Weed Control

NOTES: For the herbicides listed below, one product trade name and formulation is provided for each active ingredient along with preharvest interval (PHI), restricted entry interval (REI), resistance management group number, and example of rates and special instructions. In many cases, there are other products available with the same active ingredient. However, not all products with the same active ingredient are registered for use in a crop. Always check the product label to be sure that the crop is listed before using. 

A common strategy of potato growers is to combine one of the "grass" herbicides listed (EPTC, metolachlor or pendimethalin) with one of the broadleaf herbicides listed below (linuron or metribuzin). In more northern areas of New England, grasses are much less of a problem due to cooler soil temperatures, and there are many cases where only a broadleaf herbicide is necessary. If only a broadleaf herbicide is used, sethoxydim (Poast), described below, could be used during the growing season to provide emergency or spot treatment of any emerged annual or perennial grasses that were not anticipated.

Yellow Nutsedge: The herbicides that will provide the best control of yellow nutsedge, include a soil-incorporated treatment of EPTC (Eptam), a preemergence application of metolachlor (Dual) or a postemergence application of Metribuzin. The best strategy is to map the problem spots in a field and make an application of either Eptam or Dual before the nutsedge emerges. A postemergence application of Metribuzin can be used to clean any areas that escape the Eptam or Dual treatment. The best time to apply a postemergence treatment of Metribuzin is when the yellow nutsedge is 4-6" tall. Remember that nutsedge is not a grass and will not be controlled by grass herbicides like Poast and Select Max.

Quackgrass: The best strategy for quackgrass control is an application of glyphosate (Roundup) to actively growing quackgrass in the fall prior to planting. An application of Roundup in the spring at least 3 days prior to soil preparation will suppress quackgrass but will not kill it. EPTC (Eptam) and sethoxydim (Poast) can also be used to suppress quackgrass during the growing season. Both of these herbicides will provide greater activity if the quackgrass rhizomes (underground storage roots) are cut thoroughly with a disc prior to planting the potatoes. Also, Poast will provide better suppression of quackgrass if the lower rate is used and repeated when the quackgrass regrows (about 2 weeks after the first application). Be sure to observe the preharvest interval for both Eptam and Poast.

Stale Seedbed

See Stale Seedbed Technique for information on the use of these herbicides or flaming.

• carfentrazone (Aim EC): PHI 0d, REI 12h, Group 14.
• glyphosate (Roundup Power Max): PHI 14d, REI 4h, Group 9.
• paraquat (Gramoxone SL 2.0*): restricted use. REI 12h or 24h, Group 22.
• pelargonic acid (Scythe): PHI 1d, REI 12h, Group 0.

Preemergent Herbicides (before weeds germinate)

• dimethenamid (Outlook): PHI 40d, REI 12h, Group 15. Apply 12-21 oz/A per application, only 1 application per year. Can be applied after planting or after drag-off.  Rate based on soil texture and organic matter content. Use the lower rate on sandy soils and the higher rate on silt and clay soils. In cold and wet growing conditions, Outlook application may result in delayed emergence or early season stunting.
• EPTC (Eptam 7E): PHI 30d, REI 12h, Group 15. Apply and 3.5-9 pt/A per application, multiple applications allowed per year. Do not exceed 14 pt/A per season. Can be applied to the soil surface before planting and immediately incorporated 2-3” into the soil, or as a band where beds will be formed and then covered with 3-4” of soil. Can also be applied after planting but before potatoes emerge, immediately after drag-off, at laby-by, or after a clean cultivation. Incorporate immediately on a wet soil surface or within 36 hours on a dry soil surface. Care should be taken not to fold in the band treatment. Rate based on soil texture, application timing, and weed composition. ‘Superior’ potatoes are sensitive to Eptam and, early season stunting may occur under stress conditions.
• ethalfluralin (Sonalan HFP): REI 24h, Group 3. Apply 1.33-2.67 pts/A per application. Do not exceed 2.67 pts/A per year. Apply broadcast, immediately after planting but prior to crop emergence. Rate based on soil texture. Must be incorporated for maximum effectiveness. Incorporate 0.5-1” into the soil with water through irrigation or rainfall within 2 days of application. Or mechanically incorporate 2-3” into the soil. Ensure incorporation equipment does not damage seed pieces or unemereged shoots, or expose untreated soil.
• fomesafen (Reflex): PHI 70d, REI 24h, Group 14. Apply 1 pt/A per application, only 1 application every other year. Can be applied as a broadcast application after planting but before potato emergence. Take care not to expose untreated soil. Has not been tested on all varieties, crop tolerance should be verified before treating the entire field. Do not apply to emerged potato plants or severe crop injury will occur.
• linuron (Lorox DF): REI 24h or 8d, Group 5. Apply 1.5-3 lbs/A per application, only 1 application per year. Rate is based on soil texture and weed composition. Apply after planting but prior to crop emergence. If beds are to be dragged and/or hilled, apply after the final dragging or hilling operation.  Do not spray over top of emerged potatoes. Plant seeds at least 2" deep. Apply before grasses are 2” tall and before broadleaf weeds are 6” tall, preferably just before or when weed seedlings emerge. If emerged weeds are present, add 1 pt surfactant for each 25 gal of spray mixture.  Application should be made to moist soil, followed by 1-2” of water through irrigation or rainfall within 2 weeks of application.
• sulfentrazone (Aquesta 4F): REI 12h, Group 14. Apply 3-8 oz/A per application, only 1 application allowed per year. Rate based on soil texture, organic matter and soil pH. Apply after planting and dragoff but before potatoes emerge, with a minimum of 10 gal/A of spray. Incorporate into the soil with water through irrigation or rainfall. If dry conditions persist within 7 days of application, incorporate no more than 2” into the soil to activate. Undesirable crop injury can result from irrigation with alkaline water of 7.5 pH or greater. You can increase the amount of available sulfentrazone in the soil by irrigating with highly alkaline water. Young or stressed crops and some varieties including Sangre, Shepody, and Snowden planted on coarse soil are susceptible to adverse effects. Has not been tested on all varieties, crop tolerance should be verified before treating the entire field. Do not use on soils that contain less than 1% organic matter. Do not apply to emerged potatoes.

Pre- and Postemergent Herbicides

• metribuzin (Metribuzin 75): PHI 60d, REI 12h, Group 5. Apply 0.3-0.6 lb/A per application, up to 2 applications allowed per year. This product may be applied once before and once after crop emergence. Do not exceed 1.3 lb/A per season. Can be applied after planting or immediately after drag-off but before crop emerges. Do not incorporate into the soil. Can also apply over the tops of potato plants, but not prior to rainfall or irrigation, on recently cultivated potatoes, or within 3 days after periods of cool wet weather. Apply with 20 gal/A of spray.
  Only make postemergence applications on russet or white skinned potatoes that are not early maturing, Early maturing smooth-skinned white and all red skinned varieties may be injured with applications after crop emergence. The varieties Atlantic, Bellchip, Centennial, Chipbelle and Shepody are sensitive to Metribuzin. Applications before crop emergence on these varieties may cause crop injury under adverse weather conditions, on coarse soils, under high soil pH, with high application rates, or with mechanical incorporation. May cause some chlorosis or minor necrosis. Crop tolerance should be verified before treating the entire field.
• pendimethalin (Prowl H2O): REI 24h, Group 3. Apply 1.5-3 pt/A per application, only 1 application per year. Rate based on soil texture and weed composition. Apply as a broadcast spray after planting or drag-off but before potatoes emerge. Do not apply to the variety White Rose. If rainfall does not occur within 7 days after application, incorporate 1-2" into the soil with shallow cultivation. Be careful not to damage seed pieces when incorporating. Can also be applied from crop emergence to the 6” growth stage. Do not apply after emergence if potatoes are under stress from cold/wet or hot/dry conditions or crop injury may occur.
• s-metolachlor (Dual Magnum): PHI 60d for applications at planting to drag-off, 40d for applications at lay-by, REI 12h, Group 15. Apply 1.0-2.0 pt/A per application. This product may be applied once before and once after crop emergence. Do not exceed 3.6 pt/A per year. Can be applied before planting or after plating but before potatoes have emerged. If applying before planting, mechanically incorporate 3” into the soil. If applying after planting, you can incorporate 2” into the soil but take care not to damage seed pieces. Can also be applied after potatoes have emerged after hilling/lay-by at 1.67 pt/A.
• trifluralin (Treflan HFP): REI 12h, Group 3. Apply 1-2 pt/A per application, only 1 application per year. Can be applied after planting either before potatoes emerge, immediately following dragoff, or after potato plants have fully emerged. Rate based on soil texture and variety.  Set incorporation equipment so that the bed and furrow will be uniformly covered with a layer of treated soil. If the layer of treated soil is not uniform and the herbicide is concentrated over the bed, potato emergence may be delayed and stem brittleness can occur. When applying and incorporating Treflan HFP after potato plants have fully emerged, do not completely cover the foliage with treated soil.
• rimsulfuron (Matrix SG): PHI 60d or 30d*, REI 4 hr, Group 2. Apply 1-1.5 oz/A per application, up to 2 application allowed per year, a minimum of 14 days apart. Do not exceed 2.5 oz/A per year. Can be applied after hilling, drag-off, or reservoir tillage, to a clean, newly prepared seedbed. Activate and incorporate 2-3” into the soil with 0.33-1" of water through irrigation or rainfall within 5 days of application. Can also be applied after crop emergence, when weeds are less than 1" tall or 1” in diameter. Activate with 0.33-1" of water through irrigation or rainfall no sooner than 4 hours, but not more than 5 days after application. Weed control may be less consistent in cold, dry conditions. *Some rimsulfuron products have a 60d PHI while others have a 30d PHI. Read label carefully and use correct interval for the product you are using.

Postemergent Herbicides (after weeds germinate)

• carfentrazone (Aim EC): PHI 7d, REI 12h, Group 14. Apply 0.5-2 oz/A per application, multiple applications allowed per year. Do not exceed 11.6 oz/A per season. Can be applied as a preplant burndown no later than 1 days after planting by seed. Can also be applied to row middles of emerged crops with hooded sprayers to control emerged weeds. Prevent any spray from contacting the crop, or injury will occur.  For best results, make application to actively growing weeds up to 4” tall and rosettes less than 3” across. Can use up to 5.8 fl oz/A as a harvest aid. Adequate desiccation is achieved within 14 days after application.
• clethodim (Select Max): PHI 30d, 24hr REI, Group 1. Apply 6-32 oz/A per application, up to 2 applications per year, a minimum of 14 days apart. Do not exceed 64 oz/A per year.  Apply to actively growing grasses. Add 0.25% v:v nonionic surfactant (1 qt/100 gal of spray).  Can also be applied as a spot-spray by mixing 0.33-0.66% (0.44-0.85 oz/gal) Select Max and 0.25% v:v nonionic surfactant (0.33 oz/gal).  Spray to wet, but do not allow runoff of spray solution.
• diquat (Reglone): PHI 7d, REI 24h, Group 22. Apply 1-2 pt/A per application, up to 2 applications allowed per year, a minimum of 5 days apart. Do not exceed 4 pt/A per year. For use as a preharvest desiccant. Do not apply to drought-stressed potatoes. Apply in a minimum of 20 gal/A of spray.
• glyphosate (Roundup PowerMax): PHI 14d, REI 4h, Group 9. Apply 10 oz to 3.1 qts/A per application, multiple applications allowed per year. Do not exceed 5 qts/A per year. Rate based on target weed species. Can be applied during fallow intervals, prior to planting or transplanting, at planting, or before crop has emerged. Could cause injury when applied prior to transplanting or direct-seeding into plastic mulch. Remove residual product from plastic mulch with a 0.5” of water through irrigation or rainfall prior to planting.
• paraquat (Gramoxone SL 3.0): REI 12h or 24h (application type), Group 22. Apply 0.7-1.3 pts/A per application. Up to 3 applications allowed per year, a minimum of 7 days apart. Do not exceed 4 pts/A per year. Can be applied before, during, or after planting, but before emergence (up until ground cracking) as a banded or broadcast application in a minimum of 10 gal/A of spray. May be fatal if swallowed or inhaled. Applicators must complete an EPA-approved paraquat training listed on the following website https://www.epa.gov/pesticide-worker-safety/paraquat-dichloride-training.... The training must be completed a minimum of every three years.
• pelargonic acid (Scythe): PHI 1d, REI 12h, Group 0. Apply a 3-10% solution (3-10 gallons per 100 gallons of spray). Rate based on target weed species. Apply in 75-200 gal/A of spray or with a spot sprayer until foliage is wet but not to the point of runoff. Repeat applications as necessary. Can apply for vegetative burndown during site preparation, prior to crop emergence from soil, as a directed and shielded spray during crop growth taking care to avoid contact with foliage and green tissue of crops, or after harvest. For beets, can also apply as a harvest aid to remove crop leaves.
• sethoxydim (Poast): PHI 30d, REI 12h, Group 1. Apply up to 2.5 pt/A per application, multiple applications are allowed, a minimum of 14 days between applications. Do not exceed 5 pts/A per year. Apply to actively growing grasses. Use with crop oil concentrate (2 pt/A) or methylated seed oil (1.5 pt/A). Note that crop oil can cause injury under hot and humid conditions. Can also be applied as a spot-spray by mixing 1-1.5% (1.3-1.9 oz/gal) Poast and 1% v:v crop oil concentrate (1.3 oz/gal). Spray to wet, but do not allow runoff of spray solution.