Thanks for contacting us. Probably the easiest way to create this is to use the Falloff controls that allow you gradually reduce the density of grass as it nears an edge. You can then overlap the Include areas for the two field and ged a gradation effect between the two.
For example here is a simple set up with two spline areas and a surface
The blend effect is created by increasing the Falloff > Include
You can create each grass area as a separate Forest Object, or alternatively by controlling which items are scattered by each individual include area by using the Pick Plants
option, found in the areas rollout.
I've attached a sample file with this setup. I hope that helps, please let me know if you have any further questions.