If you intend to use the lid as well, I don't know. It may have been glued/welded before it was sold. The one my mother bought, however, was only held on by the paint sprayed over the entire can, and was removed simply by running a sharp blad over that particular seam in the paint.

I'd first spray a penetrating oil near the lid. The most common brand is WD40, most of us have a can of it around the house. Spray the seam where the lid and the can meet, trying to get some inside the seam. Wait a while and then try to slide the blade in between the lid and the can. Good luck, don't forget to make a drainage home in the can so the plant's roots don't get waterlogged.

after you try the other ideas try a rubber mallet.

Try a propane torch.(not recomended in conjunction with WD40)

"Try a propane torch.(not recomended in conjunction with WD40)"??? You're no fun...LOL!

I'm not sure if you're familiar with milk cans or not, but remember, the lid is mushroom shaped. There is a cylindrical metal ring, extending down from the visible "upside down pie-pan" part, about 3-4 inches into the top of the can.