Tenacity (mesotrione) is now labeled for sports fields in the state of Ohio (check with local suppliers and County Extension Agents for other state and country regulations).
Tenacity is a herbicide that has both pre-emergence and post-emergence efficacy on weeds, while causing negligible injury to the desirable grass. Research undertaken by Ohio State over the last few years has shown that Tenacity can be used safely in the following scenarios:
As a preemergence herbicide. Applied at time of seeding, it will prevent broadleaf and grassy weeds from germinating and taking over the seedbed. Our research on the three main cool-season grasses for athletic fields (ryegrass, tall fescue and Kentucky bluegrass) has shown that it is safe to use on desirable grass and extremely effective at preventing weed germination.
In this particular study the seedbed had been prepared by using a tiller/renovator to turn over the soil to a 4-inch depth prior to seeding. Note! Disturbing the soil prior to seeding at a time when weed pressure is high is not recommended! Soil disturbance coupled with optimal conditions for weed growth usually results in seed beds with >90%crabgrass, goosegrass and nutsedge. In this particular instance, Tenacity was applied on the same day the plots were seeded and no weed seeds germinated.
As a postemergence herbicide. Applied on newly established areas, it will remove broadleaf and grassy weeds. Weeds treated with Tenacity are typically bleached pure white, then they shrivel and die. Control is around 80-90% with one Tenacity application. A 2nd application may be necessary to completely remove aggressive weeds like goosegrass. Reducing the bleaching effect is something turf research programs are looking at.
Other weeds/grasses that may be controlled with Tenacity include nimblewill and creeping bentgrass. Research programs are also looking at the control of Poa annua and Poa trivialis.
Posted by Pam Sherratt & Dr. John Street, www.buckeyeturf.osu.edu