It's a serrated kitchen knife, costs just a couple dollars from the thrift store. And I have six or seven of them, stashed in various places as well as my gardening tote.
It cuts the roots of buttercups, digs down deep enough to loosen small dandelions and helps transplant anything smaller than a gallon pot shrub. What more could I ask? And, oh yes, it's cheap enough so when the blade gets dull I just buy another. And it's light, easy to carry.
Easy to lose, however, like most tools that aren't painted in glaring colors. But the strip of red tape wound around the handle fixes that.