California growers should consider fallowing worst weedy rice fields

• By Whitney Brim-DeForest and Tim Johnson •

weedy rice in california
Weedy rice in a California rice field — photo courtesy UCCE

The absence of a registered herbicide to control weedy rice in California leaves growers with fewer options to control this important pest. Planting clean seed, as required by the Certification Act, is an important control measure, and roguing can help control lower levels of infestation.

For heavily infested fields, growers may want to consider fallowing, as outlined in the UCCE Weedy Rice Best Management Practices during this short water year.

Although dry fallowing (without soil moisture or irrigation water) for one season will not eradicate weedy rice seed from a field, it may increase predation or desiccation (drying out) of weedy rice seeds that have imbibed water that are on or near the soil surface.

If growers notice weed growth in dry fallowed fields, shallow tillage is recommended to ensure plants do not go to seed. If weeds go to seed, this will increase the potential weed seedbank and could have the opposite effect (increasing the weed soil seedbank).

For maximum control of weedy rice in a fallow field, a “managed fallow” is recommended. This obviously is not possible in all fields in 2021. However, if growers can still run irrigation water on a fallowed field, the following is recommended:

• Flush the field with water (just enough to moisten the soil).

• Wait until weeds emerge, but before weeds start to go to seed (usually no more than one month for most rice weed species).

• Shallowly till, trying to minimally disturb the soil.

• If possible, repeat the “flush, till” process at least once more, preferably early in the season (no later than July-August, to maximize weed emergence and control).

• A managed fallow may have the added benefit of controlling other rice weed species as well. For help in implementing a managed fallow, please contact your local UCCE rice adviser.

Link to UCCE Best Management Practices (PDF)

Dr. Whitney Brim-DeForest is a University of California Cooperative Extension rice advisor for Sutter and Yuba counties. She may be reached at Tim Johnson is president and CEO of the California Rice Commission.

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