Sow at a depth of approx.
0.25 inches (0.635 cm) and aim for a distance of at least
11.9 inches (30.5 cm) between Common yarrow plants. Soil temperature should be kept higher than
18°C / 64°F to ensure good germination.
By our calculations, you should look at sowing Peggy Sue about 49 days before your last frost date.
As Peggy Sue is hardy, ensure temperatures are mild enough to plant out - wait until after your last frost date to be on the safe side.