carfentrazone (Aim EC)
Rate Up to 0.031 lb ai/A (2 fl oz/A Aim EC)
Time After crop emergence, but only to row middles. Use hooded and shielded sprayers.
Remarks Tank-mixes with other herbicides increase spectrum of control. Apply to actively growing weeds not more than 4 inches tall, or rosettes 3 inches in diameter. Coverage is essential for good control.
Caution Will burn crop if contacted; use only in row middles with hooded sprayers at slow speeds (less than 5 mph). Do not exceed 0.031 lb ai/A (2 fl oz/A Aim EC) per crop season.
Site of action Group 14: protoporphyrinogen oxidase (PPO) inhibitor
Chemical family Triazinone
dichlobenil (Casoron 4G)
Rate 2 lb ai/A (50 lb/A Casoron)
Time Apply to dormant rhubarb when temperatures are low (45°F) and rain is expected immediately after application, to activate herbicide and prevent loss by volatility.
Remarks Provides residual control of summer annuals at 50 lb/A product; suppresses some perennials. Even coverage is important for good weed control. Inhibits cell wall production.
Caution Experience is lacking in the PNW with this product. Treat small areas initially.
Site of action Group 20: inhibits cell wall synthesis Site A
Chemical family Nitrile
glyphosate (many products)
Rate Consult label.
Time Broadcast before crop emerges; after crop emerges use hooded and shielded sprayers in row middles, and wiper application in row middles.
Remarks Can apply in winter or early spring before rhubarb shoots or leaves emerge. Over-the-top wipers and directed applications are not permitted once the crop emerges. Treatment with other selective equipment (hooded and shielded sprayers) must be 14 days before harvest.
Site of action Group 9: inhibits EPSP synthase
Chemical family None generally accepted
halosulfuron-methyl (Sandea)
Consult "Use Precautions" and "For Optimum Results" sections of the EPA registered label for important usage information.
Rate 0.023 to 0.047 lb ai/A (0.5 to 1 oz/A Sandea)
Time Apply to dormant rhubarb.
Remarks Apply as late as possible to maximize efficacy, but do not apply after rhubarb leaves have begun to grow in the late winter or spring. Use a nonionic surfactant if labeled weeds have begun to grow.
Caution End-user or grower accepts any crop injury as a risk of application to this crop. Application of Sandea to rhubarb may slow growth. Start with lower rates to determine potential sensitivity of rhubarb to Sandea on your soil types and in your production system. Do not exceed 1 oz/A per year.
Site of action Group 2: acetolactate synthase (ALS) inhibitor
Chemical family Sulfonylurea
linuron (Lorox 50DF)
Rate Up to 1.5 lb ai/A (3 lb/A of Lorox)
Time Apply as a dormant application, before rhubarb leaves emerge from the ground.
Remarks Both soil residual and contact weed control. Lorox must be activated within 2 weeks of application for optimum control with winter rains or irrigation. Add a surfactant to increase contact activity. Do not exceed a total of 3 lb/A Lorox.
Caution Note ground and surface water advisories on main label. Toxic to fish and aquatic invertebrates. Do not apply directly to water or to areas where surface water is present or to intertidal areas below the mean high-water mark.
Site of action Group 7: photosystem II inhibitor
Chemical family Urea
mesotrione (Callisto)
Rate 0.188 lb ai/A (6 fl oz/A Callisto)
Time Apply to dormant rhubarb before spring green-up.
Remarks Provides preemergence and postemergence weed control. Add crop oil concentrate at 1% v/v or nonionic surfactant at 0.25% to spray solution if weeds have emerged.
Caution Applying Callisto to nondormant rhubarb may result in a temporary bleaching. Rain or irrigation after application may increase risk of injury to emerging rhubarb. Preharvest interval is 21 days. Do not exceed 6 fl oz/A or one application per year.
Site of action Group 27: inhibits 4-hydroxyphenylpyruvatedioxygenase (4-HPPD)
Chemical family Triketone
napropamide (Devrinol 50DF/DF XT)
Rate 4 lb ai/A (8 lb/A)
Time Apply to weed-free soil surface in winter, to dormant rhubarb, before weeds begin to germinate.
Remarks Do not apply more than 8 lb/A (4 lb ai/A) per crop cycle.
Site of action Group 15: inhibits very long chain fatty acid synthesis
Chemical family Acetamide
paraquat (Gramoxone, 2 lb paraquat cation/gal)
Rate 0.625 to 1 lb ai/A cation (2.5 to 4.0 pints/A)
Time Apply during dormant season before buds in crown begin to grow.
Remarks Add a nonionic surfactant or crop oil concentrate as specified on label; take care to avoid anionic formulations that form insoluble precipitates in the tank. Add nonionic surfactant at 8 fl oz/100 gal spray mix.
Caution Restricted use pesticide due to acute toxicity. For retail sale to and use only by certified applicators - not to be used by uncertified persons working under the supervision of a certified applicator. For retail sale to and use only by certified applicators - not to be used by uncertified persons working under the supervision of a certified applicator. Do not ingest or inhale spray mist. Wear protective face shields, respirators, and clothing and personal protective equipment (PPE) as specified on label. Do not exceed two applications or 3 pints/A per season.
Site of action Group 22: photosystem I electron diversion
Chemical family Bipyridilium
prometryn (Caparol 4L)
Rate 1 to 1.6 lb ai/A (2 to 3.2 pints/A Caparol on coarse-textured soil, 3.2 to 4 pints/A on fine-textured soil)
Time Dormant rhubarb, before leaves have emerged from the crown.
Remarks Burndown, and soil residual herbicide. Supplemental label. Use in at least 20 gal/A spray volume. Triazine resistant weeds will not be controlled.
Caution Preharvest interval is 40 days. Crop rotation interval ranges from 5 to 12 months. Use nonionic surfactant (0.5%) or crop oil concentrate (1%) if weeds have emerged.
Site of action Group 5: photosystem II inhibitor
Chemical family Triazine
pronamide (Kerb)
Oregon and Washington only
Rate 1 to 2 lb ai/A (2 to 4 lb/A)
Time Apply in fall or winter to established rhubarb for grass and chickweed control. Results are best if soil temperature is below 55°F, and rain or irrigation follows application.
Remarks Requires moisture from overhead irrigation or rain to activate. Inhibits root growth. Poor control of weeds in the Asteraceae (Composite) family.
Caution Restricted-use herbicide. Do not apply to newly transplanted rhubarb or to rhubarb that is actively growing. Preharvest interval is 38 days. Do not exceed 2 lb ai/A per year.
Site of action Group 3: microtubule assembly inhibitor
Chemical family Benzamide
S-metolachlor (Dual Magnum)
Rate 0.63 to 1.27 lb ai/A (0.66 to 1.33 pints/A)
Time Broadcast on soil surface before crop emerges.
Remarks Do not exceed 1.33 pints/A or one application per crop. Use lower rates on light-textured soils, higher rates on fine-textured soils. Dual Magnum will not control emerged weeds.
Caution Preharvest interval is 62 days.
Site of action Group 15: inhibits very long chain fatty acid synthesis
Chemical family Chloroacetamidem
sulfentrazone (Zeus XC, Sulfentrazone 39.6%, Aquesta)
Rate 0.14 to 0.25 lb ai/A (4.5 to 8 fl oz/A Zeus) depending on soil texture: coarse texture 4.5 to 6 fl oz/A; medium texture 6.0 to 8.0 fl oz/A; fine texture 8.0 fl oz/A
Time Apply to dormant rhubarb.
Remarks Indemnified special local needs label for Zeus (OR-150008). Rhubarb crowns must be established 1 year or more. Preemergence control of annual broadleaf weeds; postemergence control of susceptible weeds seedlings. Controls nutsedge. Irrigation or winter rainfall is needed to activate this herbicide.
Caution Before applying Zeus XC to rhubarb, users, producers, and/or applicators must read and follow the information presented under TERMS OF SALE OR USE section of the label. Significant rotation intervals for some crops, including 24 months for canola and other Brassica crops, and 36 months for beets.
Site of action Group 14: protoporphyrinogen oxidase (PPO) inhibitor
Chemical family Triazolinone