This is the conclusion put forth by LDS.About.com, which cites an April 1977 Ensign article entitled "The Do's in the Word of Wisdom" as evidence. Here's the applicable statement, republished with surrounding context:
Herbs also add variety to our meals through spices such as onions, garlic, peppers, sage, parsley, bay leaves, etc. Some herbs have medicinal properties as well. However, one should use extreme care in treating illnesses with herbs, since most have not been subjected to the careful testing required for other drugs and some contain powerful chemicals. Some people enjoy herbal teas, but even these should not replace more nutritious drinks in the diet. Always tell your physician if you are taking any herbal teas and know what the tea contains if possible. Recently an association has been identified between herbal teas and cancer of the esophagus, but further research is needed. In Utah, in the summer of 1976, three deaths were attributed to the overuse of herbs. When dealing with severe or chronic illnesses, be especially wary of herbal treatments or remedies that have not been proven effective through scientific studies. Priesthood blessings and competent medical care, not home remedies, should be obtained for treating serious disease.
Since Doctrine & Covenants 89:11 states “And again, verily I say unto you, all wholesome herbs God hath ordained for the constitution, nature, and use of man — every herb in the season thereof, and every fruit in the season thereof; all these to be used with prudence and thanksgiving”, it would be illogical to ban consumption of herbal tea. The teas considered banned by the Word of Wisdom are black, green, white, and oolong. One person nailed it down in this comment: "The tea prohibited by the word of wisdom is Camellia sinensis. Herbal teas not containing this ingredient are generally ok".
