Blazes of Colour
By Jonathan Diaper
Hooray for the bonfire colours of autumn. It is best to see autumn as a rejuvenating sight, rather than as ‘dead things dropping off’. The heavy swathes of green foliage in late summer are so abundant we can easily take trees and shrubs for granted.
With the onset of autumn the greenness changes. The green pigment in chlorophyll breaks down, revealing the pigments of carotenoids (yellow-orange) and anthocyanins (red-purple). These are always present within the leaf, as by-products of photosynthesis, but in green leaves are hidden from the human eye by chlorophyll acting as a green filter. Varying amounts of carotenoids and anthocyanins explain the huge variety of autumnal leaf colours.
Sunlight is precious at this time of year. Positioning a shrub or tree in front of a low sun highlights the leaves like stained glass. The commonest deciduous trees, such as hazel, Corylus avellana, look burnished with the sun behind them. The big purple-red leaves of Cercis canadensis ‘Forest Pansy’ have a pendant look like jewellery.
To make the best autumnal display the positioning of plants in a garden is important. As trees and shrubs are long-lived, it is best to take time to site them with autumn in mind.
Plants will look their best with space around them, seen as specimens, with a distinct outline of branches. A glade of Japanese maples (Acer) is magical, especially so if each tree has a polite space around it. The brightness of the autumn leaves will make the trees seem closer together. There are many varieties of Acer which will give a dazzling autumn display. One of the best autumn reds is Acer palmatum ‘Osakazuki’ which produces a blaze of colour. A single tree, perhaps sunlit beyond a dark stretch of yew or holly, can be the highlight of a garden.
Hamamelis (witch hazel) needs space to display its widely splayed branches but rewards its position with large leaves that turn yellow in autumn. There is the promise of its strongly scented flowers to look forward to in winter, zesty yellow in the cultivar Hamamelis x intermedia ‘Pallida’. They prefer a soil on the acidic side but will tolerate a neutral soil if plenty of organic matter is added.
If you have the luxury of space then Cercidiphyllum japonicum is a superb tree to grow in all respects, not least for the golden leaves it bears in autumn that have an uncanny caramel aroma. It will grow to 20 metres, although there is a much smaller form, C. ‘Boyd’s Dwarf’ that will grow to 2 metres.
Autumn colour can still be had if space is limited. Acers can be grown in containers with shelter from wind. There are several cultivars of our native ‘spindle tree’. Euonymus alatus ‘Compactus’ grows to only 1.5 metres in height and spread but bears the vivid red leaves of its cousins. The attractive leaves of Hydrangea quercifolia turn a plum colour at this time of year. They make a choice shrub for a partially shaded position close to a house or in a sunny position, as long as their roots do not dry out in summer. Cotinus ‘Grace’ is a spreading shrub but, once established, it can be stooled in winter to keep it within bounds. This has the effect of producing larger leaves that turn shades of orange and pink.
Vitis vinifera ‘Purpurea’ is a vigorous but well-behaved climber that can cloak a wall or fence, its leaves turning a garnet colour that will not blow away at the first breath of wind. For a larger wall Parthenocissus henryana (Chinese Virginia creeper) is a good choice, its little suckers holding it to a wall without the need for wires. Both these climbers will grow well on alkaline and neutral soils.
Vitis vinifera ‘Purpurea’
Autumns vary in intensity. The plants mentioned above will give reliable colour each year. Plants vary genetically, however, in variety but also individually. The weather plays its part. Cold, but not freezing, temperatures help to increase the levels of anthocyanins and inhibit movement of sugars out of leaves. High rainfall is thought to lower levels of anthocyanin – as are high levels of nitrogen – converting surplus sugars into proteins rather than pigments. High light levels will increase colour. Apples that receive more sunlight are redder.
All this wealth of course is short-lived but the more poignant for it.