Different flowering times of Pedicularis palustris ssp. palustris (May-July) and P. palustris ssp. opsiantha (Ekm.) Almquist (July – September) are related to variation of the life cycle; ssp. palustris is a biennial, ssp. opsiantha is a summer annual. Growth experiments confirmed the genetic basis of the difference. The biennial has rosette leaves in the first year, and forms a winter bud with scale-like leaves. These leaves or their scars are visible in the flowering plant; they are missing from the annuals. Germination requirements are similar; fresh seeds germinate at rather high temperatures (25-30 °C) only, but stratification widens this range. The adaptive nature of the life form is discussed in relation with the different habitats in which the taxa are found.