A really serious infestation may require the use of herbicides, but such chemicals are likely to damage or kill desirable plants, too. A few years ago, the Cooperative Extension service at Colorado State University published a list of herbs and other plants that have been documented as producing undesirable effects in humans when they are eaten or brewed into teas. Besides comfrey, a number of other rather common plants are listed with the negative consequences of their use. Catnip and chamomile are two herbs that many gardeners grow and sometimes brew into teas. If you find yourself having a strong urge to slither around and rub against a clump of catnip in your garden, it's obviously time to get rid of it.
Chamomile frequently produces allergic rhinitis or anaphylactic shock in some people.
Apparently those folks who are allergic to ragweed, asters, chrysanthemums or other members of the composite family are the ones seriously affected by exposure to chamomile tea. A biennial garden flower, foxglove or digitalis, has escaped domestication in the Pacific Northwest and has naturalized in shady, moist locations. It is extremely poisonous and has caused serious illness and death many times. In 1979 in the Spokane area, an elderly couple died within 24 hours after drinking tea brewed from foxglove. Marigold should also not be used by people with allergies to the composite family.