Frequent mowing encourages dense growth, so mow once a week when grass is growing strong. 

Don't mow grass if it's growing slow maybe due to a drought for example. Don't cut your lawn to short; about half an inch is fine if you have a very flat fine lawn. This is common mistake lawn owners make, wanting it as low as possible so it wont need cutting for a few weeks or more. 

If you want a weed free green lawn then cut the lawn often and not to low. A lawn that sees a lot of traffic on it, like children playing then this should be cut at no less than one inch. When your grass is growing rapidly in summer try to rake the lawn once a month. This will bring out thatch and dead moss, which if left builds up and you find you have brown patches on your lawn.

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