Some people's effn children
. The only thing I can think of that will remove the tar while the baskets are still in place is a chemical called MethylEthylKetone (MEK for short). It's a brutal chemical though.
If your going to do it with the baskets still in the ground you would need to lay out a blue tarp, cover the blue tarp with black garbage bags, get yellow latex dish gloves and double layer them. MEK will eat through powder coating, paint, tar, hell even linoleum sometimes. It will not, however, mess with any of the metal. Good luck to your endeavor.