A thatcher basically works like a lawnmower, but instead of the blades spinning horizontally, they spin vertically and instead of their only being one set of blade there are 40-120. The reason there is a huge variance in the number of blades is because there are actually 3 different types of thatching blades you could used on your lawn.
Thatching blades. (About 120 loose, rotating blades.) This type of blade is the most common and the best a removing moss and thatch out of your lawn. A thatching blade is thorough and if you go over your lawn more than 2 times, you could start to damages the lawn. It is the only type of blades that is any good at removing moss. This type of blade beats the surface of the soil and removes everything that you could get out with a thatching rake in about 1/10 of the time. Thatch blades can get rid moss very easily.
Slicing blades. Power rakes and dethatcher with slicing blades are often called "slicers". Slicers contain 40 to 50 stiff metal blades. These blades slice into the soil, to a depth of about ½ inch to and 1 inch. They are idea for reseeding because of the slide grooves they leave in the soil and because the blades trip the existing grass to allow more room on the surface to reseed. A slicer will not devastate your lawn. It can pull out thatch either by pulling it in reverse or going slowly over the lawn multiple times. However, thatching with a slicer is a time consuming project.
Thatching Spring Blades on a thatchers are springy and stretch when the machine is on. This thatching blade is not as effective for thatching or for reseeding. It is great to use when the thatch is really light. I major advantage is that on this type of reel the springs can easily be individually replaced and not having to replace the real. Another advantage is this blade if the least likely to damage sprinkler heads.
Most types of mechanical power rakes are better then thatching rakes or larger lawns.
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