The Science of Autumn Colour
Have you ever wondered what makes the leaves on trees and shrubs those beautiful hues of orange, gold and red in the autumn? You could be mistaken for thinking that they have turned those rich colours but they have been hiding there all the time. Leaf colour is made up of a range of pigments and what has in fact happened is the plant has stopped producing chlorophyll, the dominant pigment which gives leaves that distinctive green hue. This in turn affords the other pigments a chance, albeit a short one, to come to the fore and have one last hurrah before the leaves are shed. This process occurs once the days start to shorten and cooler temperatures prevail.
Chlorophyll, being the main pigment in plants is a vital component in the cycle of photosynthesis – the powerhouse of life. It is through this process that light energy from the sun is used in conjunction with carbon dioxide and water to produce sugar and oxygen. In the height of summer, the plants are producing their highest quantities of food using the energy harnessed by chlorophyll therefore perpetuating the green colour. When chlorophyll production slows down, the pigments that make carrots orange (carotenoids) and sweetcorn yellow (xanthophylls) become visible in the leaves.
In the case of the stunning reds you often see in acer, sycamore and mountain ash, this colour change is brought about by the development of anthocyanin which gives flowers their distinctive red and purple flowers but also the fiery reds in autumn foliage. There are a number of theories about why these vivid reds develop. Some suggest it is to warn insects that the leaves no longer contain as much nutrition as the plant has reabsorbed the chlorophyll to conserve energy. There are also theories that the red pigment protects the plant from the harmful rays of the sun, although the sun is essential for life it can be harmful to plants as it is to humans.
Why do the leaves fall?
In a temperate climate like ours we have distinctive seasons with spring, summer, autumn and winter creating a seasonal ebb and flow throughout the year. The act of shedding leaves by trees and shrubs is an energy saving process which is a direct reaction to the cooler, shorter days. In order to harness the energy from the sun, the plants must experience favourable periods of warm temperatures and long days. Without these, the energy necessary to continue producing chlorophyll would become inefficient leaving the plants with an energy deficit. It therefore makes more sense for the plants to reabsorb chlorophyll into the trunk of the plant in the autumn and enter a period of dormancy to conserve energy until the dawn of the next growing season. Once the chlorophyll is reabsorbed and the leaves change colour, the plant closes off the tissues that attach the leaves to the stem with the production of abscisic acid which eventually results in leaf fall. Think of the distinctive horseshoe-shaped leaf scar found on the stem of the horse chestnut tree, this was as a direct result of that process.
Best Plants for Autumn Colour
Now that the science bit is out of the way here are some of my favourite plants for autumn colour which are sure to add a veritable technicolor coat to your garden that Joseph would be proud of!
Winged spindle Euonymus alatus
My absolute favourite for fiery red in the autumn has to be the spindle tree (Euonymus alatus). No other shrub packs such a hearty punch with vibrant red foliage which goes on for days. These unusual plants are unique in the sense they have ridges which protrude longitudinally along the stem appearing like wings (alatus is Latin for winged).
Japanese maples Acer palmatum
Such elegant specimens, the Japanese maples offer some really spectacular colours at the end of the growing season. Good varieties to look out for include Acer palmatum ‘Katsura’ for golds and burnt orange tips and A. palmatum ‘Sango-kaku’ for lime green foliage that turns a soft yellow with vibrant coral stems.
Mountain Ash Sorbus aucuparia
Native to Ireland, the mountain ash or some of you may know it as the rowan tree is a relatively short-lived specimen as it is a pioneer tree but boy does it pack all it can into that short life. With delicate leaflets stood alongside plump round bunches of fruit the rowan tree develops superb orange/red foliage in the autumn and depending on the variety you can expect pink, orange or white berries which really add something special to the autumn garden.
Boston Ivy Parthenocissus tricuspidata and the Virginia creeper Parthenocissus qinquifolia
If you are short on space, why not consider a deciduous climber like Boston ivy or the Virginia creeper. Both offer exquisite wall coverage of the richest reds, a real visual treat for the autumn.
Yellows and Oranges
The fossil tree so-called as it is the only known species in its genus, Ginkgo biloba has the most unusual bi-lobbed leaves which turn a dramatic, golden yellow before shedding its foliage like a golden fleece covering the ground.
Ash Fraxinus excelsior
Native to Ireland and unfortunately currently on the wrong side of a battle with a deadly fungal disease, the ash tree can be seen showing off its golden leaflets across the country during October.
The handkerchief tree Davidia involucrata
Famous for its elegant bracts which emerge in June resembling handkerchiefs fluttering in the wind, Davidia gives you bang for your buck when its foliage turns a rainbow of colour sure to bring a smile to anyone’s face.
Larch Larix decidua
One of a handful of deciduous conifers, larch can stand up to any yellow broadleaves in the autumn with its golden whorls of soft needles which are shed once the temperatures drop each year.
I have definitely noticed a shift in the season and am seeing far more colour popping in the tree crowns so why not get yourselves out to your local park or woodland and enjoy a jump in leaves before the cold really sets in!
For anyone interested in doing a gardening course this autumn I still have places left on my October course which starts this Saturday, October 6th. Give me a call on 086 393 8467 if you would like to take part.
As always, Happy Gardening Everyone!