Comfrey was a food and feed crop, but has become weedy in gardens and fields of the Pacific Northwest. Naturally aggressive spread is accelerated by tillage that moves plant parts. A 1970s experiment in western Oregon determined the activity of several herbicides on comfrey. Relatively high rates were used, but only a few herbicides were effective. Most problems with comfrey are in gardens, so use caution in recommending any herbicide. One of the few options is to repeatedly (about every 3 weeks) remove plants down as far as practical below the soil surface.
Herbicides that gave good control were bromacil and terbacil at 6 lb ai/a; picloram at 1 lb ae/a; 2,4-D ester at 2 lb ae/a; triclopyr at 2 lb ae/a; and clopyralid at 4 lb ae/a. These rates gave nearly complete control of a new planting. Older plants might be more difficult to control.