When we receive unseasonably high heat and dry weather, these conditions cause bentgrass and other annual weedy grasses in lawns to brown out much earlier than we normally see it. If you are seeing areas on your lawn yellowing or browning right now, there is a good chance it is Bentgrass (Agrostis stolonifera), or one of its close cousins, Poa Annua (annual bluegrass) or Poa Trivialis (Rough Bluegrass).
Bentgrass and Annual Bluegrasses, can invade a lawn through use of a low-quality seed mix, from sod (more commonly found then anyone likes to admit) or from a nearby golf course. These weed grasses are encouraged by too frequent, shallow-watering. Golf courses use bentgrass because it doesn’t mind being mowed very short (so the ball can roll). They can spread quickly through a lawn during cool, wet weather, taking over areas where the rest of your grass isn’t doing well. But it is helpless against heat and drought. Once that weather hits, an invaded lawn will start to brown out in large, uneven patches.
The damage in this lawn is easily identified. The dividing line between healthy and dead grass is sharp. You can also see that the blades of the brown grass are much thinner than those of the healthy grass. Bentgrass also roots shallowly, which just makes droughts worse as its root structure quickly dries out in the top level of the soil, while the more deeply rooted desirable grasses remain protected deeper in the soil profile. This patch pulled right out when Kyle tugged on it.
The only way to deal with Bentgrass is to replace it with more desirable grass cultivars. If you have a dead patch now, there is nothing to do but rake it all out, loosen the soil, and reseed with a proper seed varietal. In the future though, you can avoid ugly brown patches like this by dealing with patches of Bentgrass before they die. In the early Spring, when everything is going well, you can find Bentgrass when it is healthy and green. Look for patches of lighter-coloured grass with the characteristic skinny blades. The shallow roots make it easy to pull out at this time, too.
Finding a patch early in the year gives you time to spot-seed the area while giving the desirable grass time to establish itself. Then by the time the heat strikes, you will have stronger grasses among the weak bentgrass. This method also avoids making the kind of large area of open earth that weeds consider an open invitation to take root.