Trying to figure out what this grass is, because I like it. Very soft. I've been cutting my grass @ 3.5" mower height so its seems to get quite matted. Depending on the grass type maybe that's too high. This picture is not just mowed, I mowed this law a week ago, but as I said, @ 3.5"
This is lovely grass! But this is a bit long. Did you really measure this length as 3.5" Grins. I adore grass that looks like this but this takes special maintenance practices to include being in partial shade and thus different fertilizers and application timing.
I can't believe this is 3.5 inches! 6" maybe! And definitely a mixture of cool season grasses to include fescue and bluegrass. You need to raise that mower as high as possible, 3.5" no lower and mow your grass asap. When it begins to lay over other members in the crop you will get dead patches, fungus and irregularities. I've only been able to do this in partial shade with fertilizer that is equal for N P and K. Not higher in N as for normal lawns. It only lasts a month or two like this and then it has to be mowed. But this needs mowing now! It is in the sun and that causes more growth than partial shade.
A great rotary mower creates a vacuum that pulls the grass blades up to be cut off equally. Any longer of blade of your grass will need someone with a good blower to blow those blades of grass to be cut evenly and no lower than 3" as you mow. In fact, if your grass is 6" that would really be too much to cut off at one time.
Using a weed wacker might work if you are able to use the lines to work like the rotary mower, sucking the blades of grass upwards before quickly cutting only an inch off the blades of grass. Then using that blower, blow the clippings out and off your lawn onto your plant beds. Not easy to do.
It will not kill your grass if you just go mow it and remove enough to get 3.5" in height. One time, no biggie. I reread your question and you say it was mowed last week at 3.5"...do it again. Please bag clippings. Clippings are great stuff but your lawn won't benefit!
Crucial are super super sharp mower blades. What are you height adjustments on your mower? I am so impressed with mowers finally being allowed adjustments of 3" not to mention 3.5 or 4.5!! I LOVE the look of lawns like this and I am the only one I know that has done this on a regular basis until you! Way cool! Please mow again and send another picture of newly mowed! At these lengths grass growth is supposed to slow waaaayyyy down. If this is one week later after 3.5" mowing height...when did you fertilize and what was the formulation?
Tell us more, J.! I'd love to promote this look but unless in shade this is tough to achieve but obviously you are achieving this! Grass this height in the sun and fertilized needs vigilance. So pretty! So soft! Right? Yummmm. And I want you to get a 'ruler' pull those grass blades up and take a picture!! Please? Amazing it grew this long in one week. Sharpen your mower blades, use alcohol every time you sharpen or change out your blades, bag clippings. Please give any detail possible. I'd like to see this done more often but having a lawn at this height is ADVANCED lawn stuff. You need to consider mowing twice per week, not once to allow your lawn almost this height. As soon as those blades begin to fall over, mow your lawn.