As autumn brings cooler, shorter days, people in Canada will rake, mow, and blow leaves from more than six million lawns. It’s a task you might want to ignore. Leaving the leaves can save you time and help pollinators like butterflies and bees.
Why do leaves fall in the first place? It starts with photosynthesis, which is how plants make their own food using energy from sunlight hitting their leaves.
You may recall that chlorophyll is the hero of the story. It’s a pigment that absorbs sunlight and gives plants and leaves their green colour. Plants use that energy to convert carbon dioxide from the air and water from the soil to create glucose, a type of sugar. They combine sugars and nutrients from the soil to grow, releasing oxygen in the process.
Once fall arrives, deciduous trees shed their leaves through a process called “abscission”, which means “to cut”. Chlorophyll molecules begin to break down, which allows other, less celebrated, pigments to reveal themselves. These include carotenoids, the same molecules that make bananas yellow and carrots orange, and anthocyanins, which give red leaves their brilliance.
Sunny and warm days, cool nights, and dry conditions can draw the colours out longer, while cold, wet, overcast weather can speed up the leaf-decomposition process, creating compounds called tannins that produce less glamorous brown foliage.
When leaves hit the ground, they almost immediately begin to break down into the soil at the base of the tree. They provide a warm blanket to shield roots from the biting cold of winter and eventually send nutrients back into the soil. This is the tree’s cycle of life: nutrients from fallen leaves are absorbed into the roots and help produce buds and leaves again next spring. Drop, decompose, absorb, repeat.
As for pollinators, while the migratory flight of monarch butterflies generates much buzz, most butterflies and moths spend their winters closer to home, overwintering as eggs, caterpillars, chrysalises, or adults. Swallowtail butterflies camouflage their chrysalises as dried leaves, which get mixed into tree leaves as they fall. Woolly bear caterpillars tuck themselves into leaf layers. Critters like bumblebee queens that have burrowed into the ground to hibernate also appreciate a layer of leafy insulation. And insects in the leaf layer provide a natural fall buffet for birds, chipmunks and squirrels, including birds called thrashers that “thrash” the fallen leaves to find insects.
What can you do to help these critters? The easiest option is to “leave the leaves”—the name of a growing international campaign led by the U.S.-based Xerces Society. Instead of mowing, blowing, raking, and bagging, consider leaving leaves where they fall.
Research shows that leaving a thin layer of mulched leaves over winter won’t smother your beloved turfgrass. Instead, it can boost soil and lawn health. Just run the mower over the leaves and allow them to break down naturally. If you’re fortunate enough to have many trees and a colourful abundance of leaves, consider using them as mulch for garden beds and around trees and shrubs. If you must keep your lawn leaf-free, perhaps find space to pile them somewhere to naturally decompose.
Fall is also a good time to think about other ways to make your yard a safe haven for beneficial insects and wildlife. Many insects overwinter as pupae in the soil, and most wild bees overwinter underground, so when you tidy your garden, avoid disturbing the soil. When cleaning up your veggies and herbs, leave the roots in the soil so they can naturally decompose and add nutrients for next spring. And be sure to leave some seed heads and berries to feed birds through the winter. Fall is also the best time to expand your wildflower garden. Seeds and bulbs should be planted once it gets too cool for them to germinate.
(The David Suzuki Foundation’s Bee-bnb program offers tips on how to be a superhost for bees and butterflies.)
I hope you get to see wondrous fall landscapes, filled with brilliant reds, yellows, and oranges. But before you start raking, know that leaves are not litter. They’re nature’s butterfly nursery and a free source of mulch and fertilizer. So, put your feet up and try a little wilderness this fall.