I would generally recommend bringing something to lay down on the ground. Sometimes the grass can be scratchy, too wet, too dry, too buggy, etc.,
I generally use an old picnic blanket or a tarp if the ground is really wet. That way I can still feel the texture of the ground beneath me, but I have a buffer between me and the bugs. I do like to step off of my blanket and practice on the grass from time to time!
If you want a dedicated yoga blanket, Open Road Goods sells one on amazon that looks really nice.