Why matted leaves kill a lawn
A dry scatter of leaves on a lawn does no harm. The trouble starts when they get wet and pack down into a layer. That layer blocks light at exactly the time cool-season grass is storing energy for winter, holds moisture against the crowns, and keeps the surface from drying out between rains. Grass under it yellows, then thins, and in spring you find a pale, flattened patch the shape of the pile that sat there.
Matted leaves also set up snow mold. Gray and pink snow mold are fungal diseases that grow in cold, wet, covered turf, under snow or under a wet leaf mat, and show up in March as circles of straw-colored, crusted grass. They are rarely fatal on their own, but on a lawn that went into winter covered, the damage stacks up. A clean lawn going into December, cut at the right height, is the main prevention.
Mulch the leaves or remove them
Where the leaf fall is light, a mower does the job. Run over dry leaves with the deck set high and a sharp blade, and they break down into small pieces that drop between the grass blades and feed the soil.
The test is simple: after one pass you should see grass, with leaf flecks in it. If you still see a layer of leaves, there were too many to mulch, and another pass or two will only make a thicker mat of chopped leaf.
Under mature oaks, maples and beeches, which is most of the older lots on the Main Line, the volume is past what any mower can mulch. Those lawns need the leaves blown out of turf and beds, gathered and either hauled away or moved to a compost or leaf pile on the property. Oak leaves are thick, waxy and slow to break down, so they are the worst for matting and the least suited to mulching in place.
A fall checklist, in order
Mulch light leaf fall as it comes
Through October, run the mower high over dry leaves so they break up between the grass blades instead of piling up.
First removal pass
When maples and the early trees are mostly down, blow leaves out of lawns and beds and haul or pile them before they mat.
Cut back what should come down
Take down hostas, daylilies and peony foliage; leave coneflowers, sedums, grasses and seed heads standing until spring.
Clear the foundation beds
Clean leaves and debris away from the house, open downspout outlets, and check that grade still falls away from the wall.
Second removal pass
Once the oaks have dropped most of their leaves, often early December, clear the lawn again so nothing sits on it through winter.
Last mow and gutters
Make the final cut a notch lower, then have gutters cleaned now that the trees are bare.
Winterize
Drain hoses and spigots, winterize irrigation before a hard freeze, and return a light layer of shredded leaves to the beds.
Timing, and why heavily treed lots need two visits
On a lot with a few young trees, one cleanup once most of the leaves are down, usually mid to late November, covers it. On a lot under mature oak canopy, one visit cannot be timed right. Maples and tulip poplars drop first, while oaks hold on late, and some oaks keep a share of their brown leaves well into winter.
Clean up once in mid November and the lawn is covered again by Thanksgiving. Wait for every oak to finish and the first drop has been sitting wet on the grass for weeks.
So the practical schedule on a heavily treed property is two passes: the first when the early trees are mostly down, the second once the oaks have done most of their dropping, often early December. Lawns that were overseeded in September may need lighter, more frequent clearing in between, because new seedlings are the most easily smothered grass on the property.
Beds, perennials and the leaves-in-beds compromise
Some perennials should come down in fall: hostas and daylilies collapse into slime, and peony foliage is best cut and removed because it can carry leaf disease into next year. Others are worth leaving standing. Coneflowers, black-eyed Susans, sedums and ornamental grasses hold seed for birds and look good in frost, and hollow and pithy stems are where a lot of native bees spend the winter. Those get cut in spring instead.
A thin layer of shredded leaves in a shrub or perennial bed is good mulch and good habitat. A deep layer of whole, wet oak leaves packed against the crowns of perennials and the base of small shrubs invites rot and gives voles cover all winter. The middle ground is to blow beds out, shred what came out, and return a light layer, kept off crowns and stems.
Foundations, gutters and the last mow
The beds along the house matter most. Leaves and debris piled against a foundation hold water against the wall, can block the grade that is supposed to carry water away, and give insects and rodents a sheltered route up to the sill.
Clear the foundation beds down to the mulch, make sure downspout outlets and extensions are open and pointing away from the house, and have the gutters cleaned after the leaves are down, not before. A clogged gutter overflows straight into the bed below it.
The last mow of the year should take the lawn down a notch, typically to around two and a half to three inches for tall fescue lawns, not scalped. Grass left long and floppy over winter mats down and is more prone to snow mold; grass cut too short loses the stored energy it needs in spring.
Other winter tasks belong on the same list:
- Drain and store hoses.
- Shut off and drain outdoor spigots.
- Have irrigation lines winterized before a hard freeze.
- Put fresh mulch down only after the ground has cooled.
When to do it yourself, and when not to
A small lot with a couple of trees is a reasonable weekend job: a mower with a mulching blade, a rake, a leaf blower and a few yard waste bags will handle it, and most townships run fall leaf collection at the curb. Check yours for the dates and the rules on bags versus loose piles.
Where it stops being reasonable is a lot under full oak canopy with large beds, slopes and a foundation planting all the way around the house. The volume there is measured in truckloads, and the timing has to be watched.