Black Under Zoysia Grass

Q: My lawn guy said that my grass needs to be scalped to keep it “healthy”.
Underneath the dead grass there’s a lot of black. My zoysia grass has been
around for decades and it’s very, very thick. 

A: The black streaks are the spores of fungi that are decomposing dead zoysia
thatch. The thatch layer is there because the lawn was not mowed often
enough. In my experience zoysia needs to be mowed every five days. The
grass clippings can decompose completely before the next mowing. If you
mow every seven days, or longer, thatch will accumulate into the thick layer
you see today. When your lawn is 50% green, you can mow and remove 1/2
inch of grass height. In five days, remove another one half-inch. Repeat until
the grass is two inches high. Maintain this height throughout the summer. If
you can aerate after a rain, when the soil is soft, the aerator soil plugs will help
decompose the thatch even further.