Achillea L.
Achillea is a genus with ca. 115-130 species (Guo & al. 2004, Oberprieler & al. 2007), mainly native in the northern hemisphere. Its center of diversity is in southeastern Europe and southwestern Asia. Two species are native in Belgium: Achillea millefolium L. and A. ptarmica L. (Lambinon & Verloove 2012). A few additional species are cultivated as ornamentals (see Thornton-Wood 2000 & Jäger & al. 2008 for very useful overviews for Europe).
In addition to the alien species treated in this account Lambinon & al. (2004) cited two more species: Achillea crithmifolia Waldst. et Kit. and A. lanulosa Nutt. The citation of the former seems to be erroneous, the second species has only been found in Luxembourg (Grand Duchy), not in Belgium (Verloove & Lambinon 2008).
Some species of Achillea are exceedingly similar to those of Tanacetum (especially species of the latter genus with outer florets ligulate). However, in Tanacetum achenes usually have a small pappus (corona).
Molecular studies showed that with the inclusion of Otanthus and Leucocyclus, Achillea s.l. is a monophyletic assemblage and appears as sister to Anacyclus (Guo & al. 2004, Ehrendorfer & Guo 2005).
Achillea millefolium is morphologically variable and has been treated as either a single species with varieties or as multiple distinct species. It is clear that non-native taxa from this complex occur in Belgium, especially in newly sown roadsides, on coal mining spoil heaps, canal banks, etc. However, their identity should be critically assessed, preferably based on flow cytometry or molecular techniques. For the polyploid taxa many hybrid links and introgression from the diploids are suggested (Guo & al. 2012). Some plants have been ascribed to Achillea collina J. Becker ex Reichenb. [syn.: A. millefolium subsp. collina (J. Becker ex Reichenb.) Weiss], an allopolyploid (2n=36) (while native A. millefolium is a hexaploid with 2n=54), partly based on cytological studies on a population from a coal mining spoil heap in Eisden. This identity, however, was subsequently rejected (F. Ehrendorfer, in lit. 06.2009). Plants (often taller) with wider, flat leaves possibly belong to Achillea distans Waldst. et Kit. ex Willd., or at least may have some blood of it. The exact identity of plants with pinkish or reddish ligules (sometimes seen on dumps and in roadsides) also requires further study; these possibly also represent non-native taxa.
Very useful and up-to-date accounts for several taxa of the Achillea millefolium complex were published for Central Europe (Danihelka 2001, Danihelka 2003a, b, Danihelka & Rotreklová 2001a, b, Danihelka & Rotreklová 2002).
1 Ligules pure yellow on both sides === 2
Ligules usually white, very rarely pink or pale yellow === 3
2 Cauline leaves entire, serrate. Corymb 20-40 mm across === Achillea ageratum
Cauline leaves deeply pinnatisect. Corymb 40-100 mm across === A. filipendulina
3 Leaves simple, divided much less than ½ to midrib === 4
Leaves compound === 5
4 Leaves glandular punctate, up to 17 mm wide. Capitula 4-8 mm across. Ligules 7-8, ca. 3(-4) mm === A. cartilaginea
Leaves not glandular-punctate, up to 8 mm wide. Capitula 8-12 mm across. Ligules up to 13, ca. 5 mm (native) === A. ptarmica
5 Middle cauline leaves rounded to ovate in outline, 1,5-3x as long as wide, plane. Plant without stolons === A. nobilis
Middle cauline leaves lanceolate to linear in outline, 3-15x as long as wide, terete. Plant (shortly) stoloniferous (native) === A. millefolium s.l.
Additional alien: Achillea setacea Waldst. et Kit. [syn.: A. millefolium subsp. setacea (Waldst. et Kit.) Čelak., A. millefolium var. setacea (Waldst. et Kit.) W.D.J. Koch] (S, SE and SC-Eur., grain alien?).
Literature:
Dąbrowska J. (1982) Systematic and geographic studies of the genus Achillea L. in Poland with special reference to Silesia. Acta Univ. Wratislav. Pr. Bot. 24: 1-223.
Danihelka J. (2001) Achillea setacea in the Czech Republic, with taxonomic remarks. Preslia 73: 97-120. [available online at: https://www.researchgate.net/publication/264423426_Achillea_setacea_in_t...
Danihelka J. (2003a): Achillea asplenifolia in Moravia (Czech Republic), with taxonomic remarks. Preslia 75: 115-135. [available online at: http://www.preslia.cz/P032CDan.pdf]
Danihelka J. (2003b) Achillea pannonica in the Czech Republic, with taxonomic remarks. Preslia 73: 213-244. [available online at: https://www.researchgate.net/publication/264423457_Achillea_pannonica_in...
Danihelka J. & Rotreklová O. (2001a) Achillea pratensis – a recently recognized species of the Czech flora. Preslia 73: 1–27. [available online at: https://www.researchgate.net/publication/271307100_Achillea_pratensis_-_...
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