Weed control in low-input or organically grown wild lowbush blueberries

Weed control in low-input or organically grown wild lowbush blueberries

Summary

Wild blueberries (Vaccinium angustifolium) are a crop with significant cultural and historical significance to the northeast, but are facing considerable competition from Little Bluestem (Schizachyrium scoparius), a weed present throughout Maine and New Hampshire. Due to the nature of most wild blueberry fields (shallow, low fertility, sandy) using conventional herbicides is not a sustainable practice as it poses a significant risk to local water bodies and drinking water.

We worked with 2 landowners who were facing significant Little Bluestem pressure to address these concerns using a sulfur treatment. While we have observed some success with the sulfur treatment, there was interest in an alternative application method. Through support of one of the landowners we have applied additional sulfur to a 2^nd site in a “dollop” method to evaluate the effectiveness of lowering the pH directly where the weed persists. This site will be monitored more loosely, but could be an effective management strategy to reduce existing weed pressure.

Objectives/Performance Targets

Two on-farm replicated trials were established in spring 2014 at two different low-bush blueberry fields. There is significant weed pressure at both sites. Plots measure 10 feet by 100 feet long and are replicated 4 times at each site. The four managements are low-pH, medium-pH, low-pH with nitrogen fertilizer, and control. Low-pH plots received enough sulfur to reduce the pH to 4.0 while 4.5 was the medium pH target. Prior to sulfur application soil samples were taken, and a weed evaluation was done. Weed evaluation was done again each spring and fall. We used a 2-foot square 5 times in each plot to measure weed size. We also rated the abundance (1=not severe, 10=very severe) in each plot during the spring and fall.

• Application of treatments – June 2014
• Weed evaluations completed
  • June 2014 (pre-treatment)
  • October 2014
  • June 2015
  • October 2015
  • July 2016
  • October 2016 (one site only)
• Post Treatment Soil pH Sampling – July 2014, June 2015, August 2015, July 2016
• Tissue sampling – August 2015, August 2016

We were not able to collect fall weed data from the “foss mountain” site. During data collection we found the site had been mowed and many of our field plot markers (flags) were destroyed during the process. We were not able to confidently recreate the plots in order to take this last data point. Since we have four data points to base recommendations, we did not feel the loss of this one data point would inhibit the outcome of the project.

Accomplishments/Milestones

• June 2014: Plots laid out by extension specialists at two sites
  • Sulfur applied, & weed data collected throughout the 2014 season with assistance from growers.
• September/October 2014 – numerous conversations with two growers about results of sulfur study
• December 2014 – presentation to landowner about 1^st year results
• September 2015 – presentation to landowner about 2^nd year results
• October 2016 – report provided to landowner about 1-3 year results

Impacts and Contributions/Outcomes

Review of the three years of spring & fall weed data indicates only minor differences by management. While this was easy for us to note visually looking at the plots during year 2, the data shows the control plots (no management) and the plots receiving fertilizer had greater weed abundance. Both data and observational information show low pH treatment has the lowest weed abundance generally, but data was messy across the two sites, making clear recommendations based on treatments difficult. After three years it is difficult to say that the sulfur treatments made a significant impact at controlling little bluestem. We noted an increase in vigor and productivity of weeds in the plots receiving nitrogen fertilizer immediately in year one, which had compounding impact on year 2 and 3.  While no statistical analyses have been done, these trends are true for both sites.

Soil data showed the target pH was not achieved, and after three years, the difference in soil pH between treatment plots and control plots was not great. As the organic matter level is quite high in these plots, it is possible that additional sulfur could have been applied during initial application to achieve the target pH. Better control could have been seen if we achieved the target pH.

We also noted that sulfur treatment as the single management strategy is not enough to maintain crop productivity when little bluestem persists. While we can see that sulfur does reduce weed pressure compared to our controls, the level of control is likely not enough for commercial production. Semi-annual burning or mowing of the crop while helpful for pest suppression of other insects and diseases does not deter little bluestem. It is advised that if little bluestem is detected in a field, an aggressive management strategy be put into place right away to control the weed. Sulfur treatment can be used as an organic strategy if detected early, but may not be the most effective approach if a field is already inundated with little bluestem.

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