Mid-season weed control options for blueberries

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.        

Spring-applied herbicides do not always provide adequate weed control through the harvest season. Summer flushes of annual grasses (crabgrass, fall panicum) or broadleaves such as pigweed often are problems. Some Michigan blueberry areas received large amounts of rain in early June, and this may reduce the effectiveness of preemergent herbicides by leaching materials below the weed seed germination zone. Easily leached herbicides are soluble materials that are held less tightly to soil particles. The leaching potential is hard to predict exactly for all soils, but common blueberry materials might be ranked as follows:

Increasing leaching potential -->
Solicam < Karmex = Princep < Sinbar < Velpar

This means that weed control from Sinbar and Velpar is more likely to be reduced by heavy rains than perhaps Solicam. Unfortunately, herbicide options for use in mid-season are limited by label restrictions, primarily pre-harvest intervals. The label restrictions for preemergent materials are:

  • Solicam: 60 days PHI.
  • Karmex DF: Before germination and growth of weeds.
  • Princep Cal-90: Not when fruit are present.
  • Sinbar: During early seedling stage of weed growth.
  • Velpar: Before budbreak.
  • Chateau: Do not apply after bud break through final harvest.
  • Callisto: Do not apply after onset of the bloom stage.

Several post-emergent herbicides can potentially be used in the summer. Some can still be used before harvest, but others cannot be applied until after harvest. Each herbicide has different characteristics that need to be considered when making choices.

Aim, Gramoxone, and Rely are burn-down materials. Pre-harvest intervals are listed on the labels for Aim (one day) and Rely (14 days), but the Gramoxone label indicates it should not be applied after growth begins. These herbicides kill treated plant parts, but do not move within the plant. As a result, perennial weeds are usually not killed because the chemical does not move to the roots. Rely may move slightly in plants, because it often provides a slightly better control of some herbaceous perennials, such as dandelion. Aim is relatively inexpensive, but does not control grasses. Rely is much more expensive, but controls broadleaves and grasses. All three herbicides kill green bark and leaves of blueberries, so take care to keep spray off blueberries.

Fusilade and Poast are selective grass killers; they have no effect on bloadleaf weeds or blueberries. Poast has a PHI of 30 days, and Fusilade is only for non-bearing plantings. These products would not be effective if applied in the summer because the grasses are too old. Grasses must be treated when they are four to eight inches tall in the spring. If grass is taller, the control is poor.

Glyphosate (Roundup) products have a PHI of 14 days, so they can still be applied to most fields. Glyphosate is the most effective postemergent herbicide for blueberries, but also the most hazardous because it is absorbed by green tissues and moves throughout the plant. Perennial weeds are killed because the chemical moves to below-ground plant parts. Translocation is a two-edged sword. Glyphosate applied to blueberry branches moves within the bush, and can kill large canes or whole bushes. The most effective time to treat perennial weeds is late in the summer because absorbed glyphosate tends to move down to the roots. This timing is also most hazardous for blueberries. Use extreme care to avoid contact with blueberry tissues.

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