Hot mop tar is a good way to waterproof projects like an indoor planter. Here's how it's applied. After it's heated in a kettle, hot asphalt is used to adhere layers of tar paper to cover the surface. Overlap each sheet to prevent leaks, applying enough pressure for bonding. Finish with a top layer of tar for a waterproof surface.
For more on waterproofing, consider:
Install Gutter Flashing
How To: Dry a Wet Basement
Radio: Basement Waterproofing