"all flesh is as grass".

"All flesh is grass and all the goodliness thereof is as the flower of the field:

 
the grass withereth, the flower fadeth: because the spirit of the Lord bloweth upon it: surely the people is grass. 

The grass withereth, the flower fadeth: but the word of the Lord shall stand forever."

-the prophet Isaiah.

Cutting grass, mowing, is just like a kind of maintenance, necessary maintenance, like vigilance in the natural against the devouring beast.  We have to keep one open sometimes because "the days are evil"; we are submerged into a world of contrasting wants, desires and needs, and not all of them need to be necessarily evil that does us harm, but things that would be necessary and providential to others, might sometimes cause us harm--such is life, with all the contrasting, wants, desires and needs.

Just like we might mow often, we keep an eye out for the devil, and when the plant goes astray, we cut 1/3 from the plant, just like the grass, we should so too treat the planting of the soul, keeping vigilance, doing maintenance, being mindful of evils that might creep-up into our perview from time to time.  Just as the husbandman works the field, so to do we tend to our own garden, that internal bit of old Eden in the soul.