If competition for pollinators is an important influence on populations of Centaurea scabiosa and C. nigra sensu lato one of the characters most likely to be affected is flowering time. Observations were made on the flowering period of several populations of the two species growing sympatrically and in isolation, and on plants from these and further populations transplanted into the Botanic Garden.
Conclusions from the study are:
- 1Where C. scabiosa and C. nigra grow sympatrically, C. nigra flowers slightly later than C. scabiosa but there is a considerable period of overlap.
- 2Populations of C. scabiosa differ slightly in flowering time, but there are large differences between C. nigra populations growing in isolation from C. scabiosa.
- 3In many populations of both species there are large differences in flowering time between individuals.
- 4The differences observed between plants in the field are maintained in cultivation.
- 5Small differences are apparent between years and between aspects.
The large range of flowering time seen in C. nigra may be partly attributable to differences between ssp. nigra and ssp. nemoralis and hybridization between them, but local ecological factors are important influences on some populations. Competition for pollinators between C. scabiosa and C. nigra may maintain the difference in flowering time between the two species in some areas.