Originally posted by Vivian, Lady of Dawn and Dusk:

Workbenches suppress spawns within 20 meters. They don't need to have a roof on them to do that. Cutting the grass will make it easier to target enemies if they are there but won't affect the spawn rate.

Player base structures all have this effect. You can look this up in the wiki.
https://valheim.fandom.com/wiki/Player_base

This, the link has all the items that prevent spawns within 20m. I have a shack in my garden center for storing tools and supplies with a work bench, then a wall around it all under 20m out.
Note no spawn zones are in a sphere so if you still have spawns inside your walls you'll need to figure out where the gap is.