As September approaches, individual gardeners can make a huge difference to the success of our bumblebee populations. Because this is the ‘hungry gap’ for bees.
In spring, we’re strongly urged to provide nectar rich flowers for emerging queen bumbles and other pollinators but perhaps surprisingly, it’s just as important to lay on an abundant supply of nectar and pollen in the autumn. The life cycle of bumblebees is the reason for this. New queens emerge from nests in September to mate and build up resources to help them survive the long months of hibernation.
The vital importance of supporting our bumblebees in autumn was highlighted in a study published in The Journal of Applied Ecology earlier this year. When trying to find ways of making farms more bee-friendly, researchers led by Thomas Timberlake asked: “What limits bumblebee populations on farmland?” They found that lack of nectar supply in September was key.
The availability of garden flowers made little or no difference to bumblebee populations in nests for most of the year, but it was critical in September and could affect population density the following year by as much as 50%.
There aren’t many wild flower varieties in autumn. The few include knapweed, Centaurea nigra, and maybe some red clover. Ivy is often mentioned but this only applies further south in England. Ivy in this country doesn’t flower till November or later. So our gardens must come to the rescue and provide many more flowers. Colonies of the Buff-tailed bumblebee, Bombus terrestris, were much stronger if their queens had had access to gardens the previous September.
As principal researcher Professor Jane Memmott explains, “finding that these small rural gardens can have a measurable effect on bumblebee populations shows us that we really can make a difference for pollinators – even as individuals – by making sure our gardens are as pollinator-friendly as possible.” This applies to the vast majority of Scottish gardens, wherever you see bumblebees in summer.
It doesn’t matter whether a flower is native or not which is just as well because not many native or near-native ornamental species flower here as late as September. Single flowered dahlias are very popular with bumblebees. They really like that huge boss of disc-florets and don’t care that dahlias originated in Central America.
Critically, single flowers are essential since nectaries have been removed in doubles. Shape and colour of flower do matter and bees can easily see large clumps of individual colours.
A great clump of sedums or Hylotelephium like Sedum spectabile presses all the buttons for bumblebees. These mostly come in bumblebees’ favourite pinks and purples. These colours, together with blues, are in the part of the spectrum that bumblebees see easily. One sedum flower head comprises lots of tiny flowers with each producing nectar. I find it riveting seeing the bees crawling hungrily over the flower heads sipping as they go. It’s almost like a pub crawl. Once an individual nectary has been emptied it will take several hours to refill so flower species that produce lots of individual flowers in a single head are ideal for the insects.
The daisy and scabious families have very similar flowerheads. This makes Michaelmas daisies, sunflowers and other late flowering Helianthus a good choice.
Bumblebees are famous for making holes in the base of foxglove flowers but even if foxgloves are over by September, some salvias will still be flowering and their smaller flowers, especially blues and purples, are useful for bumblebees. Scarlet Salvia elegans may be less popular.
Fortunately all these species grow best in a sunny spot which is where bumblebees prefer to forage. Even though their hairs keep them warm, they’re not up for chilly autumn shade.
Plant of the week
Dahlia ‘Roxy’ flowers for months, producing nice open single flowers that are perfect for bees. Bright magenta flowers and dark foliage make this a striking plant for pots.