well,again there are multiple solutions. Your way is one, but it had lack of positioning/limiting of this.
I suppose what you are really searching for is Decal Projector. There are some libs for JME like for example recent one. Example video of decal projecting:
Recent project:
(but as mentioned for terrain it can be slow, so you might search other Decal projector or adjust it for terrain)
Also you might need “synchronize it” with area that really will be affected to grow crop.
Myself i just prefer Draw 3D visual tool for area, but also thought of using decal projector