I just tested the SpeedTree 'ThickGrass' with Tenkoku and it's working OK, but I'm testing in the base Unity 5.3.4 version.
Are you adding a directional or spherical windzone? Tenkoku includes a windzone object under the 'Tenkoku_DynamicSky' object, and modulates the settings on this objects based on Tenkoku wind settings. Are you using the latest 2018 beta? It's possible there was a change made in the latest beta (I have not had a chance to test yet).
You can try deleting the included Tenkoku wind object and adding your new one as a child object. Just make sure to call it 'Tenkoku_WindZone' and to drag the object onto the top two Tenkoku wind slots in the 'Tenkoku_Library' object.