The best way to control dallisgrass in St. Augustine grass is to use a non-selective herbicide. However, you must realize that any of the non-selective herbicide that comes in contact with the St. Augustine will kill it as well. You will probably have to replant plugs of St. Augustine grass in the dead spots. The best time to spray for dallisgrass in the Dallas area will be late April to early June. Contact your local Cooperative Extension office for herbicide recommendations. Also, keep in mind that even if you kill all the dallisgrass plants in the lawn this spring, more plants can come back from seed later this summer or again next year.
If you have a serious problem with dallisgrass, we would have to question how good a job you are doing in maintaining the St. Augustine lawn. A serious weed problem indicates that the lawn is under stress. If you are mowing, watering, and fertilizing the St. Augustine grass correctly, then dallisgrass shouldn't be a major problem unless you have some other problems such as compaction, thatch, or too much shade.
