Q:
Is it necessary to fertilize trees?
Cameron O’Neill, Bloomington, IL
In addition to nutrients, a 2- to 4-inch-deep layer of composted mulch provides a host of other benefits to trees.
Photo/Illustration: Deana Tierney
A:
Rhoda Maurer, plant records supervisor at the
Scott Arboretum
of Swarthmore College, replies: Trees planted in urbanized or developed environments such as city parks, near roads or buildings, or in cultivated gardens or lawns are often exposed to stresses not usually found in woodlands or natural environments. When we rake leaves, remove topsoil from construction sites, and tidy up our gardens, we remove the natural source of nutrients available to plants: decaying organic debris. The nutrients from this debris keep trees in the wild from needing any additional fertilizing. But trees in cultivated areas often need extra attention.
Plants do not care what form of nutrients they receive, whether organic or inorganic, because they are used in their elementary form. However, along with many environmental and economic benefits, organic materials contain significant quantities of the major plant nutrients and a full range of micronutrients and trace elements not often found in inorganic sources.
Trees in cultivated areas simply need a periodic application of composted mulch to meet their basic fertilization requirements. The organic matter in the mulch acts as a slow-release fertilizer and offers improved water-holding capacity, moisture conservation, and increased microbial activity (which contributes to disease suppression) and root growth, while also reducing soil compaction and erosion. And mulches reduce the competition from weeds.
Many bark mulches are not composted—a process that breaks down pathogens often found in these mulches. It is very important to use mulch that has been composted for at least a year. Here at the Scott Arboretum, we prefer a mixture of composted leaves and bark.
It is best to apply composted mulch no deeper than 2 to 4 inches and extend it at least to the canopy drip line (roots actually extend well beyond this point). Also, be sure to keep the trunk flare and root crown free of mulch. Overapplication can lead to problems. Mulch volcanoes, where mulch is piled up around the tree trunk, promote excessive soil moisture and potential root rot. This excess moisture also sets the stage for pest and disease problems, the death of inner bark tissue, imbalances in soil pH, and anaerobic conditions, which produce alcohols and organic acids that are toxic to young plants.
If you’re in doubt of your tree’s health or if composted mulches are not available in your area, conduct a soil test before determining the appropriate inorganic fertilizer and application rate. Your local arboretum or ISA (International Society of Arboriculture) certified arborist may also be a good resource for additional information.
