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Reduced risk herbicides for horticultural crops in organic soils: supplemental registration data and herbicide screening

Project Code: MUR06-100

Project Lead

Diane-Lyse Benoit - Agriculture and Agri-Food Canada


To optimize weed management programs with flumioxazin in onions and carfentazone in celery, lettuce, and chinese cabbage with minimal crop damage and to obtain supplemental data in support of label expansion

Summary of Results

Vegetables are poor weed competitors and effective weed control is required throughout the growing season. Limited weed management options are available to growers of vegetable crop in organic (muck) soils, and this can give rise to the development of resistant weed populations. Work was undertaken in Ontario and Quebec to address the need for new weed management options for onion, celery, lettuce and Chinese cabbage grown in organic soils.


The first objective of this research was to identify herbicide combinations and timing that will optimize weed management programs with flumioxazin in onions. The present commercial practice of combinations with either pendimethalin or dimethenamid applied at the onion loop stage followed by pendimethalin at the 2 leaf stage onion and complemented with application of oxyfluorfen when onions were at the 3 and 4 leaf stages gave optimum marketable yields. The addition of flumioxazin when onions were at the 3 leaf stage following pendimethalin provided no additional benefit in weed control but did allow the introduction of another herbicide into the rotation to prevent weed resistance build-up.

There are no registered herbicides in lettuce and chinese cabbage grown in muck soils and most registered products for use on other vegetables in muck soils are older chemistries with documented impact on environment. Consequently, the second objective was to identify rates and application timing of alternative herbicides that will optimize weed control in celery, lettuce, and chinese cabbage with minimal crop damage. The goal is the reduction of weed control reliance on single older molecules, to slow the development of herbicide resistance by providing new weed control options for important vegetables crops, and minimize negative impacts on sensitive organic soils.


Results obtained in Ontario and Québec over two years identified a weed management solution in transplanted lettuce involving pendimethalin (PROWL) applied prior to transplantation followed by carfentrazon (AIM) applied postemergence. As a result of the identification of this option, work is now underway within Agriculture and Agri-Food Canada (AAFC) Minor Use Pesticides to pursue registration of PROWL in lettuce (AAFC09-014).

Chinese Cabbage and Celery

Screening of potential herbicides in Chinese cabbage was carried out in 2006 and 2007 in both Ontario and Quebec. Trials results were inconclusive and did not identify any potential herbicide for chinese cabbage growing in muck soil. Similarly, herbicide screening for celery gave limited results for viable alternative to the standard registered use of prometryn in organic soil. For both Chinese cabbage and celery, further research is required.

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