Why do leaves change colour in autumn?

09/11/2023

It’s that time of year when the leaves of trees show their best autumn colour. The bright reds, oranges and yellows can be just as dazzling as the bursts of blossom in spring. Autumn creates some of the most beautiful spectacles in the world of trees. Here at FPM we are fortunate to be surrounded by many trees that are looking truly radiant.

Like most plants, trees use chlorophyll to photosynthesise

This process creates essential sugars using sunlight and combining it with water and nutrients brought up from the soil by the roots. Leaves have evolved to carry out photosynthesis by covering the tree canopy to absorb the most sunlight possible. The sugars generated give the trees the energy they need to live and thrive. This is why they will reach up and lean towards the light. Chlorophyll is green so during the months when there is the most sap movement and the tree is growing as much as it can, this green colour is usually the most dominant shade.

As the daylight hours begin to shorten by a minute or two a day, the trees start to slow down. Millions of years of evolution has enabled plants to be supersensitive to changes in daylight and temperatures, so that incrementally everything happens in tandem. Gradually, the amount of sugars the tree can make decreases and the sap movement reduces to a minimum. Other chemicals in the leaves, such as carotenoids and anthrocyanins, are then revealed, bringing the rich autumn colours to the surface. Every individual tree and leaf will have a slightly different combination of these chemicals and the process will happen at different speeds. This is what creates the lovely range of warm reds, oranges and yellows.

Colder nights and lower daytime temperatures trigger hormonal changes in the trees and abscisic acid is created to seal the leaves at the stalk so they weaken and drop off. Trees without leaves are less likely to be blown over or damaged by winter storms, so it is a great way of providing protection until the spring. The best weather for an excellent autumn display of colour is mild days, cold nights and no wind. Recent storms have whipped off many leaves when they are looking amazing so we advise everyone to make the most of the colours whilst they last!

Below are just a few varieties we have photographed this week on the nursery:

Cercis can ‘Ruby Falls’®

A graceful new introduction of weeping habit. The heart shaped leaves appear rich purple, very similar to ‘Forest Pansy’. Clusters of bright pink pea like flowers open just before the foliage in spring followed by flattened, deep purple pods. Perfect for small gardens.

Malus ‘Scarlett’®

A superb ‘all seasons tree’ combining good leaf, flower and fruiting features. The rich pink blossom contrasts well with the emerging purple, deeply lobed leaves. Turning glossy dark green in summer, then rich scarlet with purple tints in the autumn, very rare for a Malus. The persistent small purple fruits remain long after leaf fall.

Sorbus aucuparia ‘Beissneri’

A compact, upright, deciduous tree with reddish brown bark. Orange-copper stems bear green, fern like leaves which turn yellow in autumn. Frothy, creamy white flowers are followed by heavy clusters of red berries, very popular with birds.

Sorbus ull ‘Olympic Flame’™

This columnar, deciduous tree makes a fine specimen for small gardens and confined spaces. The highly feathered leaves start out copper green, becoming green over the summer, then a fiery mix of bronze, orange and scarlet. Creamy white flowers in late spring develop into clusters of shiny orange-red berries.