Answer: To be honest I am having a hard time envisioning marsh marigolds as being so invasive in a flower bed -- unless your flower bed is unusually wet -- so you may want to double check your identification of the weed. Then, you might contact the manufacturer to see if Preen is listed or labelled as controlling it. If the plant is coming back from the roots rather than from seed, you might instead consider using an herbicide containing glyphosate to spot treat them. The wipe on method is easier to control when working in a flower bed, and be very careful not to allow it to contact desirable plants. Sometimes you can make a shield of cardboard or plastic sheeting to protect them until it dries. Also be sure to allow the treated foliage the full time frame indicated on the label so that the chemical can be translocated to the roots.
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