St. George has a couple great locations for dirt bike riding. Warner Valley and Sand Mountain OHV area are two incredibly fun riding areas. Warner Valley is an open riding area for dirt bikes and ATVs. The terrain is quite sandy and loaded with miles of washes. Sand Mountain OHV area is large open sand dunes riding area on top of the plateau overlooking Warner Valley on one side and leading into Sand Hollow on the other. This is a great winter get away if you live up north. Temperatures in St. George can be in the 60's during the winter months.
Warner Valley is a spider web of dirt bike and ATV trails on the north side of Warner Valley RD. To the south are several desert loops drop into Arizona. Most trails follow dry sandy washes that interconnect to make some fun loops.
Sand Mountain OHV area has 4x4 trails similar to Moab with sand dunes similar to Little Sahara. The area is patrolled by the rangers from Sand Hollow. If you're riding on the sand make sure you have a flag.
Directions from St. George:
Take Exit 10 and head south east on Green Springs Drive for .1 miles. Turn left on Telegraph Street and continue for 1.1 miles. Turn right on 300 East / Washington Fields Road for 1.6 miles. Turn left onto Washington Dam Road. Continue for 3.6 miles. Parking is on the right.