Leave the Leaves, But Why? Look to the Research

Leave the leaves! It’s an exhortation to gardeners to stop raking and blowing tree, shrub, and perennial leaves that drop in autumn, keeping the leaves in the garden as a natural mulch. This approach stands in contrast to the more common practice of bringing in shredded hardwood or another variety of mulch every spring, after scraping the garden clean in preparation.

If you think about how a forest functions, leaving the leaves makes sense. The annual forest leaf drop resupplies existing soil with nutrients, creates new soil, and serves as habitat for overwintering insects. But most residential gardens aren’t the size or complexity of forests. Will leaving the leaves in these spaces really help?

Looking over the current gardening and conservation literature, the answer appears to be yes. Fallen leaves insulate the soil, helping burrowing insects withstand potentially lethal fluctuating temperatures, and the leaves provide winter mulch for perennials susceptible to frost heave and other effects of cold weather. But the leaves must be left whole rather than shredded, to protect insect eggs laid on leaf surfaces and chrysalises formed from or hidden among the leaves.

What’s exciting about these recommendations is that they’re being studied as research questions, at places such as the University of Maryland’s Burghardt Lab. Descriptions of the studies and findings to date are summarized in blog posts (“My Year Playing With Litter” and “Suburban Lawn Study Launched”) and videos (“Preliminary Results: Initial findings from insect collection”).

The final results should provide evidence to help answer questions a gardener may have about leaving the leaves, questions such as:

  • Which species of insects, and in particular butterflies, moths, and bees, are reduced when leaves are removed?
  • Must the leaves be left in place for a full 12 months (essentially, forever)? If leaves can be removed, when is the optimal time?
  • Are there any negative effects of moving leaves around a property, such as distributing leaves across garden beds instead of leaving them where they fall and clearing leaves away from the crowns of perennials?
  • If leaves are removed from a garden after winter and mulch is applied, are there any benefits to applying commercial leaf litter/leaf mold rather than shredded hardwood mulch?

Traditional annual garden leaf cleanup has been described as a kind of unintentional bait and switch, where pollinators and other insects are encouraged to spend most of the year thriving on a garden’s nectar and pollen, only to be swept away in the fall. Generations and life cycles are interrupted. The garden’s usefulness as an ecosystem is reduced. Research on leaving the leaves has the potential to change this longstanding gardening practice.

If you’d like to read more . . .

Black, Scott Hoffman. “Leave The Leaves To Benefit Wildlife.” 12 November 2020. Xerces Blog, The Xerces Society for Invertebrate Conservation. Accessed June 26, 2022. https://xerces.org/blog/leave-leaves-to-benefit-wildlife

Center for Pollinator Research. Department of Entomology, PennState College of Agricultural Sciences. Accessed June 26, 2022. https://ento.psu.edu/research/centers/pollinators

Dawson, Cathy. “Delay Garden Cleanup to Benefit Overwintering Insects.” August 17, 2020. PennState Extension. Accessed June 26, 2022. https://extension.psu.edu/delay-garden-cleanup-to-benefit-overwintering-insects

Jordan, Sarah Foltz, Jennifer Hopwood, and Sara Morris. “Nesting & Overwintering Habitat for Pollinators & Other Beneficial Insects.” 2020. The Xerces Society for Invertebrate Conservation. Accessed June 26, 2022. https://xerces.org/publications/fact-sheets/nesting-overwintering-habitat

Roach, Margaret. “A Smarter Fall Cleanup.” September 30, 2020. The New York Times. Accessed June 26, 2022. https://www.nytimes.com/2020/09/30/realestate/fall-garden-cleanup.html

Truss-Williams, Anaya. “Want to help the environment? Some local residents suggest keeping leaves on yards.” October 28, 2021. The Diamondback. Accessed June 26, 2022. https://dbknews.com/2021/10/28/prince-georges-county-environment-leaves-yards/

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