In spring we sent out packets of seed to citizen scientists around the country, to explore the potential of ‘Mignon Series’ dahlias as insect friendly bedding plants and find out if bumblebees had a colour preference. Participants were asked to grow three white, three red and three purple flowered plants and use our app to record the visiting bumblebees.
192 participants submitted a total of 854 surveys, which included photographs of 1,050 individual bumblebees. 10 species of bumblebee were recorded visiting the dahlias, the vast majority of which (89%) were either Buff-tailed Bumblebees, White-tailed Bumblebees or Common Carder Bees.
As summer went on, the plants of all colours bore more flowers and bumblebee visits increased. Overall, the white flowered plants had a greater number of blooms and slightly more bumblebee visitors, though not in sufficient numbers to be statistically significant.
Nectar and pollen
We grew our own dahlias here in Coventry to assess the quantity of pollen, and quantity and sugar concentration of the nectar they produced. We found that this fluctuated hugely between individual flowers, and whilst the white blooms had on average more pollen grains and more concentrate nectar, this was due to just a few individual flowers with unusually high values, so cannot be considered significant.
What participants thought
We also asked trial participants to complete a survey. The majority thought that the Mignon Series dahlias were well visited by bumblebees compared to other bedding plants, with 56% saying they would grow them again, and 60% that they would recommend them as a bedding plant. 43% thought that certain colours were more visited by bumblebees and within this group, opinion was divided equally between the white and red, with each receiving 42% of votes.
If you took part in our dahlia trial and have not yet completed the questionnaire, then you can do so here.
Conclusions
Mignon Series dahlias were well visited by bumblebees and we think that their bright colours, dwarfing habit and long flowering period make them suitable for growing as bedding plants. Unlike many plants used in traditional bedding displays, such as pelargoniums and begonias, single flowered dahlias are a good source of nectar and pollen for insects. We were not able to draw any firm conclusions about bumblebees’ colour preferences (though previous studies have found differences).
Many participants reported that the dahlias were not always the bumblebees’ first choice when other attractive flowers were in bloom nearby. This was particularly noted in the presence of lavender and cosmos, and is a factor that could be explored in the future.
A wide variety of bumblebees visited the dahlias. These were predominately short-tongued species such as Buff-tailed and White-tailed Bumblebees and the Common Carder Bee which has a slightly longer tongue, but is known to forage on a very broad range of flowers.
Like many species of exotic origin, dahlias flower well into the autumn; later than most other garden and native plants. As such, they have the potential to be a valuable food source for insects like queen bumblebees about to enter hibernation.
What next?
In addition to the Mignon Series dahlias used in this trial, our app users have submitted records of bumblebees visiting other varieties of dahlia this summer. It will be interesting to examine how visitation rates to Mignon Series and other dahlia varieties compare. In our own dahlia trial at Ryton Organic Gardens, we also recorded the duration of each bumblebee visit. Bumblebees appeared to spend significantly longer foraging on the white flowered plants, probably due to the greater average number of open flowers, and we will be carrying out further analysis to explore this.
Thank you to all those who took part in our dahlia trial and to everyone who used our app to record bumblebees foraging on other flowers this year. There is still much analysis to be done and we will be continuing to collect records in 2018. We hope you have enjoyed growing the dahlias and observing their insect visitors. If you wish to keep your dahlias in their dormant state over winter ready to grow again next year, The RHS offers advice on this.
Visit the Plants trials pages of our website to view this and other trials in more detail.