A: Once you brought in a sample of this weed to the office with a seed head it was much easier to recognize. Your weed is one of the aquatic weeds commonly found in lawn grass called Purple nutsedge, Cyperus rotundus. It is a perennial, which means it has more than one growing season. Purple nutsedge can reproduce by seeds and underground rhizomes. The rhizomes enable the nutsedge to spread throughout the lawn.
It is easy to become over confident when hand pulling these weeds. You yank out large lush green foliage with what appears to be a small white root structure thinking the weed has been destroyed. But you would be mistaken as this plant produces a tuber which must also be dug up or the plant will continue to be productive.
Purple nutsedge, along with other common sedges such as Globe sedge or Yellow nutsedge, are difficult to manage. They also tell us a few other things about what may be occurring in your lawn. Sedges are aquatic weeds; this means they are indicators of the lawn receiving too much water. Therefore, reducing the amount of irrigation your lawn receives should be one of the cultural practices you adopt to better control their growth.
Chemical (herbicide) application will not completely control these weeds. It is important to combine chemical control with cultural changes to best manage these weeds. In addition, please focus on keeping the lawn grass healthy as the stress of too much nitrogen combined with too much water weaken the grass and enable the weeds to reproduce quickly. Attached is a publication by the University of Florida regarding weed control in home landscapes to better assist you with managing weeds. http://edis.ifas.ufl.edu/EP141