WSU Whatcom County Extension

Integrated Pest Management for Blueberries

Weed Management

 

Weed management in blueberries is critical for several reasons. Weeds compete with plants for water, nutrients, and light. They can also serve as alternate hosts for insects and diseases; for example, chickweed, dandelion, and red sorrel are alternate hosts of tomato ringspot virus. Weeds growing near the crown of blueberry plants can create a condition of high relative humidity, which favors various fungal diseases, such as Godronia canker and mummy berry. Weeds also interfere with irrigation and harvest operations, and provide habitat for vertebrate pests, such as voles. Finally, weeds that blossom at the same time as blueberries compete with the crop for bee pollination.

Weed control should begin during the year prior to planting. A combination of several well-timed cultivations and herbicide applications can greatly reduce weed populations. Timing and herbicide type used are dependent on the weed species present. In established fields, weed management must be viewed as a continuous effort, rather than a seasonal duty. Fields should be scouted at least twice during the growing season to keep abreast of changing species and locations of hot spots, by using the “weed inventory” monitoring technique described below. Perennial weeds should be removed before they can establish. Primary methods of weed management in blueberries include mulching, cultivation, and herbicide application. Blueberries are poor competitors with weeds.

 

Common weeds found in blueberry fields fall into four categories:

Annual Broadleaves

Perennial Broadleaves
Perennial Grasses

Other Weeds

 

Monitoring using the Weed Inventory Method

A “weed inventory” is a method of describing the populations of different weeds in a field so that effective treatment decisions can be made. By mapping areas with high levels of weeds, trends can be found in recurrence of certain weeds. These areas may need to be treated differently than the rest of the field. The efficacy of herbicide or cultural treatments can also be determined by keeping track of weed populations in a field before treatment, how they were treated and the weed populations after the treatment.

The weed inventory record sheet can be used to diagram the weed populations in a field. It has an area to record the field name, weather, sample date, and date last sampled. A legend of codes is provided on the sheet to use for common weeds. In the middle of the sheet is an area to draw a map of the field. When scouting for weeds, note areas with populations of weeds using the weed codes in the right hand column. Diagram areas of weeds using shapes relevant to area covered by weeds. Areas containing several weeds can be noted by writing several codes in one area.

Example of Weed Inventory Record Sheet

Weed Inventory Record Sheet (pdf)

The weed inventory should be completed at least twice per year: once before the time of each decision for herbicide application (for example, prior to bud-break and again in the fall, once the leaves drop, to get an idea of the weed pressure for the next season).

Weed profile pages are available for all noted weeds (see links in the bar at the left of this webpage). These pages include information on habitat, identification, life cycle, and management techniques as well as links to other Web pages with information about that weed.

Once the weed populations are determined in a field, a management plan can be devised for individual fields or areas in a field. This plan may include cultural management techniques (tilling, cultivation, mowing, smothering) and chemical management techniques.

 

Use of Herbicides

Herbicide recommendations and registrations change often. A matrix of weeds and effective herbicides is provided. Efficacy Ratings for Weed Management Tools in Blueberry Plantings (pdf)

These are herbicides that are believed to be effective, but this information may change. Growers need to stay up to date on new available chemistries for weed control and responsiveness of weeds to existing chemistries.

Make sure to check the chemical label carefully prior to each application to ensure correct application rates.

 

Herbicide Resistance

Herbicide resistant plants are those that can survive an application of an herbicide when the wild-type of that plant would be susceptible. In any population of weeds, plants resistant to a certain herbicide may be naturally present. By repeatedly using the same herbicide, or one with the same site of action (the biochemical site in the plant with which the herbicide reacts), resistant plants will remain living and continue to reproduce; this increases the population size of resistant weeds in a certain field.

Herbicide resistant weed populations are usually not noticed until they become approximately 30% of the population. To recognize herbicide resistant weeds, look for patches of a single weed species that are not killed when a previously effective herbicide has been applied. If the herbicide has been applied correctly, other weed species will be killed, leaving only the resistant plants in one species. If herbicide resistant weeds are identified in a field, contact your county extension educator immediately.

The risk of selecting for herbicide resistant plants can be prevented by the grower by following herbicide management practices. The following list of practices can be used along with the table on herbicide families found in the PNW 437 publication.

 

Herbicide Rotation

Prevent repeated applications of herbicides with the same site of action. Chemicals from different families may have the same site of action. Check the table in PNW 437 for information on herbicide site of action.

 

Short-residual Herbicides

Use herbicides that do not persist in the soil for long periods of time. Use of long-residual herbicides will increase the risk of resistance to herbicides since multiple generations of plants can occur during the period of herbicide efficacy. Weeds that are genetically resistant to the herbicide will remain and reproduce, which may result in a population resistant to the herbicide.

 

Cultivation

Using non-chemical methods of weed control destroys all weeds regardless of resistance to certain herbicides. The use of cultivation, tillage, and other cultural control methods should always be incorporated into a good weed control plan.

 

Accurate Record Keeping

For effective herbicide rotation, keep good records of what herbicides were applied, when, where, and how much was applied. Also monitor after the application to determine efficacy of the treatment.

 

Tank-Mixing

Tank-mixing is not an effective way to reduce resistance to herbicides. Tank-mix only when an herbicide combination is required to control the weeds in a field or a reduced level of herbicide is needed. Any other tank-mix can result in greater selective pressure for herbicide resistant weeds.

The development of herbicide resistant plants is a threat to growers and growers must do their part to reduce this risk. Following these guidelines and referring to the herbicide family table will help with good decision making on weed management so that herbicides remain effective.


General Weed References

 

Books

Whitson, Tom D., L.C. Burrill, S.A. Dewey, D.W. Cudney, B.E. Nelson, R.D.
      Lee and R. Parker. Weeds Of The West 9th edition. 2002. The Western
      Society of Weed Science in cooperation with the Western United States
      Land Grant Universities Cooperative Extension Services.

Uva, Richard H., J.C. Neal and J.M. Ditomaso. Weeds Of The Northeast.
      1997. Cornell University, New York.

Pojar, J., and A. MacKinnon. Plants of the Pacific Northwest Coast. 1994.
       B.C. Ministry of Forests, British Columbia, Canada, and Lone Pine
       Publishing, Auburn, WA.

Royer, F., and R. Dickinson. Weeds of the Northern U.S. and Canada.
       2004. The University of Alberta Press, Edmonton, Alberta, Canada, and
       Lone Pine Publishing, Auburn, WA.

Taylor, R.J. Northwest Weeds: The Ugly and Beautiful Villains of Fields,
       Gardens, and Roadsides. 1990. Mountain Press Publishing Company,
       Missoula, MT.

 

Web sites

Washington State University Extension Hortsense: Home gardener fact sheets for managing plant problems with IPM
http://pep.wsu.edu/Hortsense

University of California IPM Online Weed Gallery http://www.ipm.ucdavis.edu/PMG/weeds_common.html

Washington State University Weed Science Program at Mount Vernon
http://mountvernon.wsu.edu/WeedScience/index_WS.html

Pacific Northwest Weed Management Handbook
http://ag.ippc.orst.edu/pnw/weeds

Washington State Noxious Weed Control Board
http://www.nwcb.wa.gov

Pest Management Strategic Plan for Oregon and Washington Blueberries http://www.ipmcenters.org/pmsp/pdf/ORWABlueberry.pdf (pdf)

British Columbia Ministry of Agriculture and Lands – Field Guide to Noxious and Other Selected Weeds of British Columbia http://www.agf.gov.bc.ca/cropprot/weedguid/weedguid.htm

Weeds BC: Identification and Management of Weeds by B.C. Ministry of Agriculture and Lands
http://www.weedsbc.ca/index.html

Pacific Northwest Extension Publication: Herbicide Resistant Weeds and Their Management
http://info.ag.uidaho.edu/pdf/PNW/PNW0437.pdf (pdf)

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WSU Whatcom County Extension • 1000 N. Forest St., Bellingham, WA 98225 • 360-676-6736 • whatcom@wsu.edu