The Science of Autumn

Leaf colour is determined by three main pigments - chlorophyll for the greens, carotenoids for the yellows and oranges, and anthocyanins for the reds, blues and purples. Chlorophyll enables photosynthesis, the production of food. Carotenoids help absorb light for use in photosynthesis and also protect chlorophyll from light damage. Anthocyanins produce vivid colour in flowers and fruit that in turn attract pollinating insects and seed dispersing animals. These three elements are vital to plant life.

During the growing season leaves are in full production mode producing food for the plant. However, as ambient temperatures drop and the amount of available daylight reduces, plants begin the process of storing key elements for the following growing season. This transfer of nutrients away from the leaves and into the longer lasting parts of the plant is known as resorption. At the same time a separation layer between the leaf and tree begins to transform into a cork-like material that prohibits the flow of nutrients. This also slows down the production of chlorophyll and results in the fading out of green colouration, those great autumn colours and, as the cells in the separation layer break down, the leaves will start to drop.

That's the science bit! We all have our own favourite providers of exhilarating autumn colour, here are a few of mine.

Some of the best known autumn colours are from trees and you can't beat Acer palmatum a deciduous tree with mid green leaves that turn glorious red in autumn. But I also love two other autumn trees. First there's the Liquidambar styraciflua, a tree with resin-scented leaves and gorgeous red and purple foliage in the autumn months and Amelanchier lamarckii, a specimen shrub that offers beautiful flowers, vividly coloured berries, delicately coloured summer foliage and awesome reds and purples in the autumn months. The Amelanchier  is also a great small garden tree with fragrant early Spring flowers. And for a larger tree try Quercus rubra, a relatively fast growing and versatile oak, this tree gives structure and great autumn colour ranging from red to orange and bronze. 

But don't forget that shrubs can give some great autumn colourings. Hydrangea quercifolia  is an unusual variation on the well known hydrangea. A shrub with as much focus on its brilliant bronze, orange and red atumn leaf tones as its long plumes of creamy white flowers. Vaccinium corymbosum, a bushy shrub providing white spring flowers and dark green leaves turning glowing red in autumn and Cornus alba 'Kesselringii', a deciduous shrub with vibrant purple stems and dark green leaves transforming into outstanding reddish-purple are also favourites of mine.

Other great autumn shrubs to try out are Euonymus alatus, subtle and well behaved for most of the year, this shrub bursts into colour in autumn with a display of sumptuous crimson and scarlet foliage. Cotinus 'Grace', known as the smoke bush, has colourful leaves through the year but explodes into brilliant orange and red tones in autumn. And finally don't forget the herbaceous plants like Panicum virgatum 'Rehbraun', an upright, deciduous grass with greyish green foliage turning purple and red during the autumn months and Epimedium x rubrum, perfect for shady ground cover, this delicate and pretty plant provides subtlety in the autumn months with its reddish-brown foliage.

Unfortunately in the UK we sometimes get very short periods of Autumn colour because they sometimes come with heavy storms that take the leaves early. But a good long dry October can herald an explosion of colour that's worth a long Sunday walk.