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Hort Notes: Selecting the Right Landscape Mulch

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March 4, 2016

Mulches are important components of sustainable gardens and landscapes, providing many benefits to plants. They help to keep the soil moist and to moderate soil temperature. Mulching also helps to prevent soil compaction, improve aeration and to increase water absorption and retention. Besides the benefits to plants, mulches also make surface areas more attractive and easier to maintain.

There are many materials that are used for mulching, each having different characteristics. When selecting the right mulch for the landscape, it is important to consider how you will use it. For permanent plants such as trees and shrubs, use organic mulches that persist for more than one season. On vegetable and flower beds, use organic mulches that breakdown in one season and are decomposed enough by the end of the season and can be plowed into the soil. In this sense, “organic” refers to materials that are plant based (bark, straw, wood chips, etc.). Materials such as gravel and stone are considered inorganic mulches.

Mulching suggestions

Trees and shrubs

Newly planted trees and shrubs benefit most from mulching. Mulching gives the newly planted trees and shrubs a competitive advantage over the lawn grass and weeds so that they establish quicker and grow faster than un-mulched plants. Mulching keeps the soil moist and helps to moderate the soil temperature. A mulched area around the base of trees and shrubs makes mowing easier and helps prevent mower damage to the plant. Broad leaved evergreen shrubs such as azalea, boxwood, Japanese hollies and rhododendrons also benefit considerably from mulching, which helps prevent winter drying of these plants which in turn causes foliage to become scotched and discolored over the winter. Drying develops when foliage loses moisture faster than it can be taken up by roots from a frozen soil.

Best mulches for trees and shrubs:

1. Wood chip mulch: Wood chips make excellent mulch. They are good for moisture retention, weed control, and they resist compaction. Wood chip mulches are also slow decomposers and thus supply nutrients slowly to the system. However, wood chip mulches tend to lose color quickly, making them less appealing for highly visible areas.

2. Bark much: Bark mulches are made from the byproducts of soft wood logs such as pine, fir, and cypress, or hardwood logs such hickory, oak and elm. They are available as shredded bark or nuggets. Bark mulches from mature softwood trees such as pine and fir decompose more slowly than hardwood bark mulches because they contain high levels of lignin, waxes and tannins. Hardwood bark mulches contain high levels of cellulose, so they decompose more rapidly.

3. Pine needles: Pine needles make excellent mulch. They decompose slowly and resist compaction, allow water to easily seep through, and also prevent weed seed germination. Although they are a byproduct of trees that prefer acid soil, they do not substantially change the pH of the soil if applied in only a 2 to 3 inch layer.

4. Stone or gravel mulch: Stone or gravel mulches can be used around trees and shrubs for weed control. These do not degrade and therefore don’t need to be replenished. They are good for weed control and water permeability, but they do not improve the soil. Apply layers about 1 inch deep.

Mulches come in different colors. The color does not matter to the plants, and is only for aesthetic purposes. There is no evidence that dyes used in coloring mulches are toxic. However if planning to use colored wood mulches, it is important to know the supplier and the source of wood used to make the mulch. Avoid mulches from recycled wood if it includes pressure treated wood (this mulch could be contaminated with chromated copper arsenate).

Don’t pile mulch around the trunk of trees. Keep mulch back at least 6 inches from the tree trunk and spread a uniform layer 2 to 3 inches deep in a wide band approximately 3 times the diameter of the rootball, and tapering to 1- 2 inches over the rootball. Mulch piled up against the trunk may cause bark decay and may create entry points for insects or disease organisms. Mulch piled against the trunk may also provide a refuge for rodents, such as voles, which may then feed on and girdle the bark. Avoid "volcano mulching" where mulch is placed high around the base of the tree trunk and resembles a volcano.

Perennials

Most perennials benefit from summer mulching. Summer mulching helps preserve soil moisture and reduces soil temperature. Cold hardy perennials also benefit from mulching during the winter to prevent alternate freezing and thawing of soil which may heave the plants out of the ground and expose them to drying. Straw, hay, wood shavings, shredded leaves, and pine needles are appropriate mulch materials for perennials. Spread a uniform layer of 2 to 3 inches deep on perennial beds.

Vegetable and annual flower beds

Vegetable and annual flower beds benefit from mulching. Mulches help conserve moisture and reduce weed problems. Straw makes the best mulch for vegetable and annual flower beds as the straw is largely decomposed at the end of the season. Straw suppresses weeds, conserves moisture and insulates well.

Author: Geoffrey Njue, UMass Extension

References
Rodale’s Organic Life. November, 2010. Mulch materials research report. November, 2010.

Chalker-Scott, L. 2007. Wood chip mulch: Landscape boon or bane? Washington State University

Edwards E. 2001. (Editor). Using mulches in managed landscapes. Iowa State University.