I don’t really understand your question …
You can simply add some new materials to your landscape mat and paint it on the ground 
in the space between the two biomes you can mix these texture or just add another texture for the mid space
For the foliage you can tell for example the specific gras to only spawn on specific textures , so that green grass spawns on the normal grass texture and the bushes etc of the redwood pack spawns only in their textures
a good example how to design the space between both is the map of gta V , If you own the game just drive with a car in the near of the desert and you can see how they designed the “transition” zone