Hemp is not very competitive with weeds in the first 3-4 weeks of growth. Hemp for grain or CBD is planted at a lower density per acre compared to fiber hemp, which reduces or prevents canopy closure needed for weed suppression.
Weed control is one of the largest issues in hemp production. We have no synthetic herbicides available for use in hemp, either as a pre-emerge or post- emerge application, in the United States. Herbicide screening is underway at universities across the county, including Purdue and Vincennes University.
Choosing field sites with low weed pressure to begin with, is the best preventative measure to reduce weed competition. Rotating after crops where you can use conventional weed management may be an option, just be aware that limited research has been done on herbicide rotation restrictions in hemp. http://www.wiscweeds.info/post/herbicide-rotation-restrictions-before-planting-industrial-hemp/
Growers are using various methods for weed suppression and control including; pre-plant cultivation, in-row cultivation, and mowing. Weeding equipment common in organic systems are also an option. In CBD hemp, the laying of plastic is one method to suppress within row weed competition. Some growers are also using cover crops for weed suppression, with the greatest anecdotal success using cereal rye planting in the fall and terminated in the spring.