The program for the control of annual weeds in blueberries should consider the weed free strip under the row and the row-middles, sodded or tilled, separately. The “Weed Control Season” in blueberries starts in late fall.  The program implemented in the spring depends on what herbicides were applied the previous fall.  If herbicides were applied in late fall, applications may be able to be delayed until later in the spring.  Residual herbicides should be applied before bud break in late winter or early spring after the soil is no longer frozen if no late fall treatment was applied.

Winter annual weeds germinate in the fall or late winter, flower in the spring or early summer, then die.  Summer annuals germinate in the spring and early summer, flower, and die in late summer or fall.  Perennial weeds are weed species that live for more than two years.  Control of these weeds must be considered separately.

Emerged annual weeds under the row are controlled with a postemergence herbicide.  Annual weeds that germinate throughout the remainder of the season are controlled with residual herbicides.  Two applications of postemergence herbicide plus residual herbicides are recommended annually in the weed free strip under the row.

1. The first application should be applied in late fall, after the blueberries are dormant, but before the soil freezes, or in late winter before the buds break in the spring.  This application targets the control of winter annuals and provides early season control of summer annual weeds.  The fall can be a less busy time to apply herbicides to the fields, usually after Thanksgiving in New Jersey.  In March, growers find themselves scrambling to apply insecticides and fungicides, and prune.  Pruned branches must be removed or chopped before weed spraying can be accomplished after pruning.
2. The second application of residual herbicides should be applied before bloom or later spring, depending on the herbicides to be applied.  A postemergence herbicide may not be needed to control annual weeds in the spring if residual herbicides were applied in late fall. However, a postemergence herbicide may be included to control certain perennial weeds such as yellow nutsedge, Canada thistle, goldenrod species, or aster species.

Most residual herbicides primarily control annual grasses or annual broadleaf weeds (BLWs).  A combination of an annual grass herbicide and an annual BLW herbicide is usually recommended.  Rate ranges are recommended for most residual herbicides and will depend on soil type and organic matter content (see Table 3 in the 2017 Blueberry Weed Control Recommendations for New Jersey).  Use the lower rates in fields with coarse textured (sandy) soil low in organic matter, and the higher rate when soils are fine textured (silt and clay) and have higher organic matter. For efficient weed control, residual herbicides require a clean soil (no weeds, organic mulch or pruning residues) before spraying and need to be activated with a minimum of ½” of rain or irrigation in the week following application.

Casoron (dichlobenil), applied in late fall, followed by a spring application of a residual annual grass herbicide is the most effective residual weed control program recommended.  More different species of weeds are controlled than any other residual herbicide combination available.  Apply 4.0 lb active ingredient Casoron CS (2.7 gallons per acre) or 4.0 to 6.0 lb active ingredient Casoron 4G (100 to 150 lb per acre) in late fall when soil and air temperatures will remain below 50 degrees Fahrenheit until rainfall moves the herbicide into the soil.  The active ingredient in the granular formulation can be lost to volatilization in warm weather.  The Casoron CS formulation is encapsulated, which prevents loss due to volatilization.  Casoron provides annual broadleaf weed control until fall and annual grass control until early summer the next year.  Certain herbaceous perennials, including goldenrod species, aster species, and yellow nutsedge will also be controlled or suppressed by Casoron applied in late fall.  Late winter applications provide less consistent winter annual and perennial weed control.  Apply an additional residual annual grass herbicide in early or late spring to provide late summer annual grass control following the late fall application of Casoron.

If Casoron has not been applied in late fall, choose your residual annual grass herbicide for the coming season before the late fall or late winter herbicide application.  Options include Devrinol (napropamide), Surflan (oryzalin), or Solicam (norflurazon). All three residual annual grass herbicides can be used at the rate of 4.0 lb active ingredient per acre per year.  Apply half the yearly labeled rate, 2.0 lb active ingredient per acre, in the late fall, and the second half, an additional 2.0 lb active ingredient per acre, in the spring, or the full rate in early spring, in no late fall application was applied.

Your residual BLW herbicides should be chosen considering crop safety, effectiveness, and price.  For many years Princep (simazine) was recommended at 1.0 to 2.0 lb active ingredient per acre in the late fall, and Karmex (diuron) was recommended at 1.0 to 2.0 lb active ingredient per acre in the spring.  Both herbicides have been safe, reliable, and cost effective choices for many years, and continue to good options where their use provides good weed control.  Both Princep and Karmex share the same mode of action, inhibition of the light reaction of in photosynthesis.  Unfortunately, triazine resistant weeds (horseweed, common lambsquarters) with cross resistance to urea herbicides are present at some sites.

Where a triazine resistant weed has become established, switch to a BLW herbicide(s) with a different mode of action.  Use Chateau (flumioxazin) at 0.19 to 0.38 lb of active ingredient per acre or Callisto (mesotrione) at 0.094 to 0.19 lb of active ingredient per acre in late fall or late winter.  Chateau and Callisto must be applied before bud break in early spring to avoid crop injury.  Chateau can cause speckling and crinkling the crop’s foliage if spray drift occurs.  The activity of Chateau occurs at the soil surface as sensitive BLW seedlings emerge.  Do not disk, till or otherwise mechanically mix Chateau into the soil after application, or the effectiveness of the herbicide will be reduced or eliminated.  Callisto bleaches foliage white.  Horseweed, also called marestail or stickweed, and common lambsquarters are very sensitive to Callisto both pre and postemergence.  Chateau and Callisto can be used in combination, or either herbicide can be tank- mixed with Princep (simazine) and Karmex (diuron) to improve BLW control.

Sandea (halosulfuron) controls BLWs and yellow nutsedge in blueberries, and has postemergence and residual activity.  Sandea is an ALS inhibitor.  Herbicides with this mode of action rely on a single site of action in susceptible weeds, putting herbicides with this mode of action at high risk for weed resistance development.  Weed resistance to ALS inhibitor herbicides is already present in the New Jersey and the surrounding mid-Atlantic region.  Due to resistance management concerns, Sandea is recommended ONLY for emerged yellow nutsedge control later in spring, but not for residual annual weed control.

Stinger (clopyralid) is a growth regulator herbicide with postemergence and residual activity labeled in New Jersey for use in blueberries to control annual and perennial weeds in the legume and composite plant families.  Legume weeds found in blueberries include vetch and clover species.  Composite weeds targeted include horseweed, dandelion, aster species, goldenrod species, Canada thistle, and mugwort (also called wild chrysanthemum).  Stinger rates and application timing depend on the weed targeted.

When annual weeds have emerged before residual herbicides are applied, a postemergence herbicide should be included in the tank. Potential options include the following herbicides:

• Gramoxone or other labeled generic paraquat formulations applied at 0.6 to 1.0 lb active ingredient per acre plus nonionic surfactant to be 0.25% of the spray solution will control most of broadleaf and grass seedlings that are 2 inches tall or less. As a contact herbicide, paraquat will not be translocated and regrowth may occur from the root system of established weeds (taller than 2 inches).
• Roundup and other labeled generic glyphosate products can also be used to control emerged weeds as a spot treatment, and can be especially useful where susceptible perennial weeds are a problem. Take great care when spot treating with Roundup or other glyphosate formulations to never contact the blueberry bush, or serious crop injury could occur. The rate depends on the perennial weed targeted and the glyphosate product used.
• Rely 280 (glufosinate) is an alternative to glyphosate that is registered for use in blueberries. It is not as fast as Gramoxone, but tends to provide more complete and faster control than glyphosate without the concern for systemic movement in the blueberry bush. Similarly, to glyphosate, do not allow spray to contact desirable foliage or green bark as this would result in serious injury. Consult the label for preemergence herbicides that can be tank mixed to broaden the spectrum of weed control.

Consult the Commercial Production Recommendations for rates and additional information.