Is the grass plane the only object in your model that contains transparency? If you have multiple transparent objects that intersect each other, then you will have these kinds of issues.
To solve these kinds of problems, we usually break up the transparent objects into smaller pieces. This gives the rendering engine more fine-grained control with the order in which transparent objects are rendered.
If you continue to have problems, you can post your model here, or use our
upload form, and somebody can take a look at it.