Why do we bother mulching around trees, shrubs, and perennials? It is often expensive, has to be frequently re-applied, and doesn’t even totally stop weeds. Do we only use it because it looks pretty? While it may look nice, the real purpose behind applying organic mulch is not merely aesthetic, nor is it solely weed prevention. Mulching around plants creates one of the best environments for growing in our urban soils.

Studies conducted at the Morton Arboretum in Chicago have shown that applying organic mulch increases the organic matter in the soil. Microorganisms that are naturally present in the soil degrade the mulch depositing decomposed organic material at the plants’ roots. This organic material provides necessary nutrients and holds moisture in the soil for the plants. Access to these nutrients and moisture is visible above ground in the healthy growth and appearance of the plant. It is the microbial degradation of the mulch that necessitates reapplication every two – three years. [Scharenbroch, Bryant C. & Watson, Gary G. (2014). Wood and Compost Improve Soil Quality and Improve Growth of Acer rubrum and Betula nigra in Compacted Urban Soil. Arboriculture & Urban Forestry, 40(6).]

As this degraded organic mulch is incorporated into the soil, it reduces compaction significantly. Excessive compaction is a common issue in urban soils that have been stripped, turned, and driven on repeatedly. Compaction reduces air between soil particles, limiting the roots’ ability to breath, causing stress on the plant. “Soil compaction is one of the biggest problems a tree root can have.” [“Tree Root Systems.” Iowa State University Extension and Outreach Horticulture and Home Pest News. ISU Extension and Outreach. April 1, 1992.] Compacted soils significantly limit the growth of plants, particularly trees and predispose them to infections. Regular mulching, has been shown to reduce compaction considerably in only a few years.

The benefits of mulching: increased organic matter, nutrients, water, and reduced compaction, can be realized using a wide range of materials. All of these materials need to be readily compostable organic (carbon containing) substances. Materials that are often used are wood, straw, evergreen needles, seed hulls (cocoa beans, coconut husks), and compost. Some of these materials, such as straw and wood chips take longer to decompose and consume more nitrogen thereby reducing their benefit. Evergreen needles may acidify the soil which can be troublesome for certain plants. Some seed hulls are toxic to pets and decompose completely in the same season reducing their extended benefit. Compost does little to prevent weeds and sometimes encourages them. Our preferred organic mulch is shredded hardwood. This product provides benefits up to three years and should be applied 3” deep around the absorbing roots of the plant. When mulch has noticeably thinned, it is time to reapply to continue benefiting the plants.

Coconut husks (coir)

Wood Chips

Pine Needles

Shredded Hardwood

Thinned mulch, reapplication overdue

Mulch after reapplication with shredded hardwood mulch

Photos, Horticulture Services
 

In addition to the underground benefits from regular mulching there are other, aesthetic reasons to mulch. It provides the landscape a fresh, new look, much like newly painted siding, or clean seal-coating. It can also prevent many weed seeds from germinating depending on the product chosen. However, from the plant’s perspective, the decomposition activity occurring underground is far more important. It affords the plant the best possible access to nutrients, air space, and water in otherwise limiting soils. Reapplication of mulch will produce a substantially healthier, more productive plant in your landscape.

Catherine Nickelson | Horticulturist | Arborist
Posted 08/18/2017