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.
Stale Seedbed
See Stale Seedbed Technique for information on the use of these herbicides or flaming.
- 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)
- bensulide (Prefar 4E): REI 12h, Group 0. Dry bulb onion and dry bulb shallot only. Apply 5-6 lbs/A per application, only 1 application per year. Incorporate 1-2” into the soil with shallow cultivation or water through irrigation or rainfall within 36 hours of application. Only apply to onions growing in fine textured soil with good water drainage where the soil salinity value is less than 4 EC and soil temperature is between 55-75°F.
- dimethenamid (Outlook): PHI 30d, REI 12h, Group 15. Apply 8-21 oz/A per application, up to 2 applications allowed per year, a minimum of 14 days apart. Do not exceed 21 oz/A per year. Use the lower rate on sandy soils and the higher rate on silt and clay soils. Apply to onion that has at least 2 fully expanded leaves or significant crop damage may occur. If applying to transplants, do not apply until transplants are in teh ground and have had several days to recover.
- s-metolachlor (Dual Magnum): REI 24h, Group 15. MASSACHUSETTS, MAINE, and NEW HAMPSHIRE only. 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, REI 24h, Group 3. Apply 1.5-3.2 pts/A per application, up to 2 applications allowed per year. Do not exceed 3.2 pts/A per year. Apply as a broadcast treatment when onions have 2-9 true leaves. May be applied to direct-seeded and transplanted dry bulb onions. May be applied in the fall or spring to furrow area of land bedded in the fall in preparation for planting. Band apply and keep Prowl away from the area where dry bulb onion seed will be planted. Harrow off tops of beds after Prowl furrow application before planting dry bulb onions. Can also apply as a banded application to flag-leaf dry bulb onions. Only apply once to the furrow area and once to the dry bulb onion row per season. Rate based on soil texture.
- trifluralin (Treflan HFP): PHI 60d, REI 12h, Group 3. Apply 0.75-1 pt/A per application, only 1 application per year. Apply between rows at layby, after onions have emerged. Emerged weeds should be removed prior to application. Avoid applying directly to the tops or exposed bulbs of onions. Incorporate uniformly into the soil between the onion rows. Two incorporation passes are required, the first within 24 hours of application, the second at least 5 days after the first. Avoid covering onions with treated soil. Rate based on soil texture, percent organic matter, and weed composition. Applications made to stressed onions may lead to crop damage and reduced yields.
Pre- and Postemergent Herbicides
- bromoxynil (Broclean): PHI 30d, REI 24h or 48h, Group 24. Dry bulb onions only. Apply 1-1.5 pts/A per application, only 1 application per year. On muck soils with >10% organic matter Broclean must be applied 3-4 days before crop emergence. Rainfall or irrigation 2 days after application or 3 days before crop emergence may result in crop damage. If not on muck soils, can be applied after crop emergence with at least 50-70 gal/A of spray because concentrated Broclean will kill onions. If the waxy coating on onion leaves is insufficient Broclean can kill onions. Dry soil, dry onion foliage, high light intensity, low humidity, and high temperatures help develop a waxy coating. Do not add a surfactant.
- oxyfluorfen (Goal 2 XL): PHI 45d, REI 48h, Group 14. Dry bulb onions only. Apply 2-4 oz/A per application multiple applications allowed per year. Do not exceed 2 pts/A per year. For transplanted onions, may be applied to the soil before planting. The soil surface should be free of excessive plant residue before applying. For direct seeded onions, may be applied when the onions have at least 3 fully developed true leaves and weeds are at the 2-4 leaf stage. A chlorotic band around some of the leaves may be observed, especially in cool wet conditions.
Postemergent Herbicides (after weeds germinate)
- carfentrazone (Aim EC): PHI 0d, REI 12h, Group 14. Apply 0.5-2 oz/A per application, multiple applications allowed per year. Do not exceed 6.1 oz/A per season. Can be applied as a preplant burndown no later than 30 days before 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.
- clethodim (Select Max): PHI 45d, REI 24h, Group 1. Apply 6-32 oz/A per application, up to 4 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.
- fluazifop (Fusilade DX): PHI 45d, REI 12h, Group 1. Apply up to 24 oz/A per application, up to 2 applications allowed per year, a minimum of 14 days apart. Do not 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).
- glyphosate (Roundup Power Max): 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 2.0*): restricted use. PHI 60d, REI 12h or 24h, Group 22. Apply 1.3-2.7 pts/A per application. For bulb onions 1 application may be made before crop emergence, and 1 may be made after crop emergence a minimum of 7 days apart. Do not exceed 4 pts/A per year. For seeded onion only 1 application allowed per year. Do not exceed 2.7 pts/A per year. Use precision directed spray application equipment adjusted to prevent spray contact with crop plants is applying after crop emergence. 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 as a harvest aid and desiccant to remove crop leaves or ease harvest.
- sethoxydim (Poast): PHI 30d, REI 12h, Group 1. Apply 1.5 pt/A per application, multiple applications allowed per year. Do not exceed 4.5 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.