Google images tansy

Google images yarrow

FataMorgana

From a distance, I cannot tell the difference of the two. The foliage, height and flowers (yellow ones) look very similar. But when you get closer you will know the difference.

Yarrow is taller than Tansy. Tansy's flowers are buttons, quite different from Yarrow's. Their leaves are different. Can you take a picture?

Tansy has a much stronger scent too. Presumably the Yarrow you are talking about is Achillea filipendulina or a cultivar thereof. There is no possibility of mistaking Achillea millefolium for Tansy and that is the plant usually called Yarrow in common parlance.

Flora is correct. If we are talking about real yarrow (A. millefolium), there is no way to confuse the two.

Just go to google and type "yarrow flower" and "tansy flower" or "yarrow leaf" and "tansy leaf" you will see the difference very clearly.

FataMorgana