Thank you for your question.

Trying to identify different grass species from photographs can be very challenging. Using three different plant identification keys, the grass in your pictures gave the result that it is smooth meadow grass (Poa pratensis). Its more common name is Kentucky Blue grass. It would be interesting to know what other grass species you might have in your yard. If most of your turf is comprised of different fescues, then the bluegrass would certainly stand out. How it got there is another question. Possibly it arrived via the wind. Maybe it was in a seed mix that you had previously used?  I don’t believe these clumps are quack grass or crab grass.

If you believe your current turf is largely Kentucky Bluegrass, then what would cause certain areas of the bluegrass to be larger than the rest of the surrounding bluegrass? Although I’m grasping at straws, various possibilities come to mind:

1). Uneven application of fertilizer can cause certain areas to receive more nutrients than others, leading to more vigorous growth in those spots, resulting in clumping.

2). Compacted soil can inhibit root growth and reduce the ability of grass to spread evenly. Areas with less compaction may allow for better root development and more robust growth, creating clumps.

3). If weed control is not consistent, patches of weeds can die off, leaving gaps that Kentucky bluegrass can fill in more aggressively, leading to the appearance of clumps.

4). Certain diseases or pests can damage patches of grass, and the healthy grass around these patches can grow more aggressively to fill the space, resulting in clumping.

5). Even within a single species like Kentucky bluegrass, there can be genetic differences that cause some plants to grow more vigorously than others, leading to clumping.

See:

https://www.greenlawnfertilizing.com/blog/why-is-my-grass-growing-in-clumps - :~:text=If your lawn is watered,including clumpy areas of grass.

One other thought. Kentucky 31 tall fescue looks very similar to Kentucky bluegrass. Sometimes it is difficult to distinguish between the two types. See:

https://www.extension.iastate.edu/smallfarms/tall-fescue-alternative-kentucky-bluegrass

https://www.experigreen.com/kentucky-bluegrass-vs-turf-type-tall-fescue/

https://www.youtube.com/watch?v=4PgTscXgZfg

https://www.naturesseed.com/blog/identifying-5-common-lawn-grass-species/

I look forward to receiving your thoughts about these comments.

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