Many trimmers exist on the market today for the thickest, tallest grasses and weeds. These trimmers can cut through any size of weeds and still trim up the rest of your lawn. But you'll need to decide how much power you'll need and how much you're willing to spend to get the best trimmer for those long, tall grasses.
Enough Power
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Two-cycle and four-cycle engines will be needed to get enough power to whip those weeds. Battery operated or corded electric engines generally won't be able to handle these weeds, as their power output is too low. Depending on the thickness and the size of the area you need to trim, you may even need a four-cycle engine, which is usually the most powerful engine on the market today. These engines use two pistons instead of the single one in two-cycle engines. Keep in mind that these trimmers may be more expensive than two-cycles and electric trimmers.
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Dual Line
You'll definitely need a trimmer head that uses two nylon strings to cut the thickest and tallest grasses. Instead of a single line, these trimmers have two lines wrapped around the spool inside the cutting head. This of course will give you twice the cutting length. Dual line trimmers also offer a bump-feed system, which will allow you to quickly spit out more line as you go.
Metal Cutting Head
While nylon line is the most versatile, as you can go from hitting those tall weeds to regular, around-the-house trimming without changing lines or heads. But, some trimmer companies manufacture head attachments that are interchangeable. There are often several different choices, but they'll let you swap out the nylon line for a metal blade. These metal blades will allow you to cut down those overgrown grasses, but you can't effectively use them around more sensitive areas, such as flowerbeds and trees.
Other Considerations
Getting the head to spin fast enough will be the most critical problem when trying to cut these grasses. A higher speed requires a higher engine output and, most often, more fuel, which may add to your costs in the long run. And most often more engine power costs more upfront. You'll also need to stop the machine regularly to keep the head from getting clogged with weeds. If you don't, the gear box will get destroyed and ruin the driveshaft and even the clutch.