I'm with QQ: what's the problem with that grass? It might be a stand of tall fescue (clumpy, dark green, thick blades, and tall) which tends to be like spinning a few paper plates onto the lawn. Over here, a clump (about the size of a plate), over there, another clump. If that's the case, it's essentially a weed. Kill it with roundup, once, and then a week later. But if it's not tall fescue, give us a bigger view of the lawn, so we can isolate what the problem might be.

That appears to be a forage-type Tall Fescue (TF). Note: This is different than modified types of TF known as Turf Type Tall Fescue (TTTF) which have thinner blades and blend well into other grass types like Kentucky Blue Grass. TF is considered a weed in residential lawns because it's undesirable due to not blending in well with other turf grass. You will either have to dig it out (which can be hard given the deep roots), or use chemical means. If there are a lot of clumps, like in the image in the original post on this thread then chemical means would be the way to go. Tall Fescue is a grass, so regular weed killers that are safe for lawns wont work. You will need to use a weed & grass killer that contains Glyphosate. Home Depot has a good 41% glyphosate concentrate, there is no reason to spend more on a name brand like RoundUp, they contain the exact same stuff. Buy a 1 gallon spray can and spot spray. Be aware that Glyphosate will kill EVERYTHING so be very careful and follow the instructions on the label. TF can be hard to kill (again because of the deep roots) you might need to spray twice about a week or week and a half after the first application. Good luck! If you really want to dive deep into TF read this article from Purdue University: http://purdueturftips.blogspot.com/2015/03/weed-of-month-for-march-2015-is-tall.html

What is the horsepower rating of the mower? A mulching blade would work great IF you have enough power. 4-6hp and you are probably fine, less and it's a tough call. Thatch buildup, wet grass, or trying to mow too much at one time can significantly hamper mowing. A sharp blade is also key as a dull blade will slow down in the grass and rip instead of cleanly cutting. I know when I dethatch and then mow my zoysia center-strip of lawn that is VERY thick the difference is night and day. Before the dethatching I have to mow 1/2 the width of the deck or it tries to stall, after dethaching I can mow the entire width of the deck without a hiccup.

Well, if I'm honest with myself, it's not so much a wet bed (no, it is not near a pond or any water source) as it is poor drainage. Over the years since this house was build, the beds and land around it have sort of sunk, and the sidewalk has as well. But in a drought, this bed is not going to be wet (unless I water it). but when it gets water, it stays wet for a long time under the surface. Last year I was considering putting a rose in there, but I dug a hole and filled it with water...it had not drained 2 days later. Before it could fully drain, it rained. Then a week later it rained again...it looked terrible having a big water-filled hole so I finally filled it back in, never able to see it actually drain completely. Since then the bed has remained unplanted, except for the old Reeve's spireas planted treacherously close to the house (touching it) at the back of the bed. Last night it occured to me, those spireas are probably the termite's ticket into my house. *sigh* And so, over 1 year after moving into this house, I still have an embarrassingly ugly bed because I am frozen with indecision due to the problems there. That's why I was thinking about sticking some medium heigh grasses right there in the front, then maybe putting a row of stepping stoned behind them, so I can get back to the spireas and prune as needed (they need their post-flowering trim now).