A CR tester makes an adjustment after fitting on a new string trimmer head
CR project leader Misha Kollontai adjusts the handle on a battery-powered string trimmer.

Under normal use, you need to advance your string trimmer’s nylon cutting string whenever the tool isn’t cutting effectively. That happens every few minutes or even quicker in tall grass and weeds. What that entails varies from model to model, with some requiring far less work than others. In our tests, those that score well for ease of use reflect how simple it was for our experts to load a model’s head and advance the string. Some models also allow you to swap the original head with a different aftermarket head, about $20. No matter the type, the best-performing string trimmers cut with dual strings rather than a single string. Here’s a closer look at the different head choices.

Traditional Heads
These are the most common—and often a major source of frustration. Inside is a spool that you need to wrap long lengths of string around. Some trimmers come with the spool preloaded, but it might not be enough to get you through a season of cutting. Once the spool is loaded, you feed about 6 inches of line through holes on the trimmer head. Each time the string wears down, the trimmer head will advance a bit of string automatically or you’ll need to advance more string manually. If a sharp object cuts your string down to the base or if the auto-feed isn’t keeping up, you’ll need to open the head and advance the string manually.

More on String Trimmers

Bump Heads
These work much like traditional heads but they’re spring-loaded and have a large button on the outside of the housing. Once you’ve loaded the spool with string, you can simply bump the trimmer head against the ground while the head is spinning to auto-advance the line each time it wears down, without needing to take the head apart.

Locking Heads
These are the fastest to reload. Instead of housing a spool, there’s a simple disc that has a hole through which you feed a shorter, precut length of string that locks into place. There’s no wrestling with long lengths of line. Then simply replace the line with another precut piece each time it wears down. But you need to buy packages of precut string or cut it yourself from a larger spool.

Traditional Heads

These are the most common—and often a major source of frustration. Inside is a spool that you need to wrap long lengths of string around. Some trimmers come with the spool preloaded, but it may not be enough to get you through a season of cutting. Once the spool is loaded, you feed about 6 inches of line through holes on the trimmer head. Each time the string wears down, either the trimmer head will advance a bit of string automatically or you’ll need to advance more string manually. If a sharp object cuts your string down to the base or if the auto-feed isn’t keeping up, you’ll need to open the head and advance the string manually.

Bump Heads

These work much like traditional heads but they’re spring-loaded and have a large button on the outside of the housing. Once you’ve loaded the spool with string, you can simply bump the trimmer head against the ground while the head is spinning to auto-advance the line each time it wears down, without needing to take the head apart.

Locking Heads

These are the fastest to reload. Instead of housing a spool, there’s a simple disc that has a hole through which you feed a shorter, precut length of string that locks into place. There’s no wrestling with long lengths of line. Then simply replace the line with another precut piece each time it wears down. But you need to buy packages of precut string or cut it yourself from a larger spool.

Editor’s Note: This article also appeared in the June 2020 issue of Consumer Reports magazine.