You may have seen a “Leave the Leaves” campaign to provide winter habitat, but what exactly does leaving the leaves accomplish and what can we do to maximize its usefulness?
In the last several decades, insect populations around the world have nosedived, leaving a gaping hole in the food web. NPR reports that insect populations may be declining 8 times faster than other land animals and 40% of world insect species may be facing extinction. While the causes of this collapse are not fully understood, pesticide use and habitat loss are the most likely culprits.
While we might not be able to control, for instance, the international practice of industrial agriculture, we can immediately change our own land use, and see the results as early as next spring. This won’t just help pollinators either—other insects like fireflies greatly benefit from leaving leaves out and perennials uncut, and lots of amphibians also overwinter in leaf litter.
In a temperate climate like Chicago’s, providing habitat in which insects can hibernate is crucial. Many types of butterflies and moths lay eggs on leaves, which then fall and provide protection until spring when they can hatch. The queens of some types of bumblebee overwinter in leaf litter before emerging in the spring to start new colonies. Many other types of native bees and beneficial wasps nest in grass or flower stems or in the hollow cores of shrubs like hydrangeas, so leaving your perennial stems up over the winter is also important.
Leaving the leaves for insects has the side benefit of improving your soil as the leaves break down, helps keep your soil moist, and helps protect your perennials from frost damage.
In an urban landscape, it isn’t always possible to just leave leaves in place. You can put a moderate amount of leaves in the garden bed or on your grass, though creating too thick a layer is not ideal. Which types of leaves also make a difference: oak trees support more types of insect than any other tree in our area, so leaving as many oak leaves intact as possible is beneficial. Oak leaves also tend to keep some loft, so they protect the soil while still leaving some airflow. However, a common non-native tree like the Norway Maple doesn’t support many local insects, and also tend to form smothering mats of leaves if they are piled too thickly in a garden bed.
When you have no way to accommodate the amount you leaves fallen on your property, mowing or grinding leaves and placing them back in your garden beds is better than disposing of them, but it is likely that most insect residents already in the leaves will not survive the process.
Written by Liz Olney