Annual forbs
Indiscriminate
Moderate–high ($1000–$5000/gross)
Low risk
Low risk
Low/no risk
Low (sensitive plants, ground-dwelling animals)
Overview
Mulch is “a protective covering… spread or left on the ground to reduce evaporation, maintain even soil temperature, prevent erosion, control weeds, enrich the soil, or keep fruit (such as strawberries) clean.” (Merriam-Webster dictionary). In the context of non-chemical weed management, mulches are generally derived from fresh or composted plant materials such as wood chips or bark but can also be other materials including straw, paper, nut shells, rice hulls, or other readily available material. Inorganic mulches include gravel and polypropylene cloth.
Sheet mulching is an alternative method of covering soil with a solid material such as cardboard, typically combined with layers of degradable mulch above and below. In addition to controlling annual plants, this method is used to suppress and sometimes control perennial plants including some hard to manage rhizomatous plants.
Disturbed sites where weeds are plentiful and native plant cover is low are excellent candidates for mulching. Mulch can be applied any time of the year and in most kinds of sites. Where it can be applied easily and economically, mulch provides excellent short- to mid-term (weeks to months) suppression of annual weeds. Control can be extended if the mulch is replenished to the depth required to suppress weed germination and growth. Mulch needs to be reapplied regularly to maintain an effective depth; therefore, it is usually not used for long-term control.
There are limitations related to scale, plant form, landscape characteristics, and application that may prevent this technique from being used in some situations. Mulch is heavy and bulky, making it difficult to move to remote sites. It is also best used at small scale (less than 1 acre) or for specific uses such as in the planting basin for new plantings. It is important to remember not to push mulch directly up to base of desirable plants to avoid creating an environment for moisture loving pests such as pathogenic fungi. Conversely, mulch applied correctly has been shown to reduce root rot in some tree species.
The cost of mulching is moderate to moderately-high. Input costs include the mulch itself, trucks to carry the mulch (to cover 1000 ft2 to a depth of 4” requires about 13 cu yards, which is about the volume of a large commercial dump truck), a crew to spread, follow-up hand weeding, and replenishment. Mulch can be spread using a manure spreader or from a truck mounted mulch blower. The latter method allows for covering a large area and it can be applied more precisely than the manure spreader. Sheet mulching requires the additional cost of sheet material and labor.
How to Use
Mulch can be applied to any level or slightly-sloped area. It is a very effective choice on bare, accessible, disturbed areas. Steep slopes are usually not a good choice as mulch can move with water flow, wind, and after heavy rain or irrigation. It can be used in most locations except areas adjacent to water bodies for similar reasons.
Mulch should be free of weed seeds and plant diseases. Avoid using stockpiled chips as they may harbor pests. Where possible use locally sourced mulch rather than bringing mulch from elsewhere. Chipping fallen or cut trees on site as a mulch is highly effective in isolated areas such as those in restoration. Even Arundo mulched using a tub grinder on site has been useful as mulch to prevent annual plant establishment. This mulch was used in upland, not riparian areas. Despite popular belief, eucalyptus wood chips also make a good mulch and will not negatively affect the growth of established plants. If chipping trees to use as a mulch in place, be aware that seeds of non-native plants may be spread in the process.
While a range of mulch sizes are desired in most cases so that they pack together and reduce spaces for weeds to pop through, the smaller the mulch pieces, the less the depth required. A mix of bark or wood chip sizes that are roughly ½-2” will suppress weeds and not break down too quickly. A 4” depth is usually what is needed for weed suppression when sites can be checked regularly, but 6-8” can be applied on sites that are not maintained regularly.
Alternative mulch materials or techniques such as weed free rice straw installed 3” deep have been used successfully for control of Ehrharta spp. (Grass and coastal sage scrub species were successfully seeded over the mulched area.) Burlap sacks can also be layered (pseudo-tarping) and then covered with mulch.
Sheet mulching is a specialized technique where a solid but biodegradable material such as cardboard is layered and covered with mulch. Additional layers of organic material can be added to increase microbial activity and enhance soil. An example is a layer of rice straw, covered with sheets cardboard, covered with another layer of rice straw and finally a top layer of wood-based mulch.
Sheet mulching is best on areas less than 1000 ft2 due to the amount of material inputs and labor required (but it is also not effective on areas that are too small, where there is a high perimeter to area ratio). . Many vegetatively reproducing plants growing in patches less than 1000 ft2 can be effectively suppressed using this technique.
Cardboard decomposes quickly, so transplanting into it is easy. However, prior to planting, do not put stakes in the cardboard or otherwise puncture the cardboard barrier. Avoid wetting the sheet mulched area as that encourages it to break down faster.
Below are step-by-step instructions for conventional and sheet mulching techniques:
Conventional Mulching
- Cut all aboveground biomass except desirable vegetation
- Remove thatch to ensure good soil contact and no protruding vegetation
- Ensure equipment and mulch does not have other pests (weeds, pathogens, insects)
- Spread mulch 6-8” deep
- Monitor mulch depth over time to maintain 4”
- Complement mulching with hand weeding and adding chips as needed
Sheet Mulching
- Prep the area by cutting down existing vegetation by mowing or using a weed whip
- Apply a base layer of 2-3” straw, leaf mulch, or other organic material (optional)
- Cover with cardboard, being sure to overlap edges to avoid cracks
- Cover cardboard with straw, leaf mulch, or other organic material. This layer should be about 5” deep if compressed but more if loosely stacked. (optional)
- Completely cover with wood chips (4-6"). A little leaf material may be included in the top layer.
Special Tips
Placing mulch around desirable plants may be difficult if the plants are small or have low foliage. Mulch should not be directly against a tree trunk. Nevertheless, unmulched areas around the tree or other plant will be a place where weeds can establish.
Optimal Conditions for Use
Relatively open and level areas that are accessible by vehicles. Gently sloping sites are also suitable if precautions are taken to reduce movement of the mulch during rain or supplemental irrigations. Mulch should be used as a short-term (1-6 months) to medium-term (6-24 months) method to manage weeds in large areas until a longer-term solution is in place.
Caveats
Gophers may burrow under mulch. This is more of a problem in sheet mulching where the gophers can tear holes in the cardboard.
Mulching is non-discriminate. Native plant seeds in the soil will also be suppressed.
Potential Hazards to Humans, Environment, and Cultural Resources
Human safety. Low risk. A particle mask, eye protection, and gloves are recommended to reduce exposure to dust and fungal spores present in mulch.
Cultural resources. Low risk. This technique may even help to protect cultural sites.
Habitat. Low-moderate risk. Mulch will fundamentally change the ground that it is applied to by reducing light penetration to the soil surface, increasing surface soil organic matter content, and increasing site surface water retention. Be aware of whether these effects are desirable or not. Mulch may crowd out desirable vegetation that many species are dependent on. If mulch is very thick it may be a fire risk under dry conditions. Keep mulch away from flammable structures. Pine needle mulch is especially combustible.
Sensitive species. Low-moderate risk. Mulching may create habitat for some species by providing cover and nesting material. May also create habitat for species that are not desirable at a site, or may degrade habitat by eliminating exposed bare ground, associated soil crusts, and nesting sites for some ground-dwelling species.
Erosion. Low risk. This technique reduces soil erosion, however mulch itself may move offsite with heavy rain, wind, or irrigation.
Consider Combining with the Following Non-Chemical Methods
Mowing, shallow cultivation, weed-whipping or any other mechanical method that reduces resident vegetation prior to mulching will improve the efficacy of this technique. Manual removal with hand tools can supplement mulching by removing the few weeds that may emerge through mulch.
Don’t Use This Technique When/For
Perennial plants are rarely controlled by mulch alone. Sheet mulching may be effective for some rhizomatous plants but bermudagrass and bindweed are a couple of the more difficult weeds to control with sheet mulching as they will continue to grow upwards and also tends to exploit any seams in the sheets, even when overlapped.
Mulch should not be used where at least monthly check and maintenance cannot be done. It should not be used where there is the likelihood of it being moved, either intentionally such as for planting or unintentionally such as through water or wind.
References
Chalker-Scott, L. 2007. Impact of Mulches on Landscape Plants and the Environment — A Review. J. Environ. Hort. 25(4):239–249.
Concilio, A.L. 2013. Effectiveness and Cost of Downy Brome (Bromus tectorum) Control at High Elevation. Invasive Plant Science and Management. 6:502–511.
Downer, A.J. 2010. Use of Mulches to Control Weeds in Landscapes. In: Landscape Notes 23(1):1-3. http://ceventura.ucanr.edu/newsletters/Volume_23_No129817.pdf
Engelhardt, Blake, Popovich, Steve. 2015. King fire aerial mulching: inspections, monitoring, and lessons learned. Cal-IPC 2015 Symposium, San Diego, CA. https://www.cal-ipc.org/wp-content/uploads/2017/12/5.AnotherExamplefromKingFireMulching.pdf
Holl, K.D., E.A. Howard, T.M. Brown, R.G. Chan, T.S. de Silva, E.T. Mann, J.A. Russell, and W.H. Spangler. 2014. Efficacy of Exotic Control Strategies for Restoring Coastal Prairie Grasses. Invasive Plant Science and Management 7:590-598.
Matzek, V., and C. Sifuentes-Winter. 2020. Non-herbicidal measures of control for slender false brome (Brachypodium sylvaticum), an invasive perennial grass. Cal-IPC Symposium presentation, online.https://www.cal-ipc.org/resources/symposium//nonherbicidal-measures-of-control-for-slender-false-brome-brachypodium-sylvaticum-an-invasive-perennial-grass/cal_ipc_symposium_2020_virginia_matzek_non-herbicidal-control-measures-for-slender-false-brome-brachypodium-sylvaticum/
Nyamai, P.A., T.S. Prather, and J.M. Wallace. 2011. Evaluating Restoration Methods across a Range of Plant Communities Dominated by Invasive Annual Grasses to Native Perennial Grasses, Invasive Plant Science and Management 4:306-316.
Shive, K.L., B.L. Estes, A.M. White, H.D. Safford, K.L. O'Hara, and L. Scott. 2017. Rice straw mulch for post-fire erosion control: assessing non-target effects on vegetation communities. International Journal of Wildland Fire. 26(6): 538-549. https://doi.org/10.1071/WF16208.
UC Cooperative Extension. Mulch. https://ucanr.edu/sites/fire/Prepare/Landscaping/Mulch/
Authors and Credit
Lead Author: Cheryl Wilen, University of California Integrated Pest Management Specialist, UC ANR
Co-authors:
Pamela
Beitz, Integrated
Pest Management Specialist, East Bay Regional Parks District
Jutta
Burger, Science
Program Director, California Invasive Plant Council
Katherine
Knecht, IPM
Specialist, Marin County Parks
Marla
Knight, U.S.
Forest Service (retired)
Eric
Wrubel,Biologist,
National Park Service
Additional Contributors:
Kimberly Crispin, Vegetation Ecologist, East Bay Regional Parks
District
Steve Petterle, Principal Landscape
Architect, Marin County Parks
Kirk Schroeder, Volunteer Coordinator, Marin County Parks
Antonio Swan, Landscape Services, Marin County Parks
Efficacies
| Rating | |||
|---|---|---|---|
| Plant | Flowering Period | ||
| Winter | No Information | ||
| Spring | No Information | ||
| Summer | No Information | ||
| Fall | No Information | ||
| Multiple Seasons | No Information | ||
| None | No Information | ||
| Plant | Germination | ||
| Winter | No Information | ||
| Winter / Spring | No Information | ||
| Spring / Summer | No Information | ||
| Opportunistic | No Information | ||
| Plant | Palatability | ||
| Yes | No Information | ||
| No | No Information | ||
| Partial | No Information | ||
| Plant | Plant Growth Form | ||
| Grass | Good | ||
| Forb | Excellent | ||
| Shrub | Poor | ||
| Tree | Ineffective | ||
| Vine | Poor | ||
| Plant | Plant Type | ||
| Annual | Excellent | ||
| Biennial | Good | ||
| Perennial | Fair | ||
| Plant | Propagule Production | ||
| Low (<1000/square meter) | Excellent | ||
| Moderate (1000–10,000/square meter) | Excellent | ||
| High (>10,000/square meter) | Excellent | ||
| Plant | Rate of Spread | ||
| High (doubling in <10 year) | Poor | ||
| Moderate (50–75% increase in 10 years) | Fair | ||
| Slow Rate (25% increase in 10 years) | Good | ||
| Plant | Resprouting / Regenerative Capacity | ||
| Low | Excellent | ||
| Moderate | Fair | ||
| High | Poor | ||
| None | Excellent | ||
| Plant | Seed Life | ||
| Short (≤3 years) | Excellent | ||
| Moderate (4–10 years) | Fair | ||
| Long (>10 years) | Fair | ||
| Plant | Type of Reproduction | ||
| Seed | Excellent | ||
| Vegetative | Fair | ||
| Seed & Vegetative | Fair | ||
| Plant | Type of Vegetative Reproduction | ||
| Rhizome / Stolon / Stem | Fair | ||
| Bulb / Corm / Tuber | Fair | ||
| Root sprout / Sucker / Crown sprout | Poor | ||
| Site | Existing Desirable Plant Cover | ||
| <10% | Good | ||
| 10–25% | Good | ||
| 26–50% | Fair | ||
| 51–75% | Poor | ||
| >75% | Poor | ||
| Site | Ground Condition | ||
| Muddy | Fair | ||
| Smooth | Excellent | ||
| Cobbly | Good | ||
| Rocky | Fair | ||
| Site | Habitat | ||
| Marsh / Wetland | Ineffective | ||
| Riparian | Poor | ||
| Grassland | Poor | ||
| Shrubland | Fair | ||
| Woodland / Forest | Good | ||
| Site | Level of Tolerable Disturbance | ||
| Low | Good | ||
| Medium | Excellent | ||
| High | Excellent | ||
| Site | Slope | ||
| Flat | Excellent | ||
| Moderate (10–40%) | Fair | ||
| Steep (>40%) | Poor | ||
| Site | Target Area | ||
| <40 square feet | Excellent | ||
| 0.001–0.01 acre | Excellent | ||
| 0.02–0.1 acre | Good | ||
| 0.2–1 acre | Good | ||
| 2–10 acres | Fair | ||
| 11–50 acres | Ineffective | ||
| 51–100 acres | Ineffective | ||
| >100 acres | Ineffective | ||
| Site | Targeted Invasive Plant Cover | ||
| <1% | Excellent | ||
| 1–10% | Excellent | ||
| 11–25% | Excellent | ||
| 26–50% | Excellent | ||
| 51–75% | Excellent | ||
| >75% | Excellent | ||
| Site | Vehicle Accessibility | ||
| Roadside | Excellent | ||
| <100 feet from road | Good | ||
| 100–1000 feet from road | Poor | ||
| >1000 feet from road | Ineffective | ||
