Weed Control

NOTE:  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" to 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 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 and Seedbed Preparation

See Stale Seedbed Technique (page 107) for information on the use of these herbicides.

carfentrazone (Aim EC): REI 12h, Group 14. Apply up to 2 oz/A per application, and do not exceed a total of 11.6 oz/ per season. 

glyphosate (Roundup Power Max): REI 12h, Group 9.  Can be applied before field preparation to control emerged perennial weeds, after stale seedbed preparation, before planting or after planting but before potatoes emerge. Any crop contact will cause crop death.

paraquat (Gramoxone SL 2.0*): restricted use. REI 12h, Group 22. Use 1– 2 pts/A. Preplant or preemergence.

pelargonic acid (Scythe): PHI 1d, REI 12h, Group 17. Use a 3 -10% solution (3 to 10 gallons per 100 gallons).

Herbicides Used Preemergence to Weeds

dimethenamid (Outlook): PHI 40d, REI 12h, Group 15.  Apply preemergence (after planting or after drag-off) as a single application.  Rates are 12 to 18 oz/A on coarse-textured soils and 14 to 21 oz/A on medium- or fine-textured soils, but are also influenced by soil organic matter content. See label for rate selection. 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 8.  Apply to dry soil surface and incorporate immediately to a depth of 2 to 3”.  ‘Superior’ potatoes are sensitive to Eptam and, under stress conditions, early season stunting may occur.  Do not exceed 14 pt/A per season.

   1. Preplant:  Apply and incorporate 3.5 to 9 pt/A just before planting; use at least 4.5 pt/A for quackgrass control and a minimum of 5 pt/A to control nightshade. For incorporated applications to beds, apply as a band application and incorporate with ground or power driven tillers.

   2. Preplant Before Bed Formation: Band application: Apply as a band where beds will be formed, equivalent to 3.5 pt/A (broadcast basis). Cover beds with 3 to 4” of soil with bedding discs, middle busters or other suitable bed making equipment. Care should be taken not to fold in the band treatment.

   3. After planting but before potatoes emerge:  Apply at a rate equivalent to 3.5 to 9 pt/A (broadcast basis).

   4. Drag-Off (Come Up, Weeding Time) Incorporation: Drag-off must occur prior to application and incorporation. Apply and incorporate 3.5 to 7 pt/A after drag-off. Use the higher rate for nutsedge control. Use spike-tooth harrows or cultivation equipment for incorporation to cover with 3 to 4 inches of soil. Care should be taken not to fold in the band treatment.

   5. Postemergence, Lay-by: Apply and incorporate 3.5 to 7 pt/A after potato plants have emerged from the soil. Use lower rate on coarse-textured soils. Incorporate immediately on a wet soil surface or on a dry soil surface incorporate within 36 hours. Care should be taken not to fold in the band treatment.

   6. Irrigation: Meter 3.5 to 7 pt/A into the irrigation water after clean cultivation. May also be applied and incorporated after a clean cultivation as a directed spray to the base of emerged potato plants.

fomesafen (Reflex): PHI 70d, REI 24h, Group 14. Apply 1 pt/A as a broadcast preemergence application after planting but before potato emergence. Potato varieties may vary in their response to Reflex Herbicide. When using Reflex Herbicide for the first time on a particular variety, always determine crop tolerance before using.  Do not apply Reflex Herbicide as a preplant incorporated application in potatoes or crop injury may occur. Do not apply to emerged potato plants or severe crop injury will occur.

linuron (Lorox DF): REI 24h, Group 7. Make a single application of 1.5 to 3 lbs/A after planting but prior to crop emergence. Rate is based on soil texture. See label for rate selection.  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.  Best results are obtained when application is made to moist soil, followed within 2 weeks by 1 inch to 2 inches of sprinkler irrigation or rainfall. May be tank mixed with s-metolachlor (Dual Magnum) to improve activity on annual grasses. See label for tank mixing info.

Herbicides Used Pre- and/or Postemergence to Weeds

metribuzin (Metribuzin 75): PHI 60d, REI 12h, Group 5. This product may be applied once preemergence and once postemergence.  Do not exceed 1.3 pounds total per acre per season. Early maturing smooth-skinned white and all red skinned varieties may be injured with postemergence applications. The varieties Atlantic, Bellchip, Centennial, Chipbelle and Shepody are sensitive to Metribuzin. Avoid postemergence applications on these varieties. Preemergence applications on these varieties may cause crop injury under adverse weather conditions, on coarse soils, under high soil pH, with higher rates per acre and with mechanical incorporation. See label for other cautions.  May be tank mixed with Dual Magnum, Eptam, or Matrix.  See label for details.

   Preemergence: Apply 0.3 to 0.6 lb/A as a broadcast spray after planting or drag-off but before crop emerges. Drag-off, if used, must occur prior to application. Do not incorporate into the soil. Constant tank agitation is required.

   Postemergence: Apply 0.3 to 0.6 lb/A as a broadcast spray over the tops of potato plants.

pendimethalin (Prowl H2O): REI 24h, Group 3. Apply 1.5 to 3 pt/A  (rate based on soil texture, see label) as a broadcast spray after planting or drag-off but before potatoes emerge. If rainfall does not occur within 7 days after application, shallow cultivation to a depth of 1" to 2" will improve control. May be tank mixed with Metribuzin or Lorox to improve broadleaf weed activity. May be tank mixed with Eptam 7E before planting to obtain control of yellow nutsedge or quackgrass.

Can be used early postemergence from crop emergence to the 6-inch growth stage. Do not apply postemergence 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 application, PHI 40d for lay-by application. REI 12h, Group 15.  Apply Dual Magnum, either incorporated, preemergence, or postemergence to potatoes after hilling/lay-by.  Use the lower rate on soils relatively coarse-textured or low in organic matter; use the higher rate on soils relatively fine-textured or high in organic matter. Effectiveness will be reduced if later cultural practices expose untreated soil. If cool, wet soil conditions occur after application, Dual Magnum may delay maturity and/or reduce yield of Superior and other early maturing potato varieties. Do not use on muck or peat soils. Do not apply both as a preemergence and an incorporated treatment. Do not apply more than 3.6 pt/A of Dual Magnum in a single crop season. Dual, alone or in combination with metribuzin, is especially helpful in controlling both yellow nutsedge and black nightshade.  See label for permitted tank mixes and instructions.

   Incorporated: Apply 1.0-2.0 pt/A to the soil and incorporate into the top 3 inches before planting, using a finishing disk, harrow, rolling cultivator, or similar implement. During planting and later cultural practices, avoid bringing untreated soil to the surface. Postplant incorporated application may be made any time after planting to drag-off, but before potato emergence. Use an implement that evenly distributes Dual Magnum in the top 2 inches of soil.

    Preemergence: Apply 1.0-2.0 pt/A, either after planting as a preemergence, or after drag-off or hilling treatment, but before weeds emerge.

    Postemergence After Hilling/Lay-by: Apply 1.67 pt/A postemergence to potatoes through after hilling/at lay-by to control Dual Magnum-sensitive species for remainder of the growing season. This application will not control emerged weeds. It may be applied over a previous Dual Magnum application, but do not apply more than 3.6 pt/A per season.

trifluralin (Treflan HFP): REI 12h, Group 3. Not for use in Maine.  Apply single application after planting either before potatoes emerge, immediately following dragoff, or after potato plants have fully emerged. Apply 1 to 2 pt/A of Treflan HFP, with rate based on soil texture and crop, see label for details.  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): PHI 60d, 4 hr REI, Group 2. Can be used pre- and postemergence.  Do not exceed 2.5 oz/A per year.  Can be tank mixed with several herbicides.  See label for details.

   Preemergence: Apply 1 to 1.5 oz/A after hilling or drag-off, but before potatoes emerge. Activation by a single rainfall or irrigation event (1/3" to 1") is needed within 5 days of application. If rainfall or sprinkler activation cannot be managed, waiting for weeds to emerge and applying postemergence would result in better weed control.  If weeds are present at application, add a nonionic surfactant at 1 to 2 pt/100 gal water. Weak on lambsquarters.

   Postemergence:  Apply 1 to 1.5 oz/A to young, actively growing weeds after crop emergence. Weeds less than 1" in height or diameter at application are most easily controlled. A rainfall or irrigation event (1/3" to 1") no sooner than 4 hours, but not more than 5 days after application, will activate Matrix in the soil and help provide control of subsequent flushes of annual weeds.

Herbicides Used Postemergence to Weeds

carfentrazone (Aim EC): PHI 7d, REI 12h, Group 14.  Aim is a burndown herbicide and will injure any foliage it comes into contact with. Apply Aim to row middles of emerged crops with hooded sprayers to control emerged weeds, including crops grown on mulch or plastic.  Prevent any spray from contacting the crop, or injury will occur.  For best results, make application to actively growing weeds up to 4 inches tall and rosettes less than 3 inches across. Good coverage is essential for good control.  Apply up to 2 oz/A per application, and do not exceed a total of 11.6 oz/ per season. 

clethodim (Select Max)PHI 30d, 24hr REI, Group 1.  Will control grass weeds only. Apply to actively growing grasses.  See label for rate selection.  Multiple applications permitted of 9 to 32 oz/A per application, minimum 14-days between applications, not to exceed 64 oz/A per year.  Add 0.25% v:v nonionic surfactant (1 qt per 100 gal of spray).  Can also be used as a spot-spray by mixing 1/3-2/3% (0.44 to 0.85 oz per gallon) Select Max and 0.25% v:v nonionic surfactant (0.33 oz per gallon).  Spray to wet, but do not allow runoff of spray solution.

pelargonic acid (Scythe):  PHI 1d, REI 12h, Group 17. Use a 3 -10% solution (3 to 10 gallons per 100 gallons). Use a 3 to 5% solution for annual weeds, a 5 to 7% solution for biennial and perennial weeds, and 7 to 10% solution for maximum burndown. Delivery rate for boom applications should be 75 to 200 gals of spray solution per acre; complete coverage of weed foliage is essential. Use a DIRECTED/ SHIELDED SPRAY; contact with crop will cause injury. For hand-held equipment, spray to completely wet all weed foliage but not to the point of runoff. Repeat applications as necessary. Tank mixes are allowed with this product. See label for complete details.

sethoxydim (Poast): PHI 30d, REI 12h, Group 1.  Controls grass weeds only.  Apply to actively growing grasses (see product label for susceptible stage).  Maximum 2.5 pt/A per application, minimum 14-days between applications.  Do not exceed 5 pt/A per year. Use with crop oil concentrate (2.0 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 used as a spot-spray by mixing 1-1.5% (1.3 to 1.9 oz per gallon) Poast and 1% v:v crop oil concentrate (1.3 oz per gallon).  Spray to wet, but do not allow runoff of spray solution.