A Moment of Science

Why Mowing the Lawn Doesn’t Kill the Grass

Ever wonder how grass survives the sharp blades of a lawn mower? Learn about replenishing grasses on this Moment of Science.

Grass in the daylight

Photo: wili_hybrid (Flickr)

Unlike other plants, grasses can replenish their leaves.

If you cut down an oak tree, the stump dies. But if you cut grass, you don’t hurt it at all. That’s because new growth on an oak tree is at the tips of the branches; new growth in grass happens at ground level.

Grasses, unlike other plants, can replenish their leaves. A blade of grass is the end of a long narrow leaf. If you trace a grass blade back to the stem on a tall grass plant, you see that the blade comes from a sheath wrapped around the stem.

Anatomy Of Grass

At the base of that sheath is a node, a place where the stems of some grasses have a slight bulge. Nodes are the places where new growth happens in a grass plant. A short grass plant has at least one node near ground level, out of reach of the lawn mower; a tall grass plant may have several more nodes farther up along the stem.

When a grass blade is cut off by a lawn mower or a grazing animal, some as-yet-unknown signal is sent down to the node, stimulating it to produce more leaf. Grazing animals take particular advantage of this; eating the grass causes more to grow in its place.

Oak Leaves

This capacity to add new material to old leaves is characteristic of grasses. Other plants generally don’t have this ability. An oak tree grows new leaves every year, but can’t replace part of an existing leaf.

An oak leaf grows to a certain mature size and stops. If part of an oak leaf is cut away, it doesn’t grow back. The ultimate fate of an oak leaf is old age and death; leaves of grass remain youthful all summer long.

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