What Determines the Autum Colour of Acers

Many myth surround the requirements Japanese maples have. Recently a visitor to the garden told me they thought growing Japanese maples in acid soil improved the autumn colour, which is why it is recommended.

I have considered this question myself, and I think that the amount of sunshine the tree gets during the growing season is a much better predictor of autumn colour then the pH of the soil.
This picture, of an Acer japonicum 'Aconitifolium' was taken in Autumn 2009. The tree grows in a raised bed filled with ericaceous compost in my north facing front garden. It grows under the canopy of a Cersis canadensis 'Forest Pansy'. At the time this picture was taken the canopy of the Cersis was quite light and the tree had some sunshine in the summer month.

This is a picture of the same tree taken in October this year. The only difference is that the canopy of the Cersis is much denser and the Acer is now in full shade.

This is a picture taken this autumn of another specimen of Acer japonicum 'Aconitifolium' growing in the ground in the back garden (the soil is slightly acid clay).  In the summer month it gets at least 6 hours of sunshine. The colour at the extremities of the tree is a deep red, while the leaves further down, which don't get as much sunshine are only red at the edges and are mostly dark yellow.

This is not a scientific experiment, but while the pH soil makes a difference, I believe it is the sunshine that produces the best colour.

Autumn Colour 2012

The miserable weather this autumn resulted in early and very good autumn colour.
We have been opening the garden for 4 years and this is the first time we had decent colour on the October open day (This year this was on 20 and 21st of October).

 In the foreground is Acer shirasawanum 'Jordan'

In the foreground is Acer palmatum 'Kamagata' a dwarf acer which is usually pale orange and pink in colour. It has been in the garden for 6 years and this is the first time it is bright red.

Acer palmatum 'Orangeola'

Acer japonicum 'Aconitifolium'