Ornamental Representatives of the Genus Centaurea L. as a Pollen Source for Bee Friendly Gardens

Abstract

A b s t r a c t The flowering phenology and pollen production of three ornamental Centaurea species were investigated in the years 2009 and 2012-2013. The study objects, Centaurea montana L. = Cyanus montanus (L.) Hill, Centaurea mollis Waldst & Kit, and Centaurea dealbata Willd. were cultivated within an area of the UMCS Botanical Garden in Lublin, Poland (51° 14' N, 22° 34' E). Under the environmental conditions of SE Poland, the Centaurea species flowered continuously from mid-May to the first week of June. The mass of pollen in anthers was found to be species-related: 3.70 mg (C. montana), 4.02 mg (C. mollis), and 6.01 mg (C. dealbata) per 100 anthers. The total pollen yield was related to the mass of pollen produced in flowers and the abundance of blooming. Pollen grains were medium-sized, spheroid (C. dealbata) or prolato-spheroid (C. mollis and C. montana) in shape, and characterized by high viability (over 80% on average). The pollen provided by the plants of ornamental Centaurea species amounted to 6.0 -7.9 g per m 2 on average. The honeybee was the most frequent visitor of C. dealbata, accounting for 55.2% of the total pollinators, and bumblebee species predominated on the flowers of both C. montana (77.7%) and C. mollis (85.6%). Solitary bees and dipterans were also observed on the flowers of all species studied, but C. mollis was avoided by lepidopterans. Ornamental Centaurea species provide pollen reserves that could support communities of invertebrate pollinators, although the period of effective supply fluctuates annually due to changeable periods of blooming.

Evaluation of Selected Ornamental Asteraceae as a Pollen Source for Urban Bees

“…According to Jabłoński (1997), flowers of Dahlia variabilis supply bees with substantial amounts of nectar and pollen. The pollen production values for C. macrocephala (35.6 g·m -2 ) in the present study, were higher than those established for Centaurea scabiosa (28.3 g·m -2 ) (Denisow, 2006) or the ornamentals C. montana, C. mollis, and C. dealbata (6.81 g·m -2 , 5.85 g·m -2 and 7.92 g·m -2 , respectively) (Denisow et al, 2014). A fair pollen source was from the plants of C. sulphureus.…”

“…Insects can forage on the pollen only after it has been pushed outside the staminal tube by the stigmas of the rapidly elongated pistil -secondary pollen presentation (SPP). This mechanism of pollen presentation is typical for representatives of Asteraceae (Yeo, 2012) and has been observed e.g., in Solidago x hybrida (Strzałkowska, 2006), Ligularia , Arctium (Wróblewska & Stawiarz, 2012 b), and Centaura species (Denisow et al, 2014). Pollen grains of the studied taxa are characteristically echinate in exine ornamentation.…”

“…In all the observed plants, the onset and length of blooming depended on the weather conditions during the growing seasons. The influence of meteorological factors on the onset, duration, and abundance of flowering has been reported for many plant species, both in natural and urban habitats (e.g., Masierowska, 2006;a, Denisow et al 2014. As described earlier, in the studied Asteraceae, pollen forage is provided exclusively by disk florets whose anthers are fused together in a tube.…”

“…Asteraceae have mass flowering, showy inflorescences -capitulum type and floral resources that attract different and abundant pollinators (Käpylä & Niemelä, 1979;Suryanarayana et al, 1992;Bodnarčuk et al, 1993;Wróblewska, 1995Wróblewska, , 1997Wróblewska & Stawiarz 2012 a, b;Denisow et al, 2014). Due to these characteristics, Asteracean species are frequently used to compose flowerbeds in avenues, gardens, squares, and parks.…”

Characteristics of blooming, pollen production, and insect visitors of Polemonium caeruleum L. – a species with a potential to enrich pollinator-friendly urban habitats

“…In modern urban landscape design, a trade-off between aesthetic value and ecosystem services of plant species is desired. Ornamental plants can provide insect pollinators with considerable amounts of pollen and nectar sugars [29,31,38]. However, some ornamental plants are unattractive to native pollinators or they produce a very small amount of forage for the pollinators [39,40].…”