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. 

Stale Seedbed

See Stale Seedbed Technique in the Weed Management section. In place of herbicides, flaming can also be used.

glyphosate (Roundup Power Max): REI 12h, Group 9.

paraquat (Gramoxone SL 2.0*): restricted use. Dry bulb onion only. REI 12h, Group 22. Use 2 – 4 pts/A. Include a nonionic surfactant at 0.25% v/v, or crop oil concentrate/methylated seed oil at 1.0% v/v (1 gal/100 gal) of the finished spray volume for maximum efficacy. 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 17. Use a 3 -10% solution (3 to 10 gallons per 100 gallons).

Herbicides Used Preemergence, before weeds germinate

bensulide (Prefar 4E): REI 12h, Group 0.  Dry bulb onion and dry bulb shallot only. Apply 5 to 6 qt/A.  Can be preplant incorporated by shallow cultivation (1-2”) or incorporated by irrigation within 36 hours of application. Grass control only; should be supplemented with cultivation or another registered herbicide for broadleaf control. See label for rotation restrictions.

dimethenamid (Outlook): PHI 30d, REI 12h, Group 15.  Can be used for dry bulb onion and shallot, and also for green onion (leeks, spring onions or scallions, Japanese bunching onions, green shallots or eschalots).  If applications are made to transplanted green onions, DO NOT apply until transplants are in the ground and soil has settled around transplants with several days to recover.  This herbicide is a root and shoot growth inhibitor that controls susceptible germinating seedlings before or soon after they emerge from the soil. Do not apply until onion has reached the 2 true-leaf stage or significant crop injury can occur.  May be applied as a single application (up to 21 oz/A) or used in split applications.  For split applications do not exceed a total of 21 oz/A per season. An initial application of 10 to 14 oz/A can be followed by another application of the remaining 7 to 11 oz/A.  Applications must be a minimum of 14 days apart. Application rates are influenced by soil organic matter content. See label for info on application rates depending on soil type and organic matter content. See label for info on tank mixing with other herbicides.

DCPA (Dacthal W-75): REI 12h, Group 3.  Select rate for preemergence use based on soil type and weeds targeted for preemergence control.  Good control of most annual grasses; fair on redroot pigweed, lambsquarter, and purslane.  Apply to weed-free soil; will not control existing weeds.  Can be applied at seeding or transplanting and/or at layby.  Can be sprayed directly over transplants without injury. A layby application can be made on onions either alone or in addition to a Dacthal preemergence treatment up to 14 weeks after planting at rates up to 14 lb/A on any soil type. If weeds emerge prior to layby, the onions should be cultivated or weeded prior to application. Preplant incorporation not recommended.

s-metolachlor (Dual Magnum): REI 12h, Group 15. MASSACHUSETTS, MAINE, and NEW HAMPSHIRE ONLY. Dry bulb and green onion. Make sure the label for your state is available for download before using this product. This is a restricted label available only to growers who apply through the website www.syngenta-us.com/labels/indemnified-label-login and agree to a waiver of liability. Main target weeds for this registration are galinsoga and yellow nutsedge. All label instructions will be supplied after the application for use is completed.

pendimethalin (Prowl H2O): PHI 45d (PHI 30d for green onions), REI 24h, Group 3. Emerged weeds will not be controlled by Prowl.  Dry bulb onion and dry bulb shallot: Apply 1.5 to 3.2 pt/A.  Select rate based on soil texture.  See label for details.  Can be used preemergence after planting but before crop emergence, after crop emergence when onions/shallots are in the 1 to 5 true leaf stage, or at both timings.  For green onions, use 2 pt/A as preemergence spray or as a postemergence spray to the crop at the 2 to 3 true-leaf stage. If applied sequentially as both a preemergence and postemergence spray, applications must be separated by at least 30 days. Do not exceed 4 pt/A per year on green onions. 

Herbicides Used Postemergence, after weeds germinate

carfentrazone (Aim EC): REI 12h, Group 14.  For use between rows after crop establishment. 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 6.1 oz/ per season. 

clethodim (Select Max): PHI 45d, 24hr REI, Group 1.  Dry bulb onions and dry bulb shallots only.  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.

fluazifop (Fusilade DX): PHI varies (45d dry bulb, 14d green onion), REI 12h, Group 1. Dry bulb and green onion only. For grass weed control only. See label to select rate based on grasses targeted for control. Apply up tp 24 fl oz/A. Can make up to 2 applications per year. Allow for minimum 14 days between applications, and not to exceed 48 oz/A per year.  Apply to actively growing grasses (see product label for susceptible stage). Add either crop oil concentrate (0.5-1%, 0.5-1 gallon per 100 gallons of spray) or nonionic surfactant (0.25-0.5%, 1-2 qt per 100 gal of spray).

oxyfluorfen (Goal 2 XL): REI 48h, Group 14. Provides early postemergence and residual control of many broadleaf weed species. For direct seeded onions, apply 2 to 4 oz/A to onions that has at least 3 fully developed true leaves.  For transplanted onions, an application of up to 2 pt/A acre may be made within two days after transplanting.  If less than 2 pt/A acre is applied, a second application can be made two weeks or more after transplanting.  Do not exceed the maximum use rate of 2 pt/A per season.

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 1.5 pt/A per application, minimum 14-days between applications.  Do not exceed 4.5 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.