Is herbal tea against the word of wisdom?

This is a matter of personal opinion. The revelation known as the Word of Wisdom, published as Section 89 in the Doctrine and Covenants, says nothing about tea. It states that "strong drinks" and "hot drinks" are not to be consumed, and gives some specific directions about alcohol - but nothing is said of tea, coffee, sodas, etc.

Hyrum Smith defined 'hot drinks' as tea and coffee in 1842, but didn't go into detail about what types of tea, decaffeinated teas or coffees or iced teas or coffees. It is possible that this is simply because these drinks were not available at the time.

It is generally assumed among church members that green and black teas are not to be consumed, whether cold or hot, but that herbal teas, Hot Chocolate, and other 'hot drinks' (such as postum, ovaltine, etc) are acceptable. One reason for not counting herbal teas in the Word of Wisdom is that they do not contain the tea plant (camellia sinensis). Church leaders advise individual members that details such as this are between you and the Lord, and that prayer about the matter will bring an answer that applies to you personally.