Breakdown, It’s Alright

Photo Paul Teysen (Unsplash)

THOSE OF US GEARING UP TO “LEAVE SOME LEAVES” in earnest for the first time this fall may be wondering: What kind of commitment does this really require? If you’ve ever left some leaves in the garden before, you’re already aware that wherever you stash them — in flower beds, around the bases of trees or in a back corner — they’ll still be there, come spring.

Never fear: The forest isn’t overflowing with leaves, and your yard won’t be either. Those leaves are decomposing — the process often takes more than one winter, and for some species, can take two or three years. And that slowness is a good thing.

“The layers of leaves at different stages of decomposition put nutrients back in a continuous cycle,” says Hannah Armstrong, lead horticulturist at the Lady Bird Johnson Wildflower Center. “If you’re removing the leaves, you’re taking away all of those nutrients that the plants are trying to put back into the soil that will make next year’s plants healthier and happier.”

In nature, leaf litter forms an integral part of the ecosystem. Leaf-eating animals, like roly-polies, centipedes, earthworms and snails, dine on fallen leaves, breaking them into smaller pieces. Bacteria and fungi finish the job, returning nutrients to the soil and carbon dioxide to the atmosphere, providing sustenance for the next generation of plant life.

How long this process takes depends on a number of factors. From leaf thickness to internal compounds, like lignin and tannins, the traits that once supported a leaf on the tree also impact how quickly it returns to the earth. In fact, back in 2008, a global team of researchers conducted a meta-analysis — a study looking across dozens of previous studies — and found that even in the same climate, different plant species can have more than an 18-fold difference in their average decomposition rates.

FROM TOP: Bur oak (Quercus macrocarpa); post oak (Quercus stellata) — photo by Phillip Schulze; wafer ash (Ptelea trifoliata). PHOTOS: Wildflower Center

Through Thick and Thin

Leaves that are particularly thick, dense or waxy can take the longest to break down — think Texas live oaks (Quercus fusiformis) and southern magnolias (Magnolia grandiflora). Evergreen needles are slower too: Studies show deciduous leaves decompose, on average, 60% faster. Meanwhile smaller and thinner leaves, like those from honey locust (Gleditsia triacanthos), ash (Fraxinus spp.) or pecan (Carya illinoinensis), break down much more quickly in comparison.

Leaves high in nitrogen will decompose faster, since they make better food for decomposing microbes — it’s the same reason why you need to add green leaves to your compost pile. Legumes, which grow in partnership with nitrogen-fixing bacteria, generally have more nitrogen in their leaves than other trees. This plant family includes many familiar species, like Texas mountain laurel (Sophora secundiflora) and eastern redbud (Cercis canadensis), though actual leaf nitrogen content will vary by species and growing conditions.

Chemical Reaction

Even when leaves don’t appear particularly dense, other traits can slow their decomposition. Lignin, a tough molecule found in plant cell walls, is especially resistant to decay. High lignin content is a big reason why many oak leaves, like bur oak (Q. macrocarpa) or post oak (Q. stellata), are notoriously slow to decompose. A study by researchers at Chapman University found that some plant species have ten times the lignin in their leaves than others — like a western sycamore (Platanus racemosa) compared to an elephant ear (Alocasia odora).

Finally, aromatic compounds called phenolics can also slow down decomposition. Plants produce these chemicals as protection from herbivores and pathogens. Some of the most common phenolic compounds in tree leaves are tannins (yes, the same ones in your favorite wine). Pecan (Carya illinoinensis) and walnut (Juglans nigra) leaves are high in tannins, as are many oaks.

This is where a common myth creeps in: that certain leaves (especially those high in tannins) are toxic to plants or will acidify the soil. Not true, says Armstrong. “Oak leaves, particularly live oak leaves, get a really bad rap, and it’s unwarranted; they’re really not a problem,” she says. Native ecosystems have evolved with their trees’ leaves for millennia. As those leaves decompose, the local soil community is well equipped to handle them.

For the Long Haul

So, if you find last year’s leaves still lingering in your beds next spring, don’t worry. Slow decomposition isn’t a sign of a neglected garden — trust that the cycle is working in the time it takes. Every layer, whether fresh-fallen or two-years-old, is building healthier soil and a more resilient garden.

Want to support wildlife this fall? Learn more about our Leave Some Leaves campaign for simple tips on helping nature thrive.