Seasonal Cues or End of Lifecycle
All grasses (plant family Poaceae) are flowering plants, and do so to create seeds that will perpetuate the species for future generations. Grasses are either annual, completing its life cycle within one year, or perennial, meaning it returns each year as it continues to grow and repeatedly flower and fall in and out of dormancy. Based on the genetics of the individual grass, the plume-like flowers, called inflorescences, are naturally formed based on the age of the plant, day length, or temperatures.
Generally speaking, grasses form their seeds after their summertime flowers are pollinated by the wind. In annual grasses, the maturation of the plant during the growing season leads to the inevitable flowering and formation of seeds to ensure new grass plants appear next year.
All plants have a mechanism enacted by their genes if certain perceived life-threatening events occur during the growing season. Environmental stresses--conditions that create unfavorable growing conditions--can cause a grass to flower earlier than usual in an effort to form fertile seeds before the plant dies. Drought is a classic example of an environmental stress that can induce flowering and seed production.
Another stress is treatment with an herbicide. In some grass species, being coated with a chemical causes foliage to wilt or yellow, triggering a plant response to try to quickly grow and form flowers and seeds before the entire plant dies. Depending on the potency of the herbicide or effectiveness of its application, a grass may have ample time to grow new shoots to flower while overall the plant is starting to decline.
Lack of Mowing or Grazing
Grasses grow differently than other plants. The stems of grasses are called culms, and when cut, they continue to grow or elongate from a mid-section of their culms called the intercalary meristem. This causes the culm to grow much like how a telescope elongates when pulled apart. Cutting the top of a culm still allows the lower meristem area to elongate, making the grass again grow taller.
Turf grasses are constantly cut, prohibiting their culms and foliage from elongating past a certain acceptable height or spread. Removal of the tips of the grass culms prevents the development of flowers and subsequent seeds. Grazing of meadow grasses by livestock creates an environment similar to weekly mowing.
When lawns are neglected and not mowed, these normally carpet-like grasses quickly sprout their culms and develop flower and seeds, much to the surprise and visual disdain of homeowners. This plant response is completely normal, however.