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. 

See section on Soil Fumigation Outdoors. Proper use of Vapam can provide control of most weed species.

Stale Seedbed

See Stale Seedbed Technique.

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

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

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.

dimethenamid (Outlook): PHI 30d, REI 12h, Group 15.  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. 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.

pendimethalin (Prowl H2O): PHI 30d, REI 24h, Group 3.  Apply 2 pt/A to the soil surface as preemergence after planting but before crop emergence, or after crop emergence when leeks are at the 2 to 3 true-leaf stage, or at both timings. Emerged weeds will not be controlled.  If applied as both pre- and postemergence, applications must be at least 30 days apart. Do not exceed 2 pt/A per application, and do not apply more than 4 pt/A per season. 

Herbicides Used Postemergence, after weeds germinate

clethodim (Intensity One)PHI 14d, REI 24h, Group 1.  Leeks are NOT on the Select Max label, but are on other clethodim product labels (such as Intensity One).  Will control grass weeds only. Apply to actively growing grasses.  Multiple applications permitted of 9 to 16 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) Intensity One and 0.25% v:v nonionic surfactant (0.33 fl 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 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.