I have grouped these together for convenience and they probably are but may not be A. millefolium hybrids.
Soil & Site
average, not rich
flat terminal corymbs of small 1/4" flowers. The hybrids come in a wide range of colors besides the white or pink of the Common Yarrow.
simple, dark green, finely divided giving them a fern-like texture and strongly aromatic, larger basal leaves have long petioles and smaller upper leaves are sessile
spreads by rhizomes
Reach 18 plus inches tall and rapidly spread. If planted 18"-24" on center, plants will touch by the end of the growing season. Mat like growth habit.
flower stalks are weak and tend to flop, after flowering shear back the plants, plant will form a dense mat of foliage, aggressive spreaders especially in moist rich soils
Very easy by division. The cultivars may not come true from seed.
The species plant Common Yarrow (Achillea milefolium) is native to Europe and Western Asia and considered to be a weed in many areas.
It was in cultivation prior to 1440 and used as a herbal remedy for many aliments. The Greek name honors Achilles a warrior of the Trojan wars.
I have grown many different varieties of Achillea millefolium. There seems to be a few secrets to successfully growing them. Avoid rich, moist, poorly drained soils and plant in a full sun site. Also cut back hard after the bloom fades.