Yarrow (Achillea millefolium), like this Coronation Gold variety, is a sun-loving perennial herb. The easiest way to propagate yarrow is through division of the root ball or by collecting seeds in late summer.
Cutting the stems
Allow the blooms to fade and go to seed while still on the stem in the garden. The shape of the bloom remains the same as the seeds and chaff (the seed covering) turn brown. Once the stem has dried at least 6 inches from the flower-turned-seed-factory (the stem is dark brown to black), the yarrow seeds are ready for harvest. Clip the stem of the dried yarrow and place it in a paper bag or on sheet of newspaper (my preference).
Collecting the seeds
Yarrow seeds are tiny, smaller than a gnat. To harvest the yarrow seeds, hold the stem and turn the seed-packed flower upside-down. Tap the dried bloom on a sheet of newspaper or push the dried bloom on the the palm of the opposite hand. Gently squeeze the bloom to release the seeds and chaff. Don’t worry about separating the chaff from the seeds; nature takes care of the chaff when it’s planted. Following nature’s pattern, immediately plant the seeds outdoors in a well-drained, full sun location.