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The recipe for autumn colour

Autumn is truly a favourite season for many of us and that's largely down to the sublime shades that gradually take over trees and woody plants. For some reason, seeing leaves turn from green to bright carmine, purple, clear yellows and rusty browns just makes us go 'wow'. It's a wonder of nature that really triggers something in us.

Liquidambar (sweet gum) in its full glory
Liquidambar (sweet gum) in its full glory

Displays of autumn colour are more splendid some years than others, though. Read on to find out why trees turn different seasonal hues and why some years are awe-inspiring, some less so.

The science behind autumn leaves

During spring and summer, most deciduous trees (which lose their leaves in autumn/winter) are on a mission to feed themselves by collecting as many sun rays as possible. The chemical chlorophyll imbues their leaves not only with the colour green, but also permits photosynthesis. This is the process which turns sunlight, carbon dioxide and water into energy, the starches and sugars which feed the tree's cells and allow it to grow. In the growing seasons, leaves are constantly fed with chlorophyll to enable this process.

Cleverly, plants can tell when summer is coming to an end. As hours of sunlight reduce and temperatures drop, trees get ready to shed their leaves before they get frozen off or ravaged by windy weather. Plus, with fewer hours of sunlight, they might as well take a break from maintaining all those leaves, which take energy and water themselves.

Part of this process involves the tree re-consuming the nutrients in its leaves, so as not to waste them. For example, sugars travel back into the branches. Meanwhile, chlorophyll production ceases, and what's left in the leaves gets broken down, releasing nitrogen that the tree can use.

This is why leaves lose their green colour, though the effect is more that they are filled with another colour. Though in many cases it defies intuition, as the clear green pigments fade, others that were previously not visible become the dominant shades.

Why red, purple and bronze?

Fading from green to yellow makes some sense, but to red and burnt umbers? In green-coloured leaves, the chlorophyll content is in fact so strong that it masks all other pigments. These other pigments include carotenes (orange) and xanthophylls (yellow).

Bright reds and purples come from anthocyanins, which are involved in plant defence. They are produced when it gets cold, but while it is still bright, and are more abundant the more carbohydrates there are in the leaf.

As well as the traffic light colours, we see gold, bronze, russets and more. This is because different trees have different proportions of pigments, and they combine to make a warm rainbow of shades in the landscape. E.g. anthocyanin + carotene = deep orange; chlorophyll + anthocyanin = brown.

How the onset of cold affects vibrancy

If there is a sudden cold snap and the tree quickly begins to cut off contact with its leaves, this means more carbs are left in the leaves than if cooler temperatures come on gradually. I.e. the tree doesn't have time to re-absorb as much of the leaf sugar as it might like. More sugar makes for more anthocyanin production. For this reason, autumn colours are brighter and more wonderful in years when the mercury drops dramatically at some point in the season.

The time that the autumn rainbow begins to appear, meanwhile, is generally more dependent on light than on temperature, so it's a fairly stable annual occurrence, though different for each species. Many of our favourite seasonal trees turn in late October (hence the mile-long queues at arboreta in October half term!) and things really get into gear in November.

Other factors

There's a sweet spot for anthocyanin production, when temperatures are just above freezing. If there's an early frost and the air dips below zero degrees, this hinders things and there won't be so much red on the branches.

Dry weather can also speed up leaf drop, so they fall to the ground before they have a chance to colour up.

What you want for a stunning season is bright, warm days, followed by cool but not freezing nights, with a decent amount of rainfall. When Mother Nature gets the recipe right, it's amazing, so make sure you've got at least one autumn stunner in your garden. Think maples, snowy mespil, dogwood, sweet gum and spindle, to name but a few.

See our full list of trees for autumn colour and our article, Autumn shades and human perception for more inspiration.

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