There are 2 issues with flying off grass. Excess height of the grass, and whether the surface under it is smooth or not. My field is grass, and I could mow to 1/2 inch, but the ground under it wasn't level enough to taxi on. I would nose over constantly. Once I bought a roller, I got it smooth enough to run on, grass height is less of an issue. You just don't want to hear the "zzzzzzzzzzzzzzzz" sound while taking off. It coats the plane with grass clippings, and turns everything green. The grass for me can be up to about 3 inches.