Flowering of two Arctium L. species and their significance as a source of pollen for visiting insects

Summary This study conducted in the years 2004–6 concerned the biology and abundance of flowering as well as pollen productivity of Arctium lappa L. (great burdock) and Arctium tomentosum Mill. (woolly burdock) (Asteraceae), growing in natural habitats in the area of Lublin, Poland (51° 14' N and 22° 34' E). Arctium tomentosum started to bloom in late June or early July, with A. lappa two to three weeks later. The flowering of these plants lasted for 6 to 9 weeks, depending on the species and weather conditions. During the growing season, one plant developed 10–15 lateral branches, comprising 320–471 flower heads which produced bisexual tubular florets. The mean number of flowers in the A. lappa inflorescence reached 79.5; in A. tomentosum 45.9. A single plant of A. lappa produced 29.8 thousand florets; A. tomentosum 13.2 thousand. The mean pollen mass produced by 10 florets was very similar for both species, reaching 2.91 mg and 2.99 mg, respectively. One flower head of A. lappa may provide 22.55 mg of pollen, compared to 13.72 mg of A. tomentosum. Pollen yield from one plant averaged 8.45 g and 3.94 g, respectively, corresponding to 23.6 g and 15.4 g per 1 m2.