Never heard of a 1/3 rule. I rely on common sense. Hot temps, leave it
long. Competing weeds, leave it long. If the grass blades curl over, its
way too long. You start seeing horizontal/lateral runners, its too short
unless you put your golf ball on it. Then, none of this matters.
I know the 1/3 "rule" is supposed to be a real "rule", but it never
made much sense to me. If I followed it (never have) my lawn would never
ever look mowed.
For me, I try to never cut more than 80 % at one time.
The 1/3 rule (really a suggestion) is more or less to keep you from
creating thatch. If you let the grass blades get long then hack them down
short, the sheaf on the blades will toughen and thicken. This is what
gives your lawn a "woody" feel to it when you walk on it.
The other reason for the 1/3 rule is the when you mow off more than this,
there is a big reduction of nitrogen stored in the blade...ie removed. The
grass blade has to rebuild it's store of nitrogen, thus a heavy pull of
nitrogen from the soil. Doing this a few times a season will not cause
major harm to the lawn, but repeated hacking will cause thatch and
The 1/3 rule is just a guideline for regular cutting. That means you
should cut it so that you don't remove more than 1/3 to get it to the
height you normally want it cut to. That minimizes the stress on the
lawn. That is only a desirable guideline. Many lawns get cut once a
week by a lawn service regardless of how much they will remove. If the
lawn has been allowed to grow real long, then you may have to take off
more than 1/3. Or else cut it 1/3, wait a few days, then cut it again.
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