Leaving Leaves Be


The end of the growing season is one of the most beautiful times of year in the northwest.  The sun typically sticks around for a while even as daytime air temperatures are cooling, resulting in foggy mornings and dewy spider webs.  As day length decreases, deciduous trees and shrubs begin pulling starches and sugars out of their leaves and store all of that energy in stems and roots for the next season.  As this occurs, chlorophyll production slows and the resulting oranges, reds, and yellows of the trace pigments and minerals that are left behind start to show.  Finally, the leaves fall to the ground where nature intends them to help create deeper, healthier soils as they decompose.

If we move the setting from a native forest to an urban landscape appearance safety concerns begin to impact the natural process.  Our landscape contractors come in to rake, blow, and vacuum leaves up, struggling against fall rains and a variety of tree species, some of which give their leaves up grudgingly over a long period of time. All of this is done to make our landscapes look manicured and free from “slip and fall” hazards.  This is a work-intensive process often resulting in increased waste, transportation, and fossil fuel usage. By removing the leaves we halt the natural decomposing cycle.

While removing leaves from hard surfaces such as sidewalks and parking lots will remain a critical part of effective risk reduction and property management, aesthetic expectations are evolving in regards to the rest of the landscape.  For evidence of this, type “leaf mulch” into your internet browser. It will result in pages of articles, research, and success stories encouraging you to simply leave leaves be! This is not just a homeowner trend, commercial and municipal property managers are finding greater acceptance for a different aesthetic from their tenants and communities who increasingly embrace sustainability.

Trees source mineral elements from the soil, some of which remain in leaves when they drop. Chief among these elements is carbon, which balances nitrogen in the soil and provides a food source to many different decomposers vital to the nutrient cycle.  As leaves break down they release a complex of acids, collectively called humic acid, that help form soil colloids, relieve compaction, and aid in moisture retention. Further, leaves as a mulch on the surface will insulate the soil, protecting tender plants from the cold and help suppress weed seed germination.

There are several ways to employ leaf mulch on a site in the fall. After leaves are blown off of hard surfaces a contractor can mow leaf-covered turf areas with a mulch mower, chopping leaves up and mixing them with grass clippings. This mixture will break down faster than leaves alone since the nitrogen in the grass clippings will excite microbial activity. As with regular mulch mowing, try to avoid leaving large clumps on the surface that can smother underlying grass.

In beds, whole leaves can often be left in less visible areas such as under and around large shrub, tree plantings, and in remote areas where aesthetics are not critical. In areas of higher visibility where this may not be acceptable, consider re-applying partially processed leaves that have been diced through a commercial leaf-vacuum. Another alternative may be on-site composting which can produce an even more refined product with the help of a contractor.

Whatever methods are employed regarding leaves this fall, stay focused on the idea of closing the loop of the nutrient cycle and keep leaves on site.