Maintaining a proper lawn requires more than just running a lawn mower over the grass and calling it a day. When grass grows around trees, a lawn mower can't typically mow it effectively. If you leave this grass untrimmed, it will harbor insect pests and leave your lawn looking unsightly. A string trimmer makes quick work of trimming grass around trees, but you must be careful not to damage the tree trunk with the rapidly spinning string.
Practice using your string trimmer by trimming grass around signs, fence posts or other sturdy objects until you get the feel of how to use the tool properly.
Start the engine far from the tree around which you are trimming. Turn the string trimmer to the lowest possible speed that will still cut the grass.
Approach the tree trunk with the string spinning parallel to the ground. Move it slowly until just the grass around the tree is cut. Keep the string from touching the tree trunk.
Hold the trimmer away from the tree trunk if you need to let out additional line as you trim the grass.
Use hand-held garden shears to trim around young trees or delicate specimens. Shears may take more time to trim the grass around your trees than a string trimmer would, but will prevent any damage to the tree trunk.