By the time I get a chance to visit the gardens again in late September, there is little still flowering apart from a few Hydrangeas and the last of the Red hot pokers in the South African garden. I have to admit that neither plant is my favourite, though one of my sons really wanted to grow Red hot pokers as a small child – I suppose they are dramatic!
The bigleaf hydrangeas often grown in the UK, Hydrangea macrophylla, have two sorts of flowers and the obvious, coloured ones are actually sterile and largely made up of coloured sepals (rather like the ‘flowers’ on your Christmas Poinsettia, but that’s another story…). The fertile flowers are relatively insignificant and often sit within a ring of sterile flowers, which presumably do the heavy lifting in terms of attracting pollinators.
Maybe the best-known fact about Hydrangeas is that the flowers of some species (including H. macrophylla) change colour from blue in acidic soils to pink in more alkaline soils – in fact, the colour changes in response to the availability of aluminium ions rather than soil pH, as such. Aluminium ions (with a 3+ charge) bind tightly to negatively charged soil particles in alkaline soils and so are unavailable to the plant; the anthocyanin pigments responsible for flower colour are found in their pink form. In more acidic soils, aluminium ions are more loosely bound to soil particles and more readily available to plants. When taken up by Hydrangeas, aluminium ions bind to anthocyanins in the sepals and convert the pigment into a blue form.
Red hot poker flowers (Kniphofia sp.) don’t favour style over substance in the same way as Hydrangeas, being an important source of nectar to bees and sunbirds in their native range of East and Southern Africa.
Kniphofia are members of the Asphodel family, like the Asphodelus ramosus and A. fistulosus growing everywhere on Cyprus and the Aloe albiflora flowering in the Botanics greenhouses.
All are monocot plants, like grasses and cereals, and have brightly coloured tepals rather than the separate petals and sepals typical of eudicot plants.
The other pop of colour in the gardens in September comes from the fiery red autumn leaves of Euonymus alatus, which change colour long before anything else. Although its common name of “burning bush” fits well, the Latin specific ‘alatus’ refers to the corky wings or flanges which form on its older branches. I have a tiny plant in my garden, bought after being delighted by the colour of the one in the Botanics during a lockdown ramble, but I fear I’ll have a long wait before it puts on much of an autumn show of leaves or bright spindle berries!