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*van Donk, Simon J., Dale T. Lindgren, Daniel M. Schaaf, James L. Petersen, and David D. Tarkalson. 2011. [https://www.researchgate.net/publication/265804348_Wood_chip_mulch_thickness_effects_on_soil_water_soil_temperature_weed_growth_and_landscape_plant_growth Wood chip mulch thickness effects on soil water, soil temperature, weed growth and landscape plant growth]. Journal of Applied Horticulture, 13(2): 91-95. | *van Donk, Simon J., Dale T. Lindgren, Daniel M. Schaaf, James L. Petersen, and David D. Tarkalson. 2011. [https://www.researchgate.net/publication/265804348_Wood_chip_mulch_thickness_effects_on_soil_water_soil_temperature_weed_growth_and_landscape_plant_growth Wood chip mulch thickness effects on soil water, soil temperature, weed growth and landscape plant growth]. Journal of Applied Horticulture, 13(2): 91-95. | ||
*Venner, K. H., Preston, C. M. and Prescott, C. E. 2011. [https://cdnsciencepub.com/doi/full/10.4141/cjss09109 Characteristics of wood wastes in British Columbia and their potential suitability as soil amendments and seedling growth media]. Can. J. Soil Sci. 91: 95_106. | *Venner, K. H., Preston, C. M. and Prescott, C. E. 2011. [https://cdnsciencepub.com/doi/full/10.4141/cjss09109 Characteristics of wood wastes in British Columbia and their potential suitability as soil amendments and seedling growth media]. Can. J. Soil Sci. 91: 95_106. | ||
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This page provides information on wood chips. While providing extensive information on wood chips, there is a section focused specifically on stormwater applications for wood chips.
Wood chips are small- to medium-sized pieces of wood formed by cutting or chipping larger pieces of wood such as trees, branches, logging residues, stumps, roots, and wood waste. They include bark, wood, and often leaves. Wood chips are rich in lignin, suberin, tannins. Common mulch sources include cedar, cypress, straw/hay, pine, and spruce. Organic wood mulch is often a byproduct of the lumber industry (typically shredded bark), wood recycling centers (i.e. pallets) or processed yard waste from public landfills.
In stormwater applications, wood chips are used as a mulch to provide one or more beneficial functions. Potential benefits of wood chips include but are not limited to the following. They
Physical and chemical properties of wood chips vary depending on the source, method of production, and age. Because of this variability, this page focuses on generic properties of wood chips used as mulch, except where otherwise stated.
As discussed below, the properties of mulch vary with several factors, including the tree species, source of chips (e.g. bark, wood), and initial condition of the chips (e.g. age, moisture content, chip size). Aged wood mulch, which is preferred, is commonly available as bark nuggets, and as both shredded softwood (such as cedar or fir) and hardwood. Wood mulch feedstock may be dictated by region as softwoods are more prevalent on the west coast of the U.S. and hardwoods on the east coast. A double shredded bark 3-inch mulch layer is generally used in biofiltration practices. Triple shredded bark mulch may contain too many fines and single shredded may contain larger mulch more prone to floating. Bark nuggets should also be avoided to prevent floating as they are less dense. When softwoods are used, texture should be evaluated as some softwoods can be somewhat “mouse-nest” in appearance and contribute to floating (Hills, 2019).
Fresh wood mulch should be avoided. Composted mulch is typically free of disease, insects and weed seeds. Fresh wood mulch can remove nitrogen from the soil, which may be beneficial to nitrogen removal from stormwater, but can strip nitrogen away from landscape plants. Fresh wood mulch is typically available as wood chips and can be more prone to floating, exposing the media surface to erosion and obstructing overflows.
Shredded wood should be avoided. Shredded wood mulch tends to clog soil and form a mat which can restrict infiltration.
This section includes a discussion of chemical and physical properties of wood chips, and potential contaminants in wood chips.
Physical and chemical properties of wood chips varies by the source (e.g. species, wood or bark) and the physical dimensions of the wood chips. Chips from wood typically are 70-80 percent cellulose and 20-30 percent lignin, while chips from bark are roughly 50 percent cellulose and 50 percent lignin ([1]). Other components comprise less than 5 percent of the total dry weight mass of wood chips.
The adjacent table summarizes select physical and chemical properties of wood chips. The table does not differentiate between species or whether the chips are from bark or wood residue. Some general observations regarding these include the following.
Chemical and physical properties of wood chips. For concentrations of metals, link here.
Link to this table
Property | Range found in literature1 | Median value from literature |
---|---|---|
Total phosphorus (mg/kg) | 0.027 - 0.24 | 0.13 |
Total nitrogen (mg/kg) | 0.31 – 1.8 | 0.38 |
Total potassium (mg/kg) | 100-1600 | 709 |
Total carbon (mg/kg) | 434-498 | 47.1 |
pH | 3.34-5.07 | 3.47 |
Cation exchange capacity (cmol/kg)1 |
|
|
Total calcium (mg/kg) | 600-6200 | 1190 |
Total magnesium (%) | 60-6200 | 189 |
Bulk density (g/cm3) | 0.138 - 0.422 | 0.293 |
Water holding capacity (% by wt) | 58.5 | |
Total pore space (%) | 60-63 | 61.5 |
Primary references for this data:
1CEC increased as chip size decreased
The adjacent table summarizes concentrations of heavy metals, selenium, and arsenic in wood chips in comparison with soil reference values (SRVs) and soil leaching values (SLVs). Concentrations in wood chips were typically well below risk criteria, with the only exception being maximum observed arsenic concentration, which exceeded the residential SRV. Additional data was found in the literature but is not included in the table. Other data in the literature show similar results.
Heavy metal concentrations (mg/kg) in wood chips (New York State Energy Research and Development Authority, 2013).
Link to this table
Metal | Mean1 | Median1 | SRV – residential2 | SRV – commercial | SLV3 |
---|---|---|---|---|---|
Vanadium | 0.02 | 0.57 | 1.08 | 16 | 8 |
Chromium (III) | 0.24 | 7.36 | 23160 | 100000 | >100000 |
Manganese | 70.6 | 272 | 2104 | 26000 | 260.4 |
Iron | 18.1 | 345 | 10808 | 100000 | na |
Cobalt | 0.03 | 0.14 | 4.62 | 69 | 54.1 |
Nickel | 0.36 | 1.98 | 170 | 2600 | 352 |
Copper | 1.32 | 3.41 | 2192 | 33000 | 1404 |
Zinc | 5.93 | 17 | 4632 | 70000 | 6008 |
Arsenic | 0.05 | 1.17 | 0.08 | 1.2 | 11.64 |
Cadmium | 0.005 | 0.079 | 1.59 | 23 | 17.62 |
Lead | 0.25 | 1.12 | 300 | 700 | 5401 |
Antimony | 0.005 | 0.397 | 93 | 6.2 | 1.82 |
Barium | 17.4 | 45.9 | 41000 | 3000 | 3368 |
Selenium | 0.04 | 0.09 | 1200 | 77 | 5.28 |
1 13 samples
2SRV=Soil Reference Value (mg/kg)
3SLV=Soil Leaching Value (mg/kg); assumes 3 foot thick media and 3 foot separation from groundwater
Leachate from fresh wood chips is acidic, produces chemical oxygen demand, and releases nutrients. Negative aquatic response to leachate has been observed near wood chipping facilities and may be due to COD, phenols, organic compounds, or resin acids such as isopimaric acid (IA) and dehydroabietic acid (DHAA) (Machrafi et al., 2007; Taylor and Carmichael, 2003; Rex et al., 2016). Toxic effects associated with high oxygen demand from wood stockpiles have been observed in nearby receiving waters (Tao et al., 2005; Kannepalli et al., 2016). Studies indicate leaching of nutrients and organic compounds that contribute to oxygen demand decrease with time (Machrafi et al., 2007).
Wood chips from recycled wood may contain creosote and CCA (chromated copper arsenate). Wood chips from recycled wood is often colored with dyes. Dyes are typically organic- or iron-based and have not been found to be toxic. However, if colored wood chips are used, the wood source should be determined University of Massachusetts, Amherst.
Most studies indicate that diseased mulch cannot transmit pathogens to the roots of healthy trees. Mulch should not be incorporated into soil, under which conditions pathogens may be transmitted to trees ([2]; [3]).
In this section we provide information on effects of wood chips on pollutant attenuation and on physical properties of soil and engineered media.
Wood chips increases the water holding capacity and water retention of soil and bioretention media, though these increases are less than compost and other mulches that have greater surface area (Davis and Whiting, 2013; van Donk et al., 2011; Perry). Shaw et al. (2005) observed an increase in water holding capacity of 0.81 in/ft in a soil in San Diego, California. Salzmon et al. (1958) observed water holding capacities of 99-138 percent of dry weight for wood chips from four species (red and jack pine, aspen, birch), with the lowest adsorption being for birch.
Wood chips reduce soil temperatures by retaining soil water and blocking direct solar radiation, and reduce diurnal temperature fluctuations. Temperature effects are typically limited to the upper 10 cm of the soil or media (Abdul Bari Awan, 1964; Kotze, 2012; van Donk et al., 2011, Perry).
Research shows mixed results for effects of wood chips on soil bulk density. Wood chips appear to have beneficial effects on reducing soil compaction, particularly when incorporated, but minimal impact in uncompacted soils (Choi et al., 2005; Venner et al., 2011; Tahboub et al, 2008; Qu et al., 2019; Scharenbroch and Watson, 2014; Antieau, 2017).
Recommended values for wood chips used in a growth media (Source: see reference list in this section) | |
pH | |
Electrical conductivity (ms/cm) | |
Cation exchange capacity (meq/100g) | |
Nitrogen (%) | |
Phosphorus (%) | |
Potassium (%) | |
Copper (% minimum) | |
C:N ratio (minimum) | |
Lignin (%) | |
Total organic matter (% minimum) | |
Moisture (%) | |
Ash content (%) | |
Impurities | |
Fiber content | |
Expansion | l/kg |
Water holding capacity | l/kg |
Specifications exist for wood chips and pellets used for energy. Specific standards do not exist for wood chips used for other practices, but the Forest Stewardship Council (FSC) certifies wood sources, which guarantees that the wood and bark is responsibly sourced. The FSC Controlled Wood Standard (FSC-STD-040-005 Version 3.0) requires knowledge of where the wood comes from, an evaluation of the risk that a wood source is in violation with unacceptable categories of wood, and mitigate actions to reduce any risk from the wood source.
Products receiving Mulch & Soil Council Certification must pass rigorous screening and are periodically audited to ensure the products meet Council standards. The certification ensures the product label is accurate and all ingredients are listed, and product claims have been verified. A Mulch & Soil Certification also ensures the mulch contains no chromated copper arsenate.
Some general guidelines for material selection are provided below [5].
Prabhu and Thomas (2002) provide an extensive discussion of wood chips decomposition.
Keep wood mulch away from the trunks of trees to prevent rot. If you are concerned about termites, use cedar mulch or keep other wood mulches at least 6 inches (15 cm.) from the foundation. Let your mulch age if you aren’t sure of your source. This allows time for any sprays that were used on the tree or diseases it may have had to break down.[7]
Read more at Gardening Know How: Types Of Bark Mulch: Tips For Using Wood Mulch In Gardens https://www.gardeningknowhow.com/garden-how-to/mulch/bark-mulch-in-gardens.htm
Using locally produced wood chips is a sustainable activity and keeps a useful product out of landfills.