Here’s an example of how to achieve it. You have to do it for each material property.
This would be easier if I could access the blend weight directly, rather than having to blend between “0” and “1” just to get the blend weight 
In the preview, the part of the grass with alpha > 0.5 will start blending in; the part with alpha 1.0 will be blended at 0.5.
If you set preview weight to 0.5, you will blend the grass with alpha as normal.
If you set preview weight to 1.0, you will only get the grass texture.