PRC Controller described an unauthorized Runway 21L crossing event; indicating grass and foliage needs to be cut to improve the Tower's line of sight in the direction of the occurrence along with an airport review of current signage.

Narrative

Aircraft X called from an area of non visibility; the Bottle Neck Ramp. I taxied the aircraft using standard; normal phraseology and assigned Runway 21L via B4 and C taxiways. I continued scanning airport; talking to other positions and coordinating. I looked up to see the aircraft had taxied via A1; and had just crossed the departure end of Runway 21R. The pilot did not turn on B4 as instructed and taxied across the active runway. I scanned the Runway 1R and did not see any aircraft landing or departing at that time. Pilot continued to Runway 21L without further incident. Recommendation; ensure that the weeds and tall grasses/foliage is cut down in the area in the direct line of sight to the Tower. The area is difficult to see as the aircraft approaches A1 and even more difficult with the partial obstructed view due to the height of the foliage during this time of year. Review signage at the intersection to determine if anything else can be put in place to reduce further runway incursions. This is already a known hot spot.