You may need to clean up after your icing tip once in a while. I have NO idea why this happens. It's my first time using the grass tip! BUT, I think that the tip gets clogged and it doesn't allow for segregated strips of "grass" lines...so you have to kinda clean up the edges so it pipes well again. No obstructions.
In order to fix this effect on the cookie, just use your toothpick and scrape all over. That will take it from the totally controlled look, to the freestyle-wavy tree effect. Maybe you prefer this, I'm not sure. You can decide!