This section is from the book "Commercial Gardening Vol2", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
This large genus, containing the Milfoils and Yarrows, has only a few species of commercial value. The best known is A. Ptarmica, a white-flowered British species about 2 ft. high, often grown for cut flowers. The double form "flore pleno" and one known as "The Pearl" are most popular. They can be grown almost anywhere, and will produce quantities of cut flowers during the summer season. The best rose-coloured Yarrow is A. Millefolium roseum, which grows from 1 to 3 ft. high. It may be grown in fair quantity for cut flowers by the market grower. Other species in which a fair trade is done by nurserymen are: A. Ageratum, 6-9 in. high; A. Clavennoe, 6-9 in.; A. Herba-Rota, 6 in.; A. macrophylla, 2 ft.; A. mongolica, 1½ ft.; A. rupestris, 3 in.; A. serrata, 1-2 ft.; A. alpina, 2 ft.; and A. umbellata, 6 in. - all with white flowers. The best yellow-flowered kinds are: A. oegyptiaca, 12-18 in.; A. Eupatorium, 4-5 ft.; A. filipendula, 4 ft.; A. tanacetifolia, 2½ ft.; and A. tomentosa, 1 ft. All species are easily increased by division in spring or autumn. A. argentea, 6 in., white, forms dense rosettes of silvery leaves.