Physical Weed Control Strategies
Physical Weed Control Strategies
What are the different methods of weed control?
Weed control can be achieved by three different methods – physical, chemical, and biological. Physical weed control is the management of weeds using physical means such as hands, tools, or machinery. Chemical weed control is implemented by using herbicides to prevent weed seeds from germinating or killing weeds after they have emerged. Biological weed control methods use insects and/or plants to prevent weed growth.
Figure 1: A visual representation of the soil seedbank and the outcomes that seeds and propagules can follow. Image credit: UC Weed Science Blog.
Challenges with Physical Weed Control
There are several shortcomings associated with physical weed control methods that make it difficult to solely rely on for weed management, and emphasize the need for further improvements in physical weed control methods and further research to support physical weed control as a tool in managing weeds.
One of the key challenges with physical weed control is the variability in weed control efficacy when compared to other methods. Most herbicides are 90-100% effective at killing weeds, whereas physical control methods range from 0-100% efficacy. Research suggests that when using physical weed control methods, using a combination of physical weed control methods, or multiple physical weed control methods used in succession can improve the efficacy of weed control and reduce the variability of weed control success.
Additionally, physical weed control can be challenging when trying to control weeds growing in the same row as crops, known as the intra row region—which is a relatively narrow space. Eliminating weeds in the intra row region is difficult; it requires tools and techniques selective enough to access them while avoiding or minimizing damage to the crop.
Weed Seed Bank Management
Figure 2: A tarped field after the crop has been terminated. Sandbags are used to hold the silage tarp in place for 5-7 weeks. Image credit: Cornell Small Farms Program.
The soil has viable seeds and/or vegetative propagules that may germinate and grow in unwanted places. These seeds and propagules make up what is referred to as the seedbank (Figure 1). To reduce the amount of work that is required for weed control in the future, it is crucial to reduce the number of seeds in the soil which can be done through a targeted approach to seedbank management.
One successful strategy for seedbank management is the stale seedbed technique. According to University of Maryland Extension (Chen, G. and C.R.R, Hooks, 2021), “Stale seed bed is a relatively simple weed management tactic that generally involves four steps: 1) a seedbed is prepared, 2) weed seeds in the shallow soil zone germinate naturally or via pre-irrigation and then emerge, 3) emerged weeds are then killed with minimum soil disturbance as necessary, and 4) the crop is promptly seeded or transplanted into mostly weed free soil.”
Various methods may be used to create a stale seedbed, but regardless it is important that soil disturbance is minimized to prevent the movement of seeds from deep in the soil to the surface layers.
Researchers at the University of Maine have found 6 mil silage tarps to be very effective for stale seed bed weed management. Tarps are installed after a crop has been terminated and the field has been harrowed (Figure 2). Depending on your climate and the season, an initial irrigation event may be required prior to tarping to help initiate germination of the weed seeds while they're under the tarp.
Tarps remain in the field for 5-7 weeks and can be held in place using weights such as sandbags, cinder blocks, or soil. Over the course of the 5-to-7-week period, weed seeds will germinate under the tarp and die off from heat and light depravation. The site will be ready for planting after the tarps have been removed. If there are any existing weeds, they may be terminated using a method that minimizes soil disturbance such as flame weeding. Research has shown that following this method of using tarps for stale seed bed preparation, weed densities can be reduced by four times and time spent hand weeding can be reduced by 50% to 100 hours per acre (Rebecca Champagne, University of Maine).
Cultivation Tool Stacking
Another useful technique of physical weed control to consider involves cultivation tool stacking for both tractor mounted and hand tools. Tool stacking can be done using multiple cultivation tools in combination or individual tools in sequence with each other (Figure 3). This is especially effective when used with seedbank management practices such as the stale seedbed technique. Tool stacking has been shown to increase the efficacy of cultivation tools, while reducing the variability of weed control.
The spacing of tools is particularly important to ensure that effective weed control is achieved while crop damage and mortality are minimized. Spacing that is too close may disturb the intra row region and result in excessive damage to your crops, while spacing that is too far apart won’t provide sufficient weed control. Regardless of whether you’re using tractor mounted or hand tools it is important that you test various spacing combinations to find one that provides the most effective weed control for your farming operation.
Figure 3. An example of cultivation tool stacking. A) Sweeps are used on the first pass, B) finger weeders and harrows are used in combination, C) a disk hiller is used on the last pass. Image credit: New York State Integrated Pest Management.
Timing of Weed Control
Another important aspect to consider for physical weed control is timing. This is of particular interest when controlling weeds after crops have been irrigated. If the soil is still wet during a weeding event, efficacy may be low since weeds that aren’t destroyed may re-root in the soil and continue growing. To help avoid this issue and improve efficacy of physical weed control it is best to let the soil begin to dry. Timing weeding events when the soil is dry and the temperature is warm is likely to provide the best control, as weeds are more likely to die within a few hours after weeding.
Different Resources on Physical Weed Management
Factsheets
The Stale Seedbed Technique: A Relatively Underused Alternative Weed Management Tactic for Vegetable Production. by Guihua Chen and Cerruti RR Hooks. 2021. University of Maryland Extension.
Strategies for Improved Weed Management on Small-Scale Diverse Farms by Jim Leap.
Integrated Weed Management by Steven A. Fennimore, University of California, Davis.