LandScapeGrass automatically fill the landscape and cause performance hit

Hi y’all I’m having a problem with the landscape grass tool.
I’m following this tutorial. But in my case, I don’t know why as soon as I apply the landscape material to the landscape, the system will automatically fill the entire landscape with the grass.
It does not allow me to paint the grass then. What’s more, I get a big performance hit.
How can I fix that?