Two minutes into picking them out with a small shovel on my hands and knees I got up and walked into the garage to design and build a better tool. ...it's funny that this is exactly how software engineering and data-research works. If you have a boring or repetitive task you should find or design and build a tool to make it easier. "Necessity" isn't the real mother of invention; it's laziness. Or, more aptly described, it's a natural outgrowth of intellectual curiosity. But I digress.
Here are the parts I used, and the end result. On the left is 1.5" some steel tube, some 3/16" angle iron, and a short piece of reinforcing bar (rebar). The handle was cut from a piece of pine 2x4. The unstained handle was a quick prototype. I did a test-fit and then finished the welds while the head was on this handle (hence the burn marks and destruction on it).
This square handle was really easy to make and fit together. Everything was cut just using the table-saw fence and finishing it off with a chisel to get to the areas that the circular blade couldn't reach. Slice it down the center and slam home a wedge and this ended up being a solid way to put everything together. I think it's way better more classy than using bolts to attach the handle.
I wanted something small that I could stomp on with all of my body weight to sever or dig out stubborn roots if I had to. Here's a short video of how it works. I filmed this with my phone in one hand and the tool in the other; with the benefit of both hands the tool is really efficient.
The welding on the tool is a little excessive. Part of the reason for undertaking this project was to teach my father-in-law how to weld, so we kept taking passes on the metal. It's interesting to see how others interpret verbal instruction or how they pick up new tools and methods. His initial pass he waved it back and forth like a paint spray gun; I didn't expect him to do that. On the second try, I made sure to explain that you have to hold in the same spot until the metal starts to melt and then slowly lay a bead of weld. I didn't bother grinding things down very much afterward, just spray painted it and hung it on the handle. That's the fun part of building garden tools, they don't have to be pretty.
A note about the steel. This is mild steel, and that's ok. I had some high carbon steel that would have been harder, but I'd prefer a digging tool to turn an edge rather than chip. I know this tool will be slammed onto rocks and concrete, but any damage done can easily be pounded (preferred method) or filed out (last resort) before resharpening.