String trimmers or weed whackers--electric or two-cycle gas powered--efficiently mow down tall weeds in places lawn mowers can't reach. Though the machines won't hurt people if the string happens to contact an ankle, the bark of tender bushes and fruit trees could be stripped away. Learning to work carefully prevents most accidental damage. For many owners, starting the machine presents the most trouble.
Using String Trimmers
Read the instruction manual for the best advice on starting gasoline trimmers. Exact steps vary slightly. Use the correct mix of oil to gas. Don't use gas that sat over winter in the garage. Gasoline jells over time and gas/oil mix evaporates to a thick consistency, which won't ignite in the piston chamber. For most gas trimmers, the right starting procedure involves priming with the fuel bulb, closing the choke, closing the ignition and pulling the starter cord. When the engine fires, open the choke by half until the motor runs smoothly, then open the choke fully. Most but not all trimmers start with the throttle fully engaged.
Give the weed whacker enough throttle during use to cut efficiently. Don't operate the machine constantly at top speed. Cutting string could wrap around an obstacle and jam, damaging either shaft or gearbox.
Hold the forward handle with the left hand and operate the throttle with the right. To advance the line from an automatic feed spool, rev the motor and tap the base of the cutting head lightly on a clear surface like a sidewalk or stone. When the line contacts the shield a cutting blade trims it to the right length. Replace a short line in manual heads by hand with the machine turned off. Needle-nose pliers help grasp awkwardly short pieces stuck in the head.
Work with the tip of the weed whacker string, not the belly of the cord. Forcing the line into the weeds causes the middle of the line to wear, shortening the string and wasting expensive cutting cord. Cutting with the high-speed tip of the line gives more control of the work. Cutting with the belly creates more opportunity for wraps and unintentional damage to ornamentals.
Keep the cutting line away from fragile things. Bark strips easily from even large fruit trees--lilac bushes can be girdled in an instant. Don't use weed whackers around electrical wiring or even PVC pipe. The above-ground portion of an irrigation system's backflow preventer shatters easily when hit by high-speed cutting cord.