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Seattle University


Leaving It to Nature

Written by Janice Murphy
October 25, 2011

I sometimes think that the breathtaking display of color and light in autumn is part of a divine plan to distract us from the impending dark and cold of winter. Turn any corner on campus at this time of year and you will be awed by the sight of a brilliant display, often with our majestic evergreens as a backdrop. It seems a miracle that once green leaves can suddenly sport wildly varied hues of color that almost vibrate with intensity.  

In fact, as daylight hours lessen in the fall, chlorophyll, the green pigment in leaves, begins to break down. Yellow and orange pigments already within the leaf become suddenly visible. Red pigment will be produced and will intensify with sunlight. Leaf senescence, as this process is known, is a complex phenomenon that involves biochemical changes in the leaf, preparing it for a glorious death. 

While the peak of the autumn season is all about the spectacular glory of their color and form, the late autumn is all about leaf cleanup in the landscape.  

At the SU Grounds Department, our practice is to let leaves remain in the beds wherever possible. The energy stored in those falling leaves is returned to the plants through processes of decomposition and mineralization by microorganisms in the soil.   

This nutrient cycling is a very important piece of our sustainable landscape management practices. We are mimicking the forest floor, where nature has devised a way to provide all the necessary nutrients needed for plant growth and vigor.

In addition to landscape beds, the campus grounds also consist of street surfaces and sidewalks that need to be kept clear of leaves for safety’s sake. And leaves left on a lawn through the winter will effectively kill the grass. (An excellent strategy if that’s your aim!)  

So leaf time means blower time too. Although our backpack blowers are low decibel, low emission models, they are still intrusive and not pleasant to be around. Ask any gardener! Every effort is made to use blowers during off hours around buildings. And letting the leaves decompose in the landscape beds where they fall also reduces the use of leaf blowers which means less carbon emissions and less noise.  

We have two other pieces of equipment for leaf removal, a walk-behind vacuum and a tow-behind sweeper and vacuum. Both of these machines do an excellent job of sweeping up and slightly macerating the leaves. These leaves are then used as part of the carbon source in our composting process that includes pre-consumer food scraps from the campus kitchens of Bon Appétit. The maceration helps keep the leaves from matting together, which inhibits their aerobic breakdown. The finished compost is then returned to the landscape beds on campus. Only the leaves that have been exposed to possible contamination in the parking lots are removed from campus. All others are put back into the landscape in one form or another. 

So as you stroll through the falling leaves this autumn, think of them not just as pretty or a nuisance, but as a valuable storehouse of nutrients for our trees and shrubs.

Janice Murphy is integrated pest management coordinator in the Grounds Department.