The staghorn sumac (Rhus Typhina) should be pruned as little as possible. As this tree is famous for its outlandish growing habit, a slight correction of the crown may be desirable. This modest trimming can be done in June. If you want to prune more drastically, it is better to wait until late autumn or winter.
Root suckers or wild shoots may develop throughout the growing season and should be cut away immediately and as deeply as possible. However, this may simply encourage new suckers to develop. Unfortunately there is nothing you can do to prevent this, rather than staying alert and removing the culprits consistently. Always wear gloves when pruning Rhus, as a milky sap that can irritate the skin is discharged from the wound.