We burn out old steel drums and cut them in half to make planters in the greenhouses that stand about 1.5 feet tall. It puts soil above ground as a means of absorbing and retaining heat, and it makes harvesting more convenient.
They are hardy too since it's tough to destroy a steel drum even when you try to. Best of all, they are free from local shops that paint or apply insulation.
Go to this link about halfway down the page to see our steel drums in service in greenhouse #1.
http://www.frugal-living-freedom.com/build-your-own-greenhouse.html
To clean out the drums, we dig a pit and throw a bunch of pallets in the bottom, then load the barrels on top and set it ablaze. After a couple hours of burning, there isn't anything left in the barrels. Alternatively, if you remove the tops and bottoms, you can burn out one individually by simply making a pile of scrap wood in the bottom and let it burn real hot for an hour or so.
Drums with removable lids are easiest since that only leaves the bottom to contend with. I use a plasma cutter to cut the bottom out, but be aware that fammable contents are going to be a problem anytime you cut or burn with a source of heat and sparks.
For oil drums and the like, you might want to open the bungs and burn in a deep pit to make certain there is no explosion harzard before you cut into them.
When we cut, we make an opening from the top so the smooth lip of the drum remains in place. Some grinding might be necessary to remove burrs and sharp edges left from whatever cutter you use.
Also, be aware that insulation shops that give away their steel drums are trying to get rid of hazardous waste. One drum is the harzardous resin and the other is the activator. Read the drum label and only take the one that isn't labeled as hazardous waste.
If you don't have the tools to cut steel, then look for plastic drums. They can be obtained at no cost as well. The hazardous material warning applies here as well since some of the spray foam insulation comes in plastic barrels as well.
Clair