You plant your flowers in the light of the sun in the shape of a cross, but it's too hot—the sun is too strong—and you end up with grass. Flowers can't grow in these conditions.
The grass on top is brown and burned from the sun, so you need to take a cool saw (くさ) and hack the tops off. The saw is actually physically cool to the touch, to help heal the grass' sunburned bits. It just happens to look cool too.