Feeding our urban forest

by Mark von Wodtke, FASLA, and Ben Wise, PhD

Trees. In addition to beauty and enhancing the value of property, trees are essential for a comfortable and healthy habitat. Properly feeding our urban forest helps sustain well-being, not just for us but for all living creatures. When we spread poisons on soil and trees we are poisoning ourselves and all other creatures that contribute to the health of our community ecosystem.

Here is what we should avoid, minimize, and use to feed our urban forest:

Avoid the use of chemical herbicides. Monsanto’s Roundup is widely used on GMO crops.  Independent studies are now finding health hazards in this application.  Roundup is also being used indiscriminately in our landscape industry and by homeowners. Consequently, we are ending up with high rates of application right here in our urban forest.

Avoid chemical pesticides.  This will reduce the risk of damaging the urban forest ecosystem. For example, scientific studies are finding that bees are being adversely affected by pesticides and herbicides containing neonicotinoids.  We must also avoid risks to human health.  There have been studies linking chemical herbicides and pesticides to cancers, hormone disruption, and birth defects. 

Avoid the use of growth regulators. Claremont currently applies the commercial growth regulator, Atrimmec, to mitigate fruit drop.  According to the Precautionary Statements accompanying this product, these chemicals are “not to be applied to water or to areas where water is present.”… “Do not contaminate water when disposing of equipment washwater or rinsate.” We must be forever conscious that we, including young and old, live in our urban forest and drink water that percolates through it.  Growth regulators may also weaken trees such as Liquidambars that are already being subject to disease.

Minimize the use of chemical fertilizers in our urban forest.  Artificial fertilizers can alter the chemical balance in the soil in detrimental ways.  Chemical fertilizers can also contaminate groundwater.  For example, we already have several high-nitrate wells in northern Claremont, likely contaminated by the overuse of chemical fertilizer by the citrus industry.

Use biochar as a soil amendment. Biochar is produced by heating plant waste anaerobically to 600 degrees Farenheit.  Landscape crews can mix inoculated biochar with soil when planting new trees and also insert it into the soil under existing trees. Biochar increases the soil’s capacity to hold water, reducing irrigation needs. It also provides habitat for mychorrhizae and improves the soil’s capacity to retain nutrients, reducing fertilization needs.  Having healthier trees that are resistant to disease reduces maintenance costs. Biochar also sequesters carbon, keeping it out of the atmosphere. (See www.biochar-international.org)

Also use biochar to filter storm water runoff from pavements. This keeps pollution from going into storm drain systems, impacting groundwater, streams and beaches here in Southern California. Cities are now required to do this under new clean water regulations (MS4) to deal with the pollution in runoff from pavements and urban landscapes.

Compost litter from trees and shrubs.  The heat from decomposition (usually about 135-160º F.) naturally controls many pathogens. Mix good quality compost with biochar and work into the soil to retain moisture, recycle nutrients, and provide beneficial microorganisms that can inhibit disease and pests.

Make mulch from woodchips that come from pruning healthy trees and shrubs.  Mulch, which is not heated to mitigate the transfer of pathogens, does help retain soil moisture and control weeds when it is spread around the base of trees and shrubs.  Mulch will eventually decompose in the planting bed providing nutrients to soil microorganisms which help sustain plants.

Use biological pest control. For example, ladybugs eat aphids. There is a statewide Integrated Pest Management program that Claremont can become a part of.  (See www.IPM.UCDavis.edu/index.html.)

Sweep up litter and recycle green waste.  Why would we accept the risks of applying plant growth regulators just to control the drop of seedpods from Liquidambars or fruit from trees like olives? We can easily sweep up debris or harvest fruit from our urban forest. Groups such as “Teen Green” could take on this task much like the volunteers who already do this at Pilgrim Place.  Green waste can be made into mulch, compost, and biochar to feed to our urban forest.  Fruit, like olives, has many uses.

Use pyrolysis to decontaminate what is trimmed from diseased trees and shrubs. This involves heating green waste from diseased plants at temperatures in excess of 600° F without air, to prevent diseases from spreading.  Pyrolysis produces sterile biochar that can safely be incorporated into the soil. Biochar can be made from any biological material, which might otherwise be carted off to the landfill.

Our judicial system makes the presumption of “innocent until proven guilty.” When it comes to ecosystems, however, it makes sense to presume that chemical interventions are “harmful until proven benign”

Act Now! Feed your trees properly and, as a community, let’s adopt what Tree Action Group of Sustainable Claremont is proposing for the City Tree Policy and Guidelines to make our urban forest healthier.


Mark von Wodtke, FASLA, is a professor emeritus of landscape architecture. Ben Wise, PhD, is a professor emeritus of microbiology.


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