Weed management for blueberry fields in the Pacific Northwest

Weeds are always a difficult problem in blueberry production! They start to show up shortly after transplanting, and tend to become more problematic as time goes on. There are, however, strategies that help to control weeds in blueberry. This presentation will discuss some thoughts about weed control in blueberry, and some of the strengths, and weaknesses, of currently registered herbicides.

Site Preparation

Let’s start first with fields that are soon to be planted to blueberry. Site preparation is perhaps the most important single strategy for combating perennial weeds in blueberry. It is best to start the year prior to transplanting blueberry plants. If perennial weeds are present, do whatever it takes to remove them before transplanting!

There are several herbicides available for use for site preparation in blueberries. You can use glyphosate (Roundup and several other products) for both broadleaf and grass weeds. If you just have grasses, Fusilade (fluazifop) may be a good choice. One proven method would be spray emerged and actively growing perennial weeds with a translocating herbicide, such as glyphosate. These products do not have a soil residual, so carryover damage to fall-planted blueberries is avoided. Cultivate the field about five days after application, when translocation has moved the herbicide into the farthest reaches of the weed’s root system. The physical damage to the weed from cultivation will aid in control as the herbicide hampers the weed’s ability to re-sprout. Depending on the infestation, however, there will be at least a few plants that are able to re-grow. These should be treated again, and as necessary through the summer to kill as many of the perennial weeds as possible. If your primary weeds are annuals, cultivation alone will probably be adequate, although herbicide may also improve control of older plants than cultivation alone.

As good a herbicide as glyphosate is, however, not all weeds sprayed with glyphosate are killed. A major weed species that isn’t killed is horsetail (Equisetum spp.). For that species, it’s best to use several applications of an auxinic herbicide (such as 2,4-D). These products are not strongly active on that weed (only about 30% control from each application), but repeat applications during the same growing season can add up to some good control. Cultivation alone, unless applied every three weeks or so throughout two or more growing seasons, will not control horsetail. Casoron (diclobenil) has good activity on horsetail, but it tends to persist too long in the soil to allow for safe transplanting of blueberry plants even six to eight months later. Contact herbicides such as Gramoxone Inteon (paraquat), Rely (glufosinate), or Scythe (pelargonic acid), or use of mowers or flamers, can remove emerged horsetail fronds. But as with cultivation, several applications will be necessary for complete control of this weed.

For organic fields, herbicides may still be an option for this phase of the production. Herbicides applied to control established perennial weeds prior to blueberry transplanting would, of course, start the certification process at zero, so three years will have to elapse from the date of application until the field can be certified organic. But since most blueberries won’t produce much fruit until the third year anyway, this might not be a hardship for all organic producers. If you do not wish to use herbicides to aid in this step, intensive cultivation will likely be necessary to control those established perennial weeds. For some, such as Himalaya blackberry, cultivation will do a great job of control. For others, such as Canada thistle or horsetail, you’ll need to cultivate frequently (every three weeks or so) to prevent the fragmented root system from re- establishing itself. Use of spring-tooth harrows may aid in bringing rhizomes and roots to the surface where they can desiccate.

It is important not to skimp on site preparation step! Selective removal of perennial weeds from blueberries with existing herbicides is difficult, if not impossible, so the more perennials you can kill prior to transplanting, the easier it will be to maintain your blueberry block.

First-year Blueberry Plantings (non-bearing)

After transplanting, maintain a three- to four-foot weed-free zone during the first two or three years to allow blueberries to establish effectively. Sawdust mulch can be effective for preventing seed of annual weeds from germinating the first few years, but no control of most established perennial weeds should be expected. Some growers are using plastic/fabric laid in the crop row, with blueberries planted into holes in the strips. Drip irrigation beneath the fabric is normally used for these systems. Weed mat may be the best choice for organic blueberry producers, or for those fields with extensive infestations of perennial weeds.

For new plantings (non-bearing), several broadcast herbicides are available. These include selective products like Devrinol (napropamide), Snapshop (isoxaben + trifluralin), Solicam (norflurazon), Surflan (oryzalin), simazine, and Trellis (isoxaben) for broadleaf and grass weeds, or Poast (sethoxydim), Select Max (clethodim), and Fusilade for grasses only. Non-selective products include glyphosate, Gramoxone Inteon, and Scythe.

If using glyphosate or other non-selective herbicides as spot treatments in your blueberry field, try to keep your sprayer pressure low to avoid upward movement of the spray mist due to turbulence. Avoid getting herbicide on leaves or areas of smooth bark either due to drift or accidental overspray of lower branches. For glyphosate in particular, it is best to use a shielded sprayer or wiper applicator—even small amounts of glyphosate drifted to blueberry leaves and bark can slow blueberry growth and reduce productivity. If milk cartons or plastic wraps are used around the new blueberry plants, the spray shield is less important, provided the carton/wrap gives complete coverage of stems and the lower canopy.

Established Plantings (over one year old)

For dormant-season applications to established plantings (over one year old and often bearing some harvestable fruit), broadcast herbicides include Casoron, Chateau (flumioxazin), Devrinol, diuron, Dual Magnum (s-metolachlor), Kerb (pronamide), simazine, Sinbar (terbacil), Solicam (norflurazon), Surflan, Velpar (hexazinone), and Zeus XC (sulfentrazone) for broadleaf and grass weeds. In addition, Callisto (mesotrione), Matrix (rimsulfuron), and Sandea (halosulfuron) can be used after weed emergence and still provide some residual activity. If only grasses are present, Poast and Select Max can be used on bearing blueberries. Spot treatments with glyphosate via spray or wiper, acetic acid (vinegar), Aim (carfentrazone), Gramoxone Inteon, Green Match (limonene), Rely (glufosinate), Scythe, and Stinger (clopyralid) are also allowed. Grass strips between rows can be treated with Saber (2,4-D), but only if using a shielded sprayer to prevent drift damage to blueberries.

Prior to selecting a herbicide to use, it is important to first identify the weeds you are trying to control! Not every weed is controlled by every herbicide, so you need to choose herbicides that will control the weeds you actually have in your field, and that begins with a positive weed identification. Never be content letting a plant go unrecognized—it is always easier to kill a pioneer plant than to have to kill several hundred (or thousand) weeds once they established themselves in your field. If you don’t know what it is, find out what it is by bring a sample to your county extension office or local agricultural professional.

More Specific Information About Blueberry Herbicides

Of the herbicides mentioned above, let’s categorize our next discussion on the two timings used to apply these products: preemergence herbicides and postemergence herbicides.

Preemergence products 

These are herbicides that should be applied prior to weed seed germination. If applied to weeds that are already out of the ground, most of these products will not control them. Preemergence products normally should be applied to fully dormant blueberries, although some can be applied after budbreak. These products include Casoron, Chateau, Devrinol, diuron, Dual Magnum, Kerb, simazine, Sinbar, Solicam, Surflan, Trellis, Velpar, and Zeus.

  • Casoron has both granular and sprayable formulations that should be applied during winter. Apply to moist soils, just prior to a rain event to move the herbicide into the soil. If left on the soil surface or if applied to warm soil, Casoron can lose much of its activity. Casoron has activity on many annual weed species (broadleaves such as common chickweed, shepherd’s-purse, pineappleweed, common lambsquarters, and common groundsel as well as most annual grasses). Casoron also gives suppression or control of several perennial weed species such as Canada thistle, curly dock, horsetail, and yellow nutsedge. Be sure not to over-apply, particularly on young blueberry plantings—plants must be at least one year old before Casoron should be used.
  • Chateau was registered for use in blueberry relatively recently, although it has been used in strawberry for several years. Chateau has appreciable postemergence activity, but is primarily used as a preemergence product. It controls a wide variety of broadleaf annual weeds such as common lambsquarters, horseweed (marestail), shepherd’s-purse, and redroot pigweed; it also helps suppress wild buckwheat and ladysthumb. Blueberry plants must have been established at least two years prior to use of Chateau, unless the young plants are protected by milk cartons or plastic wrap.
  • Devrinol may be applied from fall to spring, and is registered for use immediately following blueberry planting or on established blueberries. Devrinol is rapidly degraded if left exposed on the soil surface, so it should be applied less than 24 hours before a rain event to incorporate the herbicide in the soil. This product has activity on many annual weed species (broadleaves such as common chickweed, pineappleweed, common lambsquarters, and common groundsel as well as most annual grasses).
  • Diuron (several trade names) should be applied in the spring just prior to budbreak. Diuron is primarily a broadleaf product with activity on common lambsquarters, wild mustards, annual sowthistle, and redroot pigweed. Diuron is much more active in coarse textured soils, and can leach under periods of heavy rainfall shortly after application, so use lower rates under those conditions. This herbicide is absorbed and translocated exclusively in the xylem tissues of weeds, so it must be absorbed by roots for best effectiveness. Diuron does, however, control certain weeds if applied early postemergence.
  • Dual Magnum is a dormant-season herbicide that is widely used in agriculture. In blueberry, the supplemental labels OR-110005 and WA-120002. This herbicide controls many annual grass weeds such as barnyardgrass, green and yellow foxtail, Italian ryegrass, and witchgrass; broadleaf annual weeds controlled with this product include redroot pigweed and common purslane. Dual Magnum also suppresses emergence of yellow nutsedge. Use Dual Magnum only on plants established for more than one year, and lower rates are suggested on 2- to 3-year-old plantings.
  • Kerb should be applied during the winter dormant season. If applied to warm soils, Kerb persistence (and weed control) is much reduced. Do not use Kerb on blueberries that have not been established for about a year. Kerb is active on common chickweed, but it is primarily a grass herbicide, providing control of most annual grasses and the perennials quackgrass, orchardgrass, and tall fescue.
  • Simazine (several trade names) should be applied either all in the spring or as a split application with half in the fall and half in the spring. Higher rates are used on established blueberries, but lower rates are safe for use immediately following planting. Simazine is much more active in coarse textured soils, and can leach under periods of heavy rainfall shortly after application, so use lower rates under those conditions. This herbicide is absorbed and translocated exclusively in the xylem tissues of weeds, so it must be absorbed by roots for best effectiveness. The product does, however, control certain weeds if applied early postemergence. Simazine is effective on a many broadleaf weed species, including common chickweed, common lambsquarters, common groundsel, henbit, nightshade, redroot pigweed, pineappleweed, shepherd’s-purse, smartweed, and some mustards. Grasses controlled include most of the annual grasses as well as suppression of quackgrass.
  • Sinbar should be applied in the fall or spring to established, dormant blueberries. The herbicide should probably be applied to small areas at first to evaluate the potential for crop injury before spraying the entire block. Do not use Sinbar on coarse textured soils. Also, you’ll want to minimize consecutive years of use or applying Sinbar within two years of rotating out of blueberries, as the product has considerable soil residual. This herbicide is absorbed and translocated exclusively in the xylem tissues of weeds, so it must be absorbed by roots for best effectiveness. Sinbar does, however, control certain weeds if applied early postemergence. This product is effective on a many broadleaf weed species, including common chickweed, common lambsquarters, common groundsel, henbit, nightshade, pigweed, shepherd’s-purse, and smartweed. Grasses controlled include most of the annual grasses as well as suppression of quackgrass.
  • Solicam should be applied during the winter dormant season to established blueberries. If applied to warm soils, Solicam persistence (and weed control) is much reduced. Do not use Solicam on blueberries that have not been established for about a year. Solicam is active on common chickweed, false dandelion, pigweed, pineappleweed, shepherd’s- purse, and some mustards. Grasses controlled include most annual grasses and suppression of quackgrass.
  • Surflan, if used in newly-planted blueberries, should be applied to firm soil prior to applying sawdust. On established blueberries, apply either all in the spring or as a split application with half in the fall and half in the spring. Rainfall incorporation is necessary to prevent herbicide volatilization. Surflan is effective on a many broadleaf weed species, including common chickweed, common lambsquarters, pigweed, and shepherd’s-purse. Most annual grasses are controlled.
  • Trellis is currently registered for nonbearing (but established) blueberry. It may become registered for fruiting blueberry in the near future. Trellis primarily controls annual broadleaf weeds, such as horseweed, common lambsquarters, wild mustards, shepherd’s- purse, annual sowthistle, purslane, and common chickweed; higher rates may also suppress field bindweed and curly dock. Apply Trellis prior to weed seed germination, as it will not control emerged weeds. The product must be incorporated by a half-inch of rainfall or irrigation for best weed control.
  • Velpar is an early-season herbicide that is best applied before the lowest leaves of the blueberry plant have fully expanded. It has some postemergence activity on susceptible weeds that are less than about 2 inches tall/wide. Broadleaf weeds controlled by Velpar include common lambsquarters, several species of mustard family weeds, and common chickweed; it also provides some suppression of fireweed, dandelion, and Himalayan/evergreen blackberry. Velpar has activity on barnyardgrass, velvetgrass, yellow foxtail, and quackgrass. Use Velpar only on blueberry plants that have been established for three or more years.
  • Zeus XC is the same product as Spartan herbicide registered in strawberry, while Zeus Prime is a mixture of sulfentrazone and carfentrazone (think of it as a pre-packaged mix of Spartan and Aim). Do not apply to blueberry fields established for less than two years. Zeus is primarily a preemergence product, while the carfentrazone in Zeus Prime helps kill emerged weeds. Many species of broadleaf weeds are controlled with Zeus products, including redroot pigweed, catchweed bedstraw, common mallow, common lambsquarters, ladysthumb, wild mustard, and shepherd’s-purse. Some grasses may also be controlled, including the foxtails and witchgrass; it may also provide some suppression of yellow nutsedge.

Postemergence products.

These are herbicides that should be applied after the spring flush of weed seed germination is complete, or when perennial weeds have fully expanded leaves. These products include acetic acid, Callisto, citric acid, clove oil, Fusilade, glyphosate, Gramoxone Inteon, Green Match, Matrix, Poast, Saber, Sandea, Scythe, Select Max, and Stinger.

  • Acetic acid, citric acid, clove oil, Green Match, Scythe. These are contact herbicides that can be used in newly-planted or bearing blueberries. Since they have no residual activity, multiple applications will likely be necessary to control flushes of annual weeds as they germinate throughout the year. Broadleaf annual weeds are more susceptible than grass seedlings. Try to apply before weeds are in the 4-leaf stage of growth for best results. Control of perennial weeds is not likely, unless used repeatedly on shoots. These products are best used on warm days. Care must be taken to avoid movement of mist to blueberry plants. Acetic Acid formulations are available in several formulations, usually as ready-to-use concentrations. There is a general consensus that the product should contain least 20% acetic acid to provide acceptable weed control (provided there are no other active ingredients in the product). This product is generally acceptable for organically certified blueberry, but be sure to check local product lists to be sure it’s OK to use in your state and for trade names.
  • Callisto is a recently registered herbicide in blueberry, although it is widely used in corn and small grains. While listed here as a postemergence product, the herbicide also has some residual preemergence activity. Callisto works best on broadleaf weeds, including redroot pigweed, common chickweed, common lambsquarters, ladysthumb, and wild mustard; most grasses aren’t controlled by Callisto. The use of crop oil concentrate at 1% is recommended if weeds have already emerged. Do not apply Callisto to bearing blueberries after the onset of the bloom stage.
  • Citric Acid (see entry for acetic acid for additional information). Citric acid is rarely used as a single active ingredient, but rather is pre-mixed with other active ingredients. Citric acid is available in several formulations, usually in a ready-to-use product mix. This product is generally acceptable for organically certified blueberry, but be sure to check local product lists to be sure it’s OK to use in your state and for trade names.
  • Clove Oil (see entry for acetic acid for additional information). This product is generally acceptable for organically certified blueberry, but be sure to check local product lists to be sure it’s OK to use in your state and for trade names.
  • Fusilade is a grass herbicide that is active on a wide variety of annual and perennial species. Fusilade is registered only for use in non-bearing blueberries. Grasses are controlled best when actively growing; if grasses are stressed due to drought or other environmental factors, control will be reduced. Mix Fusilade with 1% crop oil concentrate or 0.25% nonionic surfactant prior to application for improved performance.
  • Glyphosate is active on many different weed species, from annuals to perennials and including grasses and broadleaves. Glyphosate only has activity if applied to foliage, so if weeds are not emerged, they will not be killed. Glyphosate actively translocates from leaves to roots in perennial plants. If treating perennial weeds, don’t apply glyphosate too early! Wait until shoots have become self-sufficient, photosynthetically-speaking. This usually occurs after the weeds begin to send up flower stalks (known as “bolting”). Glyphosate will not translocate as well when applied to perennial weeds shortly after emergence as when applied immediately prior to flowering (when the weeds are in the bud stage of growth) or in the fall on photosynthetically active plants. Glyphosate should only be applied as a shielded spray or using a wiper applicator. Care must be taken to avoid movement of mist to blueberry plants.
  • Gramoxone Inteon is a contact herbicide that can be used in newly-planted blueberries or as a spot treatment in bearing blueberries. Multiple applications of Gramoxone will likely be necessary to control flushes of annual weeds as they germinate throughout the year. Control of perennials is not likely, unless used repeatedly on shoots. The product is best used on small annual weeds and is most active on sunny days. Care must be taken to avoid movement of mist to blueberry plants. Performance is enhanced when mixed with 0.25% nonionic surfactant.
  • Green Match (see entry for acetic acid for additional information). This product is generally acceptable for organically certified blueberry, but be sure to check local product lists to be sure it’s OK to use in your state.
  • Matrix is a recently registered herbicide in blueberry, although it is widely used in tree and vine crops and in potato. Matrix has both postemergence and preemergence activity on many weeds, including common groundsel, common mallow, wild mustards, shepherd’s-purse, purslane, barnyardgrass, annual bluegrass, and green and yellow foxtail; it also offers good postemergence control of quackgrass. Matrix should not be used until blueberry plants are at least one year old. The use of nonionic surfactant at 0.25% is recommended if weeds have already emerged.
  • Poast is a grass herbicide that is active on a wide variety of annual species, but has limited activity on perennial grasses. Poast may be used in newly-planted or established blueberries. Grasses are controlled best when actively growing; if grasses are stressed due to drought or other environmental factors, control will be reduced. Mix Poast with 2 pints/acre crop oil concentrate prior to application for improved performance.
  • Saber is registered for use with a shielded sprayer in the grass strips between blueberry rows. Saber actively translocates from leaves to roots in broadleaf plants. When applied to perennial weeds, effectiveness is maximized after shoots have become self-sufficient, photosynthetically-speaking. Avoid cutting the grass for at least seven days before or after the application to aid in herbicide deposition and translocation. Care must be taken to avoid movement of mist or vapor to blueberry plants.
  • Sandea is a recently registered herbicide in blueberry, although it is widely used in tree and vine crops and in potato. Matrix has both postemergence and preemergence activity mostly on broadleaf weeds, including common lambsquarters, common mallow, prickly lettuce, wild mustards, redroot pigweed, shepherd’s-purse, ladysthumb, and willow- weed; it also can be used postemergence for control of yellow nutsedge, and may help suppress horsetail. Sandea should not be used until blueberry plants are at least one year old. The use of nonionic surfactant at 0.25% is recommended if weeds have already emerged.
  • Scythe (see entry for acetic acid for additional information). Gowan may gain a registration for Scythe in organically certified crops, but it is not currently registered for that use.
  • Select Max is a grass herbicide that is active on a wide variety of annual and perennial species. Select Max may be used in newly-planted or established blueberries. Grasses are controlled best when actively growing; if grasses are stressed due to drought or other environmental factors, control will be reduced. Mix Select with 0.25% nonionic surfactant prior to application to get the most out of the product.
  • Stinger is used in a number of agricultural crops and on rangelands/forests for noxious weed control. In blueberry, supplemental labels OR-100011 and WA-120002 have been registered. Blueberry plants are more sensitive to Stinger if applied prior to bloom and before and/or during the crop’s annual flush of growth compared to after bloom, so avoid using Stinger from one week prior to bloom until one week after bloom. Stinger is very active on weeds in Asteraceae (including Canada thistle, dandelion, and goldenrod), Fabaceae (such as clover), and Solanaceae (nightshades), but doesn’t have much activity on weeds from other plant families.

Problem Perennial Weed Species

The most problematic weeds in established blueberries are perennial species, particularly those species with widely-spreading, creeping roots and rhizomes. As discussed above, start clean! Few herbicides are selective enough to remove established perennial weeds from blueberry, so get rid of them prior to transplanting. Next, stay clean! Remove pioneer weeds before they establish, maintain good blueberry health, use good irrigation practices, proper disease and insect management, and pruning to insure good blueberry canopy development, and implement a good fertility program. All these will help your blueberries better compete with the weeds.

Four major weeds of blueberries are Canada thistle (Cirsium arvense), horsetail (Equisetum spp.), quackgrass (Elymus repens), and field bindweed (Convolvulus arvensis). Let’s look at control options for each of these weed species separately.

Canada Thistle 

Blueberry herbicides with the best activity on Canada thistle are Casoron, glyphosate, and Stinger. Apply Casoron to moist soil during dormancy and rainfall-incorporate. For best results with glyphosate, apply using a wiper applicator or shielded sprayer when Canada thistle plants are over twelve inches tall in spring or summer; fall applications are also good if thistles are still green. Stinger can be applied either as a foliar spray or wiped onto thistle foliage. Like for glyphosate, it’s best to wait until thistles are at least a foot tall before treating them. Stinger or Saber applied to grass strips may also help reduce Canada thistle encroachment from beside the rows. Contact herbicides with some effectiveness against Canada thistle (such as Gramoxone Inteon and Scythe), hoeing, or flame can kill top-growth of weeds, but these treatments must be applied repeatedly to be effective. Weed fabrics help to prevent emergence of Canada thistle, although this species is very good at finding holes in the plastic mulch through which it can grow.

Horsetail

The only herbicide registered for use in blueberry with good activity on horsetail is Casoron. Apply Casoron to moist soil during dormancy and rainfall-incorporate. Sandea applied postemergence may help to suppress horsetail. Like for Canada thistle, Saber applied to grass strips may help reduce horsetail encroachment from beside the rows. Also like for Canada thistle, contact herbicides (Gramoxone Inteon, Scythe, etc.), hoeing, or flame can kill top- growth, but these treatments must be applied repeatedly to be effective. Weed fabrics help to prevent emergence of horsetail, although this species is very good at finding holes in the plastic mulch through which it can grow.

Quackgrass

Some herbicides with good activity on quackgrass include Casoron, glyphosate, Kerb, Matrix, and Velpar. Apply Casoron, Kerb, or Velpar to moist soil during dormancy and rainfall-incorporate. For best results with glyphosate, apply using a wiper applicator or shielded sprayer when quackgrass plants are over twelve inches tall in spring or summer; fall applications are also good if grass leaves are still green. Matrix can be spot-applied to blueberry either preemergence or early postemergence, although the latter is generally considered to perform better. Grass herbicides with good activity on quackgrass include Fusilade (nonbearing only) and Select Max (both bearing and nonbearing); use similar application timing as discussed for glyphosate. Poast may also be used in bearing and nonbearing blueberries, but quackgrass control with this product is usually only fair to poor unless used repeatedly. Contact herbicides (Gramoxone Inteon, Scythe, etc.), hoeing, or flame can kill top-growth, but these treatments must be applied repeatedly to be effective. Weed fabrics aren’t particularly helpful for control of quackgrass, as this species bears sharp-tipped rhizomes that can penetrate all but the thickest plastic mulches.

Field Bindweed

The only herbicide registered for use in blueberry with good activity on field bindweed is glyphosate. For best results, apply using a wiper applicator or shielded sprayer when field bindweed plants are in full bloom to the early seed stage of maturity, application in the fall may also provide some control. Trellis at higher rates may also help to control field bindweed in nonbearing blueberry. Like for Canada thistle, Saber applied to grass strips may help reduce field bindweed encroachment from beside the rows. Also like for Canada thistle, contact herbicides (Gramoxone Inteon, Scythe, etc.), hoeing, or flame can kill top-growth, but these treatments must be applied repeatedly to be effective. A not-yet-registered herbicide with good activity on field bindweed is quinclorac. It has been tested in IR-4 and may soon gain registration in the near future, so stay tuned for news on registration progress. Weed fabrics help to slow emergence of field bindweed, although this species is very good at finding holes in the plastic mulch through which it can grow.

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