Spare the rake, spoil the lawn? Getting at the root of our fall ritual

Raking leaves is a fall tradition — but is it really necessary?

Recently friends have sent me news columns arguing that removing leaves from the lawn in the fall is less a necessity than a reflection of our own neat-freak tendencies.

Live and let live, these stories say. It's natural for leaves to fall. Leave them on the ground, and they'll decay over winter, providing organic matter for the lawn. Everything will be just fine in the spring.

While that's true in a woodland setting, where no one is worried about what's growing under trees, we can't be quite so complacent if we're trying to grow grass, says a University of Minnesota turf expert.

If you don't rake before the snow flies, in the spring, you'll find a matted layer of wet leaves that hasn't rotted and may smother the grass just as it's trying to grow again.

An unraked lawn can lead to other problems. A layer of leaves under snow provides ideal conditions for a fungal disease called snow mold, as well as cover for pests like voles and mice that damage lawns over the winter.

But that doesn't mean you have to remove every scrap of leaf before it snows.

Chopping up leaves and leaving them on the lawn not only helps the environment by reducing yard waste but increases moisture retention and aeration in the soil, said Sam Bauer, a University of Minnesota Extension Service turf-grass science educator.

Leaf residue is an organic fertilizer and, depending on the type of tree, can reduce weed seed germination. Finely chopped maple leaves are good for that. Leaves of deciduous trees help feed the grass when their mulched leaves are left on the lawn. Silver maple, honey locust and white birch are among trees with higher nitrogen levels in their leaves.

The key is to mulch those leaves correctly.

Mulching mowers are great for chopping up leaves, but a regular mower, with its discharge chute closed, also works. If you have a lot of trees in your yard, Bauer said it's wise to mow more often than usual to efficiently chop up the leaves. You may need to make several passes over a lawn to cut leaves into little bits; you want to cut the leaves in small enough pieces that they can drop down into the grass. Mulching blades can be purchased to aid chopping.

When you're done, you should be able to see 80 to 90 percent of the lawn. If you can't, put the bagger on your mower for the last mowing of the season, and suck up excess debris.

You should continue to mow as long as the grass is growing, Bauer said. But never mow when there is frost on the ground.

One frequent question Bauer gets is whether mulching oak leaves into the lawn will acidify the soil. He said that's not a concern here, where soils tend toward the alkaline side. Grass thrives in soil that is a bit more acidic than average soils in the Twin Cities, so mulching oak leaves on lawns is beneficial in most locations.

There are other options for leaf disposal, if you don't want to mulch them into the lawn or put them out for city collection. Many gardeners leave leaves on their perennial beds for winter protection or dig them into vegetable beds to add compost for future years (chopping them up first will aid decay). And there's always the compost pile. Mixing leaves in compost with soil will hasten decomposition.

Mary Jane Smetanka is a Master Gardener and Minneapolis freelance writer.