Will leaving oak leaves on the ground improve your garden?
Every autumn, I leave the leaves in my garden as a way to help invertebrates over-winter safely. Invertebrates (bees, butterflies, beetles, bugs of all kinds) are an extremely important part of our ecosystem. Without invertebrates, in fact, our entire ecosystem would collapse. And one of the main reasons is that they're a major food source for a variety of animals. So, every winter, I leave the leaves.
What I didn't know was leaves are not all made the same. And that can have an impact on how or where you use them in your garden.
Oak leaves are unique because they contain higher levels of tannins, slowing down their decomposition process. In my experience, when left to fall and sit totally on their own, they can take a couple years to break down and decompose, versus the several months that it would take a typical leaf.
This creates a bed of leaves that blocks out sunlight. The leathery leaves lay on top of one another, creating a thick, warm bed of leaves for inverterbrates that overwinter or lay their eggs in the leaves. The fact that oak leaves don't break down quickly is great for invertebrates looking for a safe place from the cold and wet winter.
You can see the difference in this photo of a leaf from a Red Pin Oak (Quercus palustris) on the left, next to that of a Vine Maple (Acer circinatum). The oak leaf is completely in tact after one full season, while the maple leaf's skeleton is starting to show through!
These leaves from Red Pin Oak blanket my yard every year.
Each year I leave any leaves that have fallen in the garden bed. I don't mulch them because bugs will often lay their eggs on leaves in the leaf litter and I don't want to destroy these eggs. If the leaf falls on the path or in my fountain though, it gets thrown in the bin. But for the leaves that make it to a garden bed, they're allowed to stay.
I leave them over the winter, then after night time temperatures reach 50 degrees for at least a couple of weeks, I clear them out. But now that I know more, I will likely leave some of them through the summer to really help those bugs out!
I know the soil I have is healthy because each year in the area where I've left the leaves I get big mushrooms. In areas where I've seeded wildflowers for the next year, I don't get as many weeds, but the wildflowers still come through. My guess is that since the seeds need cold stratification (exposure to cold temperatures and moisture to enhance germination), they're not bothered by the lack of sunlight. And since I remove them in the spring, the soil isn't getting acidic enough to impact their growth.
This year and last year I also saw a notable increase in the amount and variety of bees, butterflies, and other bugs in the yard.
As I slowly build out my garden, the goal is to provide lots of plants and materials for invertebrates so that I can attract more birds. Birds almost exclusively feed insect larvae to their young because it's so high in protein. So, the more food sources I can offer, the more birds I'll have in the yard. And I need as many birds as I can get so I can keep adding to my backyard bird list and drawings!
If you'd like to learn more about why oak trees—particularly oak trees that are native to your area—are considered a keystone species, check out Doug Tallamy's book "The Nature of Oaks". Keystone species are species that help prop up the entire food web and without them, things would collapse.