Mustard Cover Crop Growth and Weed Suppression in Organic, Strawberry Furrows in California

in HortScience

Strawberry (Fragaria ×ananassa Duch.) production in California uses plastic mulch–covered beds that provide many benefits such as moisture conservation and weed control. Unfortunately, the mulch can also cause environmental problems by increasing runoff and soil erosion and reducing groundwater recharge. Planting cover crops in bare furrows between the plastic cover beds can help minimize these problems. Furrow cover cropping was evaluated during two growing seasons in organic strawberries in Salinas, CA, using a mustard (Sinapis alba L.) cover crop planted at two seeding rates (1× and 3×). Mustard was planted in November or December after strawberry transplanting and it resulted in average densities per meter of furrow of 54 and 162 mustard plants for the 1× and 3× rates, respectively. The mustard was mowed in February before it shaded the strawberry plants. Increasing the seeding rate increased mustard shoot biomass and height, and reduced the concentration of P in the mustard shoots. Compared with furrows with no cover crop, cover-cropped furrows reduced weed biomass by 29% and 40% in the 1× and 3× seeding rates, respectively, although weeds still accounted for at least 28% of the furrow biomass in the cover-cropped furrows. These results show that growing mustard cover crops in furrows without irrigating the furrows worked well even during years with relatively minimal precipitation. We conclude that 1) mustard densities of ≈150 plants/m furrow will likely provide the most benefits due to greater biomass production, N scavenging, and weed suppression; 2) mowing was an effective way to kill the mustard; and 3) high seeding rates of mustard alone are insufficient to provide adequate weed suppression in strawberry furrows.

Contributor Notes

We thank Rogelio Fuentes and the crew at Fuentes Berry Farms, Laura Murphy, Patricia Love, and Gustavo Reyes for their assistance with fieldwork. We also thank Michael Cahn, Richard Rosecrance, and Jim Leap for their review of the manuscript.

Corresponding author. E-mail: eric.brennan@ars.usda.gov.

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Article Figures

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    Saturated furrow bottoms and runoff from bare furrows in a commercial-scale organic strawberry field at the USDA-ARS in Salinas, CA. The plastic mulch–covered beds are 132-cm wide (furrow center to furrow center) and are ≈100-m long. The slope at this site is ≈1% perpendicular to the bed direction. The field drains toward the bottom of the photograph and then exits to the right toward the Salinas River which flows to the Monterey Bay. This bay has been a National Marine Sanctuary of the United States since 1992. The photograph was taken on 12 Dec. 2014 several hours after a 10-mm rain storm had occurred earlier that day. This illustrates the problem of plastic mulch concentrating rainfall, sediment, and nutrient laden runoff into the furrows. The dashed lines indicate the approximate high water mark on the ends of the beds.

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    Cumulative precipitation and average daily air temperatures from strawberry transplanting through the mustard cover crop growth period (shaded grey) during 2 years in Salinas, CA. Data were collected from the closest active station of the California Irrigation Management Information System; [South Salinas station no. 89 (Year 1), South Salinas station no. 214 (Year 2)]. The accumulated growing degree days (GDDs) during the mustard growth period were calculated with the single-sine method with a baseline threshold of 4 °C using the online calculator at the University of California Statewide Integrated Pest Management (http://www.ipm.ucdavis.edu). This calculator used temperature data from the Salinas municipal airport weather station that is ≈7 km from the experimental site. The X-axis on both plots runs from 28 Oct. to 9 Feb. The only irrigation that was applied to the cover crop occurred on 18 Dec. of Year 2 and was applied with a linear irrigation system.

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    Population densities of mustard cover crops planted in furrow bottoms of strawberry beds at two seeding rates and their difference during 2 years in Salinas, CA. Raw data points are shown in order of the replications (from left to right) as clusters around error bars that are 95% confidence intervals with the mean at the center horizontal line on the bar. The percentage above the difference is the average percentage change from the 1× to the 3× seeding rate. The population densities were determined at the end of the mustard growth period during Year 1 (9 Feb.) (A) and 55 d after planting (DAP), but during Year 2 (B) were determined at Dec. 9 (17 DAP).

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    Oven-dry shoot biomass of mustard cover crops in furrow bottoms of strawberry beds at two seeding rates and their difference during 2 years in Salinas, CA. Raw data points are shown in order of the replications (from left to right) as clusters around error bars that are 95% confidence intervals with the mean at the center horizontal line on the bar. The percentage above the difference is the average percentage change from the 1× to the 3× seeding rate. Biomass was determined on 9 Feb. (Year 1) (A) and 31 Jan. (Year 2) (B).

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    Shoot height of mustard cover crops in furrow bottoms of strawberry beds at two seeding rates and their difference during 2 years in Salinas, CA. Raw data points are shown in order of the replications (from left to right) as clusters around error bars that are 95% confidence intervals with the mean at the center horizontal line on the bar. The percentage above the difference is the average percentage change from the 1× to the 3× seeding rate. Year 1 (A) measurements in each replication were the average of three randomly chosen plants from the soil surface to the top of the apical tip, whereas during Year 2 (B), the measurements for each replication were the average of the two tallest plants from the cotyledon mark to the top of the apical tip. The dotted horizontal line is the approximate height of the bed top (37 cm).

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    Height to first true leaf bud of mustard cover crops in furrow bottoms of strawberry beds at two seeding rates and their difference during 2 years in Salinas, CA. Raw data points are shown in order of the replications (from left to right) as clusters around error bars that are 95% confidence intervals with the mean at the center horizontal line on the bar. The percentage above the difference is the average percentage change from the 1× to the 3× seeding rate. Year 1 measurements (A) in each replication were the average of three randomly chosen plants from the soil surface to the first true leaf bud, whereas during Year 2 (B), the measurements for each replication were taken from the cotyledon leaf scar to the first true leaf bud.

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    Shoot nutrient concentrations (AC) and C:N ratio (D) of mustard cover crops planted in furrow bottoms of strawberry beds at two seeding rates during Year 1 in Salinas, CA. Raw data points are shown in order of the replications (from left to right) as clusters around error bars that are 95% confidence intervals with the mean at the center horizontal line on the bar. The percentage above the difference is the average percentage change from the 1× to the 3× seeding rate.

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    Oven-dry shoot biomass of weeds in strawberry furrows with no cover crop vs. furrows planted with mustard cover crops at two seeding rates during Year 2 in Salinas, CA (A). Plot B illustrates the differences in weed biomass between the three treatments. The percentage above the differences in plot B is the average percentage change from the no cover crop to the 1×, no cover crop to the 3×, 1× to the 3×, respectively. Raw data points are shown in order of the replications (from left to right) as clusters around error bars that are 95% confidence intervals with the mean at the center horizontal line on the bar.

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    Relationship between mustard cover crop shoot biomass and weed shoot biomass in strawberry furrows planted at two seeding rates (1× and 3×) in Salinas, CA during Year 2. The solid line is the fitted linear regression equation and the dashed lines are the 95% confidence interval bands.

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    Total shoot biomass from weeds and mustard cover crops in furrow bottoms of strawberry beds with no cover crop and two cover crop seeding rates during Year 2 in Salinas, CA. Raw data points are shown in order of the six replications (from left to right) as clusters around error bars that are 95% confidence intervals with the mean at the center horizontal line on the bar. The percentage below each data cluster is the average percentage of total shoot biomass from weeds.

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