There was a good thread on this over on the Perennial Forum not too long ago. And, if you scroll down on the Cottage Garden discussion page, you will find another thread on this topic called "Yarrows.".

I have given up on all Yarrows. The one type dies out in our damp weather, and the other spreads waaaaay to much. Not my type of plant.
CMK

Here is a link that might be useful: Achilleas/Yarrows- opinions? Perennial Forum

cziga,

It's not exactly a small plant. One little plant can grow about 4 feet tall and possibly equally as wide. It's something that I would call a dramatic plant and a bit on the aggressive side although I would hesitate to describe it as invasiv There are other forms of yarrows that do migrate far from it's original site too, although not quite as tall.

But I do like them and so I have a couple in my yard which I surrounded with equally deep rooted plants to hinder its growth. However I did make the mistake of planting it as a foreground plant when it should either be in the middle or to the back of the border plant. Right now my yarrow has completely obscured my veronicas and my phlox. I will have to reduce it in fall and perhaps move it to a better location (it's third move)

ianna

Thank you for the link, that other thread was very helpful and I don't read the Perennial forum very often :)

Ianna, I wasn't aware that yarrow would grow that large! I do like some dramatic plants, but the thread I read suggests that many forms of yarrow are either very invasive (millefolium species) or the colours either dont last or wash out quickly.

Terra Cotta seems to be a popular choice for most, for both colour and for being a fairly well behaved plant. I think maybe I'll try that one and see how it goes ;)