Fallen leaves accumulated on lawns is a fact of fall. Typically, care is taken to rake them up before snow covers the landscape. Some leaves are composted, either right in the backyard or hauled away to community sites. Leaves may also be used as mulch on gardens and landscape beds. But what if leaves remain on lawns all winter – will there be damage next spring as a result?
Among lawn winter damage concerns is development of snow mold disease. During the wet, wintry weather of early spring, snow mold may be highly visible as matted, crusty looking areas. As conditions dry out, snow mold gradually disappears, but infected areas often remain in the form of weak or even dead turf. Avoiding excess nitrogen fertilizer in fall and removing excessive debris, such as leaves or straw, are commonly suggested to help reduce damage.
Accumulated leaves sitting on lawns all winter also can potentially smother grass. Shredding scattered leaves on lawns while mowing does not create problems, as the leaf material readily decomposes into beneficial organic matter. However, typically more than “scattered” leaves are on lawns by late fall. These situations beg the question, “How many leaves are too many to have on the lawn over winter?”
An interesting evaluation study to help answer this question was recently conducted at the University of Wisconsin-Madison OJ Noer Turfgrass Research and Education Facility. The three-year study evaluated different tree leaf mulching techniques and leaf depths for their effect on snow mold development and winter survival in a home lawn scenario. The study looked at leaf layers of two and four-inch depth, with both mulched (shredded leaves) and unmulched (whole leaves), sitting on lawns all winter. Plots with grass clear of leaves all winter were included in the study. As we all know, every Wisconsin winter is different in temperature and snowfall extremes, thus three consecutive years provided a suitable time frame this study.
Each spring, winter damage, turf (grass) quality, and turf color were evaluated after removing the leaf cover. Although results of the study have not gone through a final analysis yet, preliminary data makes it safe to say a two-inch cover of mulched (shredded) leaves is reasonable to allow on lawns over the winter. However, accumulated leaves of a four-inch depth are highly likely to lead to winter damage issues on the lawn.
Ideally mulch or shred leaves rather than have whole leaves sitting on lawns. Leaves may be shredded with various leaf blower attachments, or just by running over them with the lawn mower. Allow a maximum depth of two inches to remain on the lawn.
By Bruce Spangenberg, Horticulture Outreach Specialist, UW-Madison Division of Extension
