It’s about that time of year again- when cooler temperatures and shorter days signal that winter is coming, and our deciduous trees drop their leaves. For most residents, this also means hours of labor collecting the fallen leaves and piling them on the curb to be picked up along with other municipal waste. We have traditionally used the words “debris” and “litter” to describe dead plant material, indicating it is trash that should be bagged up and sent to the landfill. But this is certainly not the case! Dead leaves are a valuable resource, and the common practice of removing them from the home landscape has unintended environmental impacts.
The Natural cycle
Forested ecosystems have evolved with the annual input of fallen dead leaves. Here in Maryland, deciduous forests have been around for at least 10,000 years- since the last ice age! When trees drop their leaves in a forest, they are not wasted. Fallen leaves (and other organic materials like dead wood) nourish soil microbes and decomposers. They also add structural complexity (think air pockets) to the soil surface, which increases stormwater retention and provides habitat for innumerable small animals. Those small animals, including insects, feed on the litter, breaking it down into smaller pieces. Over time, fungi and other microbes completely transform the remnants of leaf litter into nutrients such as nitrogen that are recycled through the ecosystem year after year and carbon that can be sequestered belowground.
Figure 1: Removing leaves can negatively impact soil health and beneficial insect populations through many potential pathways. Illustration credit: Maggie Lin
Leaf Removal Impacts on Soil Health
Our lab has recently published research that humans may disrupt this important natural cycle when they rake (or blow) and remove fallen leaves from their yards each fall (Figure 1). This research compared areas of suburban Maryland yards where fallen leaves had been historically removed (such as in front yards) or retained (such as in back yards). In areas of Maryland yards where leaves were consistently removed each year, decomposition rates decreased by 17%, and soil carbon storage decreased by 24%. Removing leaves leads to poorer soil health!
Max Ferlauto (lead on the project) also performed an experiment: areas where leaves had been historically removed were retained and vice versa. Concerningly, two years of newly retained leaves was not enough to restore those soils' decomposition rates and carbon storage. This highlights that our yard management choices can have lasting impacts, and restoring ecosystem functions like decomposition and nutrient cycling in depleted soils may take time.
Leaf Removal Impacts on Insects and Other Animals
In contrast, insect communities seem to bounce back quickly when you start retaining leaves. While a few species of insects are pests, most insects benefit humans because they serve as pollinators, predators, decomposers, and food sources for birds and other wildlife. In the same way that providing flowers helps beneficial insects in the summer, providing shelter helps beneficial insects in the winter. The most common locations for overwintering insects are on the ground inside dead leaves or wood, underground in shallow chambers, or on twigs (Figure 2). Removing fallen leaves directly kills any insects sheltering inside the leaves, while any insects that remain near the soil surface may be exposed to more variable temperatures without the buffering effect of a leaf layer (Figure 1). Together these processes may result in smaller and/or less diverse insect communities. In fact, in the same yards where soil processes were measured, there were 40% fewer moths and butterflies emerging in the spring. Fewer moths and butterflies mean fewer caterpillars for birds, which rely on the spring insect bonanza to fuel successful migrating and breeding.
Figure 2: Examples of common overwintering locations for insects include tree holes, branches and twigs, in fallen leaves, and burrowed below the soil surface. Illustration credit: Maggie Lin
Landscaping Recommendations
Ironically, we may create even more work for ourselves when we remove fallen leaves because many processes that naturally occur on their own must now be actively managed. Fertilizer and mulch must be purchased and applied; soil must be aerated; stormwater runoff infrastructure must be maintained, etc. We spend a lot of time and money, yet ultimately degrade the quality of the environment for other plants and animals.
If you already manage an area for leaves, great! If you don’t, consider tweaking your landscape management this fall. Are there zones of your yard where removing all leaves is unnecessary? While it is often not feasible or desired to totally “leave the leaves” it is worthwhile to leave them where possible. To make things look tidier, you can rake fallen leaves from grass lawns into garden beds with borders (Figures 3 and 4) or mulch some of them to reduce the overall volume. However, you will be surprised at how much the leaves decrease in volume over the winter. This is one instance where doing less is certainly doing more- it’s easier for us and better for the environment!
Figure 3: Dead leaves can be used as an alternative to mulch in garden beds. Leaves provide nutrients and water retention for plantings, and new growth pushes up through the leaf layer in the spring. Image credit: Karin Burghardt
Figure 4: Another strategy is to create “canopy shadows” or “soft landings” underneath canopy trees. Creating borders with stones or logs keeps leaves where you want them and provides a cue of care to neighbors. Image credit: Karin Burghardt
For more information
Nesting & Overwintering Habitat For Pollinators & Other Beneficial Insects
Waiting on Spring Garden Cleanup to Support Beneficial Insects
Written by Kelsey McGurrin, Maggie Lin, and Karin Burghardt