Trash cans are subject to warrantless searches if the citizen has no reasonable expectation of privacy. California v. Greenwood, 108 S.Ct. 1625.
A closed trash can may be searched when the cans are at the curb for pick up.
A trash can in the curtilage of a home cannot be searched without a warrant.
The issue in trash pulls is whether the trash cans were within the curtilage of the house.
Curtilage of a home is afforded the same 4th Amendment protection as inside the home by the U.S. Supreme Court and is applicable to the states by application of the due process clause of the 14th Amendment.