Blueberry weed management: early summer options

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Many blueberry fields in Southwest Michigan received heavy rains in late April that affected weed control. If fields were treated with preemergent sprays prior to the rains, efficacy may be reduced by runoff and leaching. Growers who did not get their preemergent treatments on before the rains could not get back into the fields until after many weeds had emerged, so control was reduced.

Leaching potential of preemergent herbicides

Heavy rains can reduce efficacy by leaching herbicides down in the soil, or by carrying the chemical away from the treated area by runoff. Leaching losses are most likely in blueberry fields because soils are often sandy. Runoff losses usually are associated with the movement of soil particles, and are less likely in blueberry fields with little slope. Estimating the leaching potential is not straight forward because it depends on both the solubility as well as how tightly it is adsorption to soil particles. Based on solubility levels and general field observations we would consider Velpar and Sinbar as highly prone to leaching (Table 1). Callisto is also very soluble, but we have limited field observations to draw from for this newer product. Princep, Solicam, Chateau and Casoron are expected to be the lease prone to leaching.

Table 1. Solubility and leaching potential of some blueberry herbicides.

Product Chemical Solubility (ppm) Leaching potential
Princep simazine 2 low
Karmex diuron 47 medium
Sinbar terbacil 710 high
Solicam norflurazon 28 low
Velpar hexazinone 30,000 high
Chateau flumioxazin 2 low
Casoron dichlobenil 21 low
Callisto mesotrione 15,000 high

Although these herbicides are unlikely to move out of blueberries fields with surface runoff, they may move within fields and concentrate in lower areas as water recedes. This could result in herbicide injury to bushes in low areas of fields treated with soluble herbicides such as Velpar or Sinbar. Bushes in low areas may also be stressed by prolonged periods of saturated soils as well.

Early summer herbicide options

Most fields of early season varieties are within a month of harvest so pre-harvest interverals (PHI) limit which herbicides can be used. The PHI’s for some blueberry herbicides are well defined on the labels, whereas others are vague (Table 2). We are now too close to harvest to apply preemergent herbicides. However, in fields where preemergent treatments were not applied, or where efficacy is poor due to rain, postemergent products Aim, Rely, and Roundup can still be applied, if used with caution. These chemicals will damage any green bark, new shoots, or leaves of blueberries, so minimize contact with bushes. Orient nozzles so they do not contact the base of bushes, or spot-spray by hand taking care to avoid the crown of plants. New canes are now emerging from buds on the crowns of bushes and these can be killed by Aim, Rely, or Roundup.

Table 2. Preharvest intervals or restrictions for some herbicides on blueberries.

Product chemical Pre-harvest restrictions
Preemergent herbicides  
Princep simazine Do not apply when fruit are present.
Karmex diuron Apply before germination of annual weeds.
Sinbar terbacil Apply before weeds emerge or in early seedling stage.
Solicam norflurazon Do not apply within 60 days of harvest.
Velpar hexazinone Apply before bud break.
Chateau flumioxazin Apply before bud break.
Casoron dichlobenil No clear restrictions stated.
Callisto mesotrione Apply before first bloom.
Post-emergent herbicides  
Aim carfentrazone Apply up to harvest
Gramoxone paraquat Apply before new canes or shoots emerge
Rely glufosinate Apply up to 14 days pre-harvest
Roundup glyphosate Apply up to 14 days pre-harvest

The work of Dr. Hanson and Dr. Zandstra is funded in part by MSU's AgBioResearch.

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