Onion pre-emergence weed control in 2015

Prowl H20, a pre-emergent herbicide for onions, is expected to be in very short supply for 2015. Other options are available.

Prowl H2O 3.8 ACS (pendimethalin) is the primary pre-emergence herbicide for dry bulb onion production in Michigan. Prowl H2O is applied pre-emergence at up to 2 quarts (1.9 pound active ingredient) per acre, and once or twice more at the same rate. Prowl H2O may be applied in tank mixes with other pre-emergence and post-emergence herbicides, and has been very safe on onions in all types of situations as long as label rate restrictions are followed.

Prowl H2O is in very short supply for 2015. Evidently, the West Coast dock strike resulted in inadequate supplies of some ingredients needed in the manufacture of Prowl H2O. Michigan State University Extension advises growers to purchase as much Prowl H2O as possible soon if they do not already have their 2015 supply in their possession. Prowl H2O and Prowl EC are labeled on many field crops and fruit crops. Vegetable growers may be able borrow or exchange some Prowl H2O from growers of crops for which the formulation is not as critical as in onions.

For those who are unable to obtain sufficient Prowl H2O, there are other options for pre-emergence weed control. There are generic pendimethalin 3.8 ACS products available. They probably will disappear quickly. Prowl 3.3 EC is still available, as well as several generic pendimethalin 3.3 EC products. The difference between the EC and H2O formulations is the carrier. Prowl H2O uses a water-based carrier whereas the EC uses a petroleum-based solvent system. The solvent-based carrier is potentially more toxic to plants than the water carrier. For pre-emergence applications before onions are up, the EC works equally well as long as the rate of active ingredient per acre is the same. For instance, 2 quarts of Prowl H2O is equivalent to 2.4 quarts of Prowl 3.3 EC.

After onions have emerged, do not mix Prowl 3.3 EC with other herbicides unless you are sure of compatibility. Prowl 3.3 EC should not be mixed with Chateau, Goal 2 XL, Buctril, Starane, Dual Magnum, Outlook, Fusilade, Poast or Select Max. Tank mixes with any of these herbicides may cause excessive crop injury and yield reduction. Onions normally outgrow stunting from Prowl 3.3 EC applied alone. Before introduction of Prowl H2O, we only had Prowl 3.3 EC available, and many growers have experience with it.

If you have questions about specific weed problems, please contact your local MSU Extension educator or chemical supply representative.

Dr. Zandstra’s work is funded in part by MSU’s AgBioResearch.

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