Janet,

Bentgrass (Agrostis spp.) can be challenging to control in home lawns because it spreads aggressively and tolerates close mowing. One of the more effective selective control options is Tenacity herbicide, which contains the active ingredient mesotrione. Mesotrione is derived from a natural compound found in the bottlebrush plant (Callistemon citrinus) and works by inhibiting carotenoid production in susceptible plants. Without these protective pigments, the plant tissue becomes bleached and eventually dies.

Bentgrass is sensitive to mesotrione, but control is not immediate. Multiple applications (4 or more) are typically required to achieve acceptable suppression or removal. Typically in Oregon we just get suppression of the bentgrass and not complete removal.  You’ll often see whitening of the bentgrass foliage within 1–2 weeks after application, which indicates the herbicide is working. 

For best results, apply Tenacity according to the label directions and plan on a series of applications spaced 2–3 weeks apart in late summer to early fall. This approach can selectively remove (*suppress) bentgrass from desirable turf species such as tall fescue or perennial ryegrass with minimal injury.

Because bentgrass removal can leave thin or bare areas, it’s important to combine herbicide applications with a reseeding program. This timing provides optimal conditions for establishing desirable turf and helps prevent reinvasion of bentgrass or other weeds.

You may also want to consider cultural management practices such as dethatching to help remove the bentgrass from your lawn following the herbicide applications.

Keep in mind that patience and follow-up are key—complete conversion away from bentgrass often takes multiple years.

Chas