The archetypal flower reads from outside in – sepals (often green), then petals, male stamen and finally the central female parts.
In the single dahlia stamens with their yellow pollen which is available for honeybees to collect, can be seen clearly . Pollen provides most of the protein in a bee’s diet.
In double flowers, stamens have been transformed into extra petals for a fuller, showier bloom. The lack of pollen means pollination cannot occur and the flower remains open for longer, waiting. Both of these features have made double flowers attractive to horticulturists and much energy has been put into breeding double varieties. Highly bred cultivars are much more likely to be doubles than their species (natural) counterparts. Plants with the name ‘flore pleno’ should ring warning bells as it means ‘with a full flower’ and will almost certainly mean it is a double.
Nectaries (rarely visible) store nectar which provides the carbohydrate part of a bee’s diet. They are easier to access in single flowers than in doubles.
Some plant species are good honey bee plants in their single form but not when bred as doubles: Hawthorn (Crataegus ‘Paul’s Scarlet‘), Japanese anemones (Anemone x hybrida), Geums – semi-double cultivars ‘Miss Bradshaw’ and ‘Lady Stratheden’, Cinquefoil (Potentilla), Clematis (such as the strange ‘Viennetta’) and Hollyhock (Alcea rosea).
It is understandable that having as much colour, for as long as possible, has been a priority in British gardens prone as they are to the blanketing green of a wet summer. The fact that plant breeding has followed the demand of gardeners makes simple economic sense. It is nature that is beginning to suffer however. Breeding away pollen serves neither the plant species, as it can’t reproduce itself, nor the insects whose main source of protein it is.
The question is, can designers influence public taste sufficiently for nurseries to change their ways or would it put the breeders out of a job?
Sarah Raven’s Plants for Pollinators has a good selection of single flowered Dahlias.