If you are harvesting fresh yarrow leaves and they have any moisture on them, you have to let them dry out before using. So, if your yarrow is dusty, you can certainly wash it lightly, but then it needs to dry out. The dried aromatic leaves can be used to make a tea but we would add mint or it can be quite a bitter tea, often used as a cold remedy.
If you are harvesting fresh yarrow leaves and they have any moisture on them, you have to let them dry out before using. So, if your yarrow is dusty, you can certainly wash it lightly, but then it needs to dry out. The dried aromatic leaves can be used to make a tea but we would add mint or it can be quite a bitter tea, often used as a cold remedy.