The quality of autumn colour is dependent upon soil type, soil pH, soil fertility, tree species, tree provenance and the weather.
There are three pigments present in the leaves of trees and shrubs namely
It is the production and retention of these pigments that determines the colour of autumn leaves.
Leaf Change and Leaf Colour
Leaves change colour as the levels of pigments change in response to senescence, the process by which trees shed their leaves in autumn. Senescence is triggered by the amount of available daylight.
All leaves gradually lose pigments during the growing season but the rate of loss accelerates during senescence. When the rate of pigment loss is unequal, this inequality creates the variation in autumn colour.
a) anthocyanins produce the red colours,
b) carotenoids produce the yellow colours,
c) chlorophylls produce the green colours,
d) anthocyanins and chlorophyll produce bronze colours, and
e) anthocyanins and carotenoids produce orange colours.
Soil Type and Soil pH
Trees growing on their preferred soil type and pH are typically healthier and more vigorous than trees not growing on their preferred soil type. There is general belief that acidic soils, i.e. with a pH of less than 7.0 produce the best autumn colour. Whilst for many trees and shrubs this is the case, there are substantial numbers of plants which are either indifferent to soil type or thrive on alkaline soils and still produce good autumn colour.
Research conducted by Emily M. Habinck, a graduate of the University of North Carolina, found that trees growing in places where the soil was relatively low in nitrogen and other essential elements produced more anthocyanins.
Parentage & Geographical Origin (Provenance)
Trees selected from areas recognised for good autumn colour continue to display good autumn colour when grown elsewhere.
The best autumn colour is achieved when trees of good provenance are growing on their preferred soil type, preferred soil pH and poor soil fertility.
The ideal weather conditions are mild wet summers followed by a cold dry autumn with high light intensity.