Weed Control

Weed maps of field areas are extremely helpful in planning weed control strategies. A weed map can illustrate problem areas so that growers can target specific problems in specific areas. A weed map can also indicate shifts in weed pressure if kept continuously over years and indicate the possible need for a strategy change.

NOTE:  For the herbicides listed below, one product trade name and formulation is provided for each active ingredient (common name) as an example of rates, days to harvest (dh), REI, and special instructions. In many cases there are other products available with the same active ingredient. Please see Table 17 and Alphabetical Listing by Trade Name for more information on products with the same active ingredients. 

Preplant and Stale Seedbed Application

glyphosate (Roundup 4S): 1 to 5 qt/A (REI 12h, Group 9). Apply either before field preparation to control emerged perennial weeds or as a replacement to the drag-off operation as described above. Must be applied either before planting or before potatoes emerge; any crop contact will cause crop death. See also Stale Seedbed Technique.

paraquat (Gramoxone Inteon* 2S): 1.0 to 2.0 pt/A (REI 12h, Group 22) in 100 gal water plus 8 oz of nonionic surfactant. Apply at or before ground crack on round white and before ground crack on Russet Burbanks. Follow all precautions on the label. If the bed is adequately shaped and the potatoes are planted at the proper depth, Gramoxone Extra can be used to replace drag-off as a means of controlling emerged weeds prior to potato emergence. This application, also called the Stale Seedbed Technique.

pelargonic acid (Scythe 4.2): 3 to 10 gal/A (REI 24h, Group 27). See Stale Seedbed Technique.

Preplant Incorporated and Preemergence Control of Annual Grasses and Certain Broadleaf Weeds

EPTC (Eptam 7E): 4 1/2 to 7 pt/A (45 dh, REI 12h, Group 8). Use the higher rate for control of yellow nutsedge and quackgrass. “Superior” potatoes are sensitive to Eptam and, under stress conditions, early season stunting may occur. Apply to loose mellow-moist or drier soil. Incorporate to a depth of 2” to 3” within 20 minutes after application. Drag-off, if used, must occur prior to application and incorporation. May also be applied and incorporated after a clean cultivation as a directed spray to the base of emerged potato plants.

EPTC (Eptam 20G): 15 to 30 lb/A (REI 12h, Group 8). All of the same precautions listed above for Eptam 7E apply. This formulation has the greatest utility as a layby application for nutsedge control after potatoes are emerged. Do not apply when the soil is wet. If applied over emerged potato plants, apply when vines are dry and use a cloth drag to knock granules off plants.

metolachlor (Dual Magnum): 1 to 2 pt/A (REI 12h, Group 15). Use the lower rate on light soils. If cool, wet soil conditions occur after application, Dual may delay maturity and/or reduce yield of ‘Superior’ and other early-maturing potato varieties. Dual, alone or in combination with metribuzin, is especially helpful in controlling both yellow nutsedge and black nightshade. May be applied preplant and incorporated to a depth of 3”. May also be applied after planting as a preemergence or delayed preemergence treatment to clean soil before potato plants emerge. Drag-off, if used, must occur prior to application. May be tank mixed with metribuzin (Sencor or Lexone) or with linuron (Lorox) to improve broadleaf weed activity. Do not incorporate these tank mixes. A prepackaged tank mix of metolachlor plus metribuzin is available as Turbo 8EC. The use rate is 2 to 4 pt/A. Please follow all precautions for both metolachlor and metribuzin.

pendimethalin (Prowl 4E): 1 1/2 to 3 pt/A (REI 12h, Group 3). Apply as a broadcast spray after planting or drag-off but before potatoes emerge. If rainfall does not occur within seven days after application, shallow cultivation to a depth of 1” to 2” will improve control. May be tank mixed with metribuzin (Sencor or Lexone) or linuron (Lorox) to improve broadleaf weed activity. May be tank mixed with Eptam 7E before planting to obtain control of yellow nutsedge or quackgrass.

NOTE: A common strategy of potato growers is to combine one of the “grass” herbicides listed above (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.

Preemergence and Early Postemergence Control of Many Broadleaf Weeds

linuron (Lorox DF): 1 1/2 to 4 lb/A (REI 24h, Group 7) in 40 to 100 gal water. Make a single application as a broadcast spray after planting but before crop emerges. Drag-off, if used, must occur prior to application. Do not spray over top of emerged potatoes. Plant seed at least 2” deep. Constant tank agitation is required. See label to select proper rate for your soil type. Heavy rain after application may cause injury to potatoes. Do not repeat application or plant other crops within four months of treatment. Do not overlap spray. Needs moist soil to activate. Will also control small emerged annual weeds if present at the time of application. One pt nonionic surfactant/25 gal spray mixture may be added to improve activity on emerged weeds (see label). May be tank mixed with metolachlor (Dual) or pendimethalin (Prowl) to improve activity on annual grasses.

metribuzin (Sencor or Lexone DF): 2/3 to 1 1/3 lb/A (REI 12h, Group 5) in 20 to 40 gal water. Apply 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. See label to select proper rate for your soil type. Do not overlap spray. Will also control small emerged annual weeds present at the time of application. May be tank mixed with metolachlor (Dual) or pendimethalin (Prowl) to improve activity on annual grasses. Sencor or Lexone may also be applied as a post-emergence spray in certain situations (see postemergence section below). A prepackaged tank mix of metribuzin plus metolachlor is available as Turbo 8EC (2 to 4 pt/A). Follow all precautions for both metribuzin and metolachlor.

rimsulfuron (Matrix 25 DF): 1 to 1 1/2 oz/A (60 dh, REI 4h. Group 2). Apply after hilling or drag-off, but before potatoes emerge. Activation by rainfall or irrigation (1/4” to 3/4”) is needed within 1 week of application. If weeds are present at application, add a nonionic surfactant at 1 to 2 pt/100 gal water. May be tank mixed with other soil applied herbicides; see the label for advice. Weak on lambsquarters.

Postemergence Weed Control

clethodim (Select 2 EC): 6 to 8 oz/A (30 dh, REI 24h, Group 1). Apply to actively growing grasses. See label for grass height and herbicide rate. Always use a crop oil concentrate at a rate of 1 gal/100 gal spray mix. See both container label and supplemental label available from dealer.

metribuzin (Sencor or Lexone 75DF): 1/3 to 2/3 lb/A (60 dh, REI 12h, Group 5) in 20 to 40 gal water. Apply only after three days of sunny weather. Do not use on BelRus, red-skinned or early maturing, white-skinned varieties. Treat before weeds are 1” tall. Treatment may cause chlorosis or necrosis of potato vines. Do not reapply. If Sencor or Lexone 75DF is used before crop emergence, do not apply more than 1 1/2 lb/A per year.

metribuzin (Sencor or Lexone 75DF): 1/3 to 2/3 lb/A (60 dh, REI 12h, Group 5) plus sethoxydim (Poast 1.53EC): 1 to 1 1/2 pt/A (30 dh, REI 12h, Group 1). Add 2 pt/A of crop oil concentrate. Observe all precautions listed above for both products.

pelargonic acid (Scythe 4.2): 3% to 10% (REI 24h, Group 27). Use a 3-5% solution for annual weeds (4-6 oz/gal water), a 5-7% solution for biennial and perennial weeds (6-9 oz/gal water), and 7-10% solution for maximum burndown (9-13 oz/gal water). Delivery rate for boom applications should be 75 to 200 gal spray solution/A; 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. These include tank mixes with glyphosate (Roundup), sulfosate (Touchdown), and residual herbicides. See label for complete details.

rimsulfuron (Matrix 25 DF): 1 to 1 1/2 oz/A (60 dh, REI 4h, Group 2). Apply to young, actively growing weeds before crop exceeds 14” in height. Small weeds (less than 1” in height or diameter) are easiest to kill; however, quackgrass is more easily controlled when it is 4” to 6” high and actively growing. Add a nonionic surfactant at 1 to 2 pt/100 gal water. Application to moist soil is best. Rainfall or irrigation (1/4” to 3/4”) within 1 week of application will activate Matrix in the soil. May be tank mixed with metribuzin (Lexone); see the label for advice. Weak on lambsquarters. More weed species are controlled postemergence than preemergence. Do not apply after June 30. Apply only after at least three days of sunny weather. See label for other postemergence precautions.

sethoxydim (Poast 1.5EC): 1 to 1 1/2 pt/A (30 dh, REI 12h, Group 1). Apply with crop oil concentrate at 2 pt/A. Apply to actively growing grasses (see product label for susceptible stage of growth). Do not cultivate for five days prior to or seven days after Poast application.

Problem Weeds

See the relative susceptibility chart of weeds to herbicides at the end of this manual to better assess the performance of an individual herbicide on each weed species.

Yellow Nutsedge: The herbicides listed above, which 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 (Sencor or Lexone). 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 Sencor or Lexone can be used to clean any areas that escape the Eptam or Dual treatment. The best time to apply a postemergence treatment of Sencor or Lexone is when the yellow nutsedge is 4” to 6” tall. Remember that nutsedge is not a grass and will not be controlled by sethoxydim (Poast).

Quackgrass: By far the best strategy for control of quackgrass is with an application of glyphosate (Roundup) to actively growing quackgrass the fall prior to planting. An application of Roundup in the spring at least three 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 two weeks after the first application). Be sure to observe the preharvest interval for both Eptam and Poast.