Weeds are opportunist! Meaning they grow when the conditions (opportunity) is right for them. Too dry, too wet, poor soil drainage, compacted soil, temperature, etcetera, weeds will find a way! Why do weeds invade your lawn?
Mowing too low and too infrequently thins the turf, allowing weeds to get started.
Frequent watering encourages weed seed germination, disease, thatch, and a shallow-rooted turf that is less competitive with weeds for soil moisture and nutrients.
Fertilizing too much, too little or at the wrong time may benefit weeds more than grass.
Insect and disease injury.
Weeds rapidly invade lawn areas that are thinned by insects and diseases.
Soil compaction is a hidden stress on the turfgrass root system. The grass is unable to compete effectively with weeds. Clay and silty soils are especially prone to compaction.
Turf areas that are used for recreation and sports are subjected to wear and compaction. Weeds become a problem in these areas, requiring intense weed control and turf management.
Wrong kind of grass.
The wrong kind of grass for the location will gradually decline and be invaded by weeds.
Weeds often take over a lawn after it has been weakened and thinned from weather-related stresses.
Excessive thatch causes shallow-rooted grass and contributes to insect and disease problems, which are followed by weed invasion. Thatch also can reduce the effectiveness of some soil-applied weed control chemicals.