Wrong time of year to collect. The amount of trunk to leave depends on the trunk. Where are the interesting parts? Best time to collected is early spring. Best soil is rich deep soil mixed half and half with bonsai soil (not straight bonsai soil). Wisteria like to grow with their leaves in full sun and roots in shade. They over top larger tree in the wild to do this so shade the lot and keep it moist. Give the top as much sun as possible