What Leaving the Leaves Actually Does

What the leaves are actually for

“Leave the leaves” has become a slogan, which makes it easy to dismiss. The mechanism behind it is specific.

A great deal of garden wildlife overwinters at or just below the soil surface, under a layer of leaf litter. Xerces Society notes that fireflies and native bees hide in leaf litter or burrow underground and stay within an inch or two of the surface.

Clear the leaves and you remove the insulating layer those animals are relying on, at the point in the year when they cannot relocate.

Xerces also notes the secondary benefit: leaving litter to build up over time improves the soil and provides cover for overwintering queens.

Seed heads matter too

The same applies to what you cut down.

Dead seed heads are a food source for overwintering and migrating birds. A border cut to the ground in October is tidy and produces nothing for a goldfinch in January.

Hollow and pithy stems are also nesting material — NC State Extension recommends trimming perennial stems in their first winter to a 12 to 24 inch stubble, which cavity-nesting bees then use the following year.

Where to leave them, and where not

This is where the slogan needs qualifying, because leaves genuinely cause problems in some places.

Leave them:

• Under shrubs and trees, and in borders — where they act as free mulch and habitat.
• In an out-of-the-way corner, raked into a loose pile.
• Around the base of perennials.

Remove them:

• From lawns, in quantity. A thick mat of wet leaves on turf excludes light and air and kills the grass beneath. Mowing over them to shred them is a good compromise — the pieces break down into the lawn.
• From paths and steps, where wet leaves are a genuine slip hazard.
• From ponds, where decomposing leaves consume oxygen.
• From the crowns of plants prone to rot — many alpines and silver-leaved plants dislike sitting under wet litter.
• Where the leaves carry disease. Rose black spot, apple scab and many leaf-spot diseases overwinter on fallen foliage. Removing infected leaves genuinely reduces next year’s inoculum.

That last exception matters. “Leave the leaves” is not an instruction to leave diseased material lying under a susceptible plant.

Spring is the part people get wrong

The habitat value is destroyed if you clear everything the first warm week of March.

Penn State Extension recommends delaying garden cleanup to benefit overwintering insects, and waiting as long as you can before clearing dead stalks and grasses, because they may still be occupied.

A common rule of thumb is to wait until temperatures are consistently above about 50°F, by which point most overwintering insects have emerged. Later is better than earlier.

If you cannot wait, move material to a pile at the edge of the garden rather than bagging it — anything still inside can then emerge.

The honest version

Leave leaves in beds and borders. Get them off lawns, paths and ponds. Remove diseased foliage. Delay the spring tidy as long as you can bear.

That is a real practice rather than a slogan, and it is compatible with a garden that still looks looked-after.