Although lawn mowers are primarily designed to trim grass, it is possible to use one for taller weed-like growth, such as brush. This saves you money because you don't have to buy a separate lawn appliance--but it's more than likely that you'll have to mow over the brush area several times to reduce it to a suitable level. This is because the extra tall brush builds up inside the mower as it cuts, preventing all of the growth from being cut down at one time.
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Remove the bag from your lawn mower. There's no point in trying to bag all the material you are mowing--you would just have to stop and empty the bag every few minutes due to the tall growth of the brush.
Set the mower height as high as possible. This prevents the mower from becoming clogged while using it.
Wait for the brush to dry--it is often wet first thing in the morning. Wet brush sticks together and clogs up the mowers, preventing the rotating blades from cutting the brush.
Cut over the entire mowed area a second time. This chops down any pieces of brush you failed to trim the first time.