Yarrows (Achillea spp.) are perennial herbs that are favorites for butterfly gardens. The plants spread easily and grow in U.S. Department of Agriculture plant hardiness zones 3 through 10, depending on the variety. Yarrow begins to bloom in early summer and often continues to flower until the the first frost in fall. When the flowers fade and the the entire stalk has turned completely brown, the seeds are mature and ready to harvest.
The easiest way is to collect seed from yarrow is to place a brown paper bag over the seed head and secure it lower down on the stalk with a piece of twine. Snap the stalk off with the seed head inside and leave it in a dry place for a week or two to make sure the seeds have completely dried out. This way, the tiny seeds will be collected in the bottom of the bag. Knock the stalk against the side of the bag to free the last of the seeds.
Yarrow can be sown outdoors in early spring or fall. Germination is most successful when soil temperatures are between 60 and 65 degrees Fahrenheit. For best results, sow directly on the surface in a bed of fine soil. Sow at a rate of 1/3 ounce per 1,000 square feet of growing area. Because the seeds are so tiny, mix them with some coarse sand to help spread them evenly. Spread the mixture over the surface and keep the soil moist until germination.
Yarrow seed may be sown indoors eight to 10 weeks before the average date of last frost in spring. Sow in flats directly on the surface of the soil. Yarrow seeds need light to germinate and should not be covered. If you don't have a sunny spot for the seed tray, use a fluorescent lamp to provide the light necessary for germination. Germination rates are high, but the seed may sprout slowly and sporadically -- anywhere from two weeks to three months is common.
Transplanting and Care
When sowing seed indoors, carefully prick out individual seedlings when they are 2 inches tall and plant them in 3-inch pots. These should fill out quickly and be ready to plant in the ground in four to six weeks. To form a dense stand, plant seedlings 8 to 10 inches apart in a diagonal pattern. Keep them well watered and by fall they will have formed a solid group. Yarrow adapts to most soil types and is quite drought tolerant once established.