Achillea

42 ACHILLEA MILLEFOLIUM AND HYBRIDS
Achillea millefolium and hybrids common yarrow Asteraceae
perennial, Zones 2–9 Europe many colors 2–2½'/3' (60–75 cm/90 cm)
Large color selection makes this one of the most popular yarrows for cut flowers,
although attention to post production is important. Common yarrow is a matforming
species with deeply cut, dark green foliage. The flower heads are small
and arranged in flattened inflorescences (corymbs). The species itself is predominantly
creamy white, but nobody grows it: selections and hybrids abound in
pink, rose, mauve, and bicolors. Plants spread rapidly, and beds fill in within 2
years after planting. They are highly productive and may be propagated readily.
Propagation
Seed: Seed germinates in 10–15 days at 70–72F (21–22C) under humid (sweat
tent or mist bed) conditions. The small seed should be covered lightly with vermiculite
or clean sand. Approximately 1/64 oz (0.4 g) of seed yields 1000 seedlings
(Nau 1999).
Division: Plants may be divided at any time but preferably in early spring or
immediately after flowering. Even root pieces (essentially root cuttings) reproduce
a new plant. A 2- to 3-year-old plant yields hundreds of single crown divisions.
Place divisions in a propagation bed for growing-on.
Growing-on
If seedlings are planted in plugs, grow for 3–5 weeks at 55–62F (13–17C) in full
light. Fertilize with 50–100 ppm N to maintain green foliage. Do not over-fertilize,
or plants become spindly. Transplant to field when plants are large enough
to handle.
If seeds are sown in a seed flat, transplant seedlings to cell packs after the first
true leaves have formed. Grow on as with plugs.
Sort divisions by size. Transplant large divisions directly to the field; place
the smallest ones in pots or cell packs and grow on in the greenhouse or cold
frame.
Environmental Factors
Cold is not necessary for optimum flower development, although 3–4 weeks of
40F (4C) temperatures may contribute to uniformity and stem quality. Cool
temperatures delay flowering time, however. Plants are particularly cold hardy
and are common from the prairies of Canada to the Piedmont of Georgia. Stem
strength and flower color is enhanced by cool night temperatures, and stems
are of higher quality in the North than in the South.
Work with ‘Summer Pastels’ showed that long days were necessary for flowering.
Zhang et al. (1996) determined that plants grown under 8-hour photoperiods
remained vegetative and that the critical photoperiod was between 12
and 16 hours. Photoperiod is an important consideration when considering
forcing for winter production.