Apr 27, 2015
MSU tips for better blueberry weed control

Blueberry growers have several long-residual herbicides available for use in first-year and established plantings. Complete weed control is critical the first year to ensure high survival rates and quick establishment. Weed competition or herbicide injury can slow first year growth and reduce plant survival.

In established blueberries, effective preemergence weed control is important for production of quality fruit and new canes. Excessive weed pressure reduces new growth and eventually causes plants to regress. Early in the spring before new buds break and new growth emerges Roundup (glyphosate) may be applied to kill emerged winter annuals and perennials. Roundup, Rely 280 (glufosinate,) and Gramoxone (paraquat) may be applied with preemergence herbicides before bud break with little risk for crop injury. However, buds are breaking in most fields now so take care not to spray these materials over the crown of plants.

Several long-residual herbicides with different modes of action (MOA) are labeled for established blueberry. As a general rule, apply two residual herbicides with different MOA at any preemergence application (fall, spring or during the season). In addition, select herbicides that have activity against the weeds in the fields. Check herbicide labels for weeds controlled. Normally, weed suppression after premergence application lasts 6-8 weeks, after which another residual application is needed to suppress weeds through harvest. Postemergence herbicides may be applied to kill emerged weeds.

Herbicides for the year of planting are listed in Table 1. The herbicides labeled for first year blueberries have very low toxicity to woody plants, so there is little potential for crop injury. Once the plants have established a deep root system and the soil has sealed around the plants, more effective herbicides may be applied. Make extra effort the year of planting to keep the blueberry plants free of weeds.

In second and subsequent years, additional herbicides can be used. Observe label instructions about herbicide rates and soil type. Plants on light sandy soil and areas with high water tables are more susceptible to herbicide injury from herbicide movement into the root zone. Even herbicides with very low solubility can move into the root zone with heavy rains or saturated soil conditions.

Residual herbicides labeled for established blueberry are listed in Table 2. The photosynthesis inhibitors (PS II inhibitors) have a broad spectrum of control and provide 6-8 weeks or residual control. They control many broadleaves and annual grasses.Karmex (diuron) and Princep (simazine) have relatively low solubility and have been very safe on blueberries. Sinbar (terbacil) has a longer residual life in the soil and also is more soluble, so it should be used infrequently on light, wet soils. Velpar(hexazinone) is very soluble and should only be used once in 2 years on any field, and should not be used on very sandy or wet soils. It should only be used for control of woody vine, brush and perennial weed control. It controls or suppresses difficult weeds such as white heath aster, wild raspberry, blackberry, wild carrot, goldenrod, orchardgrass, and red sorrel. It has some activity against hemp dogbane and multiflora rose. Velpar does not control Virginia creeper or wild grape.

Kerb (mitosis inhibitor) may be applied in fall or spring for quackgrass suppression. Kerb suppresses most annual and perennial grasses for 6-8 weeks. Surflan (mitosis inhibitor) is effective against many annual grasses for 4-6 weeks. Solicam(pigment inhibitor) may be applied in fall or spring for annual grass, broadleaf and nutsedge control.

Chateau (PPO inhibitor) is effective in spring or fall for control of many annual broadleaves and grasses. If Chateau is used alone, horseweed will germinate in June. Zeus XC (sulfentrazone) and Zeus Prime XC (sulfentrazone plus carfentrazone) are PPO inhibitors with activity against many broadleaves. They have good postemergence activity, so application in the crop should kill many emerged weeds. Chateau has a 7 day preharvest interval (PHI) and Zeus/Zeus Prime have 3 day PHI, and can be applied close to harvest to extend preemergence weed control into late summer.

Casoron (cellulose synthesis inhibitor) may be applied in the fall or spring, for control of many annual and perennial broadleaves, grasses and yellow nutsedge.

Callisto (HPPD inhibitor) should be applied in spring for broadleaf control. It controls large crabgrass but no other grasses. It may be used as a broadcast spray in the aisles to take out broadleaves and crabgrass without injuring the fescue or ryegrass strip.

Matrix and Sandea (ALS inhibitors) are related chemically and should not be used together. Matrix controls most annual broadleaves and grasses but is weak on common groundsel, common lambsquarters and eastern black nightshade. Sandea controls yellow nutsedge and several broadleaves, including pigweeds, groundsel and ragweeds, but control of common lambsquarters and eastern black nightshade is poor. Matrix has a 21 day PHI and Sandea has a 14 day PHI, so both may be used during the fruit development period. Both herbicides have preemergence and postemergence activity. Sandea has no grass activity.

Dual Magnum (long chain fatty acid inhibitor) has a short residual period compared to the other blueberry residual herbicides. However, it provides good yellow nutsedge, eastern black nightshade and pigweed control. The PHI is 28 days. If eastern black nightshade is a problem, Dual Magnum applied 4 weeks before first harvest will help suppress nightshade berry production and avoid potential contamination.

Approximately 8 weeks after application, most of the preemergence herbicides will have lost their activity in the soil. Another preemergence herbicide may be applied then, assuming the PHI is less than the time to first harvest. Postemergence herbicides also should be applied during this period. Roundup (glyphosate) may be applied until 14 days before harvest. However, extreme care must be taken to avoid application on new shoot growth. Rely 280 (glufosinate) and Gramoxone (paraquat) are safer for use during the growing season for foliar burndown. They are not translocated within the blueberry plants so they cause no lasting injury, although they will burn off new growth. Aim (carfentrazone) also may be used postemergence in the crop, but it has a narrower spectrum of broadleaf control and no grass control. If grasses are present, include Poast(sethoxydim) or Select Max (clethodim) in the tank mix.

Blueberry growers have many excellent herbicides available for weed control in their crops. By selecting products based on their mode of action and weed spectrum, growers should be able to maintain a high level of weed control and avoid herbicide resistance in their plantings.

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Bernard Zandstra, Eric Hanson, Michigan State University

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