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 2nd 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
• Post Treatment Soil Sampling – June 2015
• Tissue sampling – August 2015
• 2016 goals include spring/fall weed analysis, summer leaf tissue sampling, and a final soil sample from each plot

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

Impacts and Contributions/Outcomes

Comparing the spring and fall weed data has shown some differences by management. While this was easy for us to note visually looking at the plots, the data shows the control plots (no management) and the plots receiving fertilizer had greater weed abundance. The low pH treatment has the lowest weed abundance generally, while medium pH while lower at some points, doesn’t differ greatly than the control plots. The plots receiving sulfur showed similar size & number of weeds in the fall as in the spring. Additionally, we noted an increase in vigor and productivity of weeds in the plots receiving nitrogen fertilizer in addition to lower pH, to the point where after 2-years, there are more weeds than blueberries. The weeds that received nitrogen looked healthier and were producing more seed heads. While no statistical analyses have been done, these trends are true for both sites.

Soil data showed that the target pH was not achieved. 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 lower the pH. Better control could have been seen if we achieved the target pH.

We also note 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. 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.

• Weed control images

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