Hey there farmer!
Currently, it is not possible to play Grass Roots without a connection to either Wi-Fi (recommended) or wireless data (4G, LTE, etc).
We totally empathize and appreciate the fact that not all players of Grass Roots are going to have the strongest internet connection, or data to use. Right now, many of the core features of the game (like Budbook, floydslist, and visiting fellow farmers) would not be possible without the internet, and we wouldn't want to make a watered down version of Grass Roots that worked offline at the cost of eliminating so much of what makes the game special.
More questions? Submit a request so we can help you out!