About 200,000 Black & Decker GH1000 Grasshog XP string trimmer/edgers, manufactured in China by Black & Decker U.S. Inc. of Towson, Md., because some parts can come loose and fly out of the tools.

To find out how a cordless string trimmer performs, I asked Black & Decker to let me try the LST220, which has a 12-inch cut and is powered by a 20-volt battery. The trimmer comes with two ...