brings the weeds you don’t want.