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Fall Colors are the Result of Genetics & Environment

By Robert Cox, Colorado State University Cooperative Extension agent, horticulture

September and October bring on "Aspen Days" in Colorado, when families head to the mountains for a glimpse of gold.

There's nothing quite like the brilliant gold color of leaves poised against a background of deep azure-blue sky. Enjoying the color is one thing; knowing why leaves change from green to brilliant hues is another.

Fall color of aspens and other deciduous trees is determined by genetics and environment (weather, altitude and soil conditions, for example). Brightest color results from bright sunny days, cool nights and drier soils high in iron content.

Leaves are green because of chlorophyll, a complex pigment involved in photosynthesis. Chlorophyll continually must be synthesized as sunlight degrades it. As days shorten and nights cool, synthesis of chlorophyll ceases and the remaining chlorophyll in leaves is broken down by enzymes and sunlight. As leaves lose chlorophyll, other pigments that were masked by chlorophyll become apparent. Yellow colors are the result of carotene pigments; reds and purples from anthocyanins. Orange fall color is caused by mixtures of carotenes and anthocyanins.

Aspen color in the mountains almost always is showier than that of aspen transplants in metro landscapes. The high altitudes means higher sunlight intensity and cooler nights.

Clay soils of metro areas retain much moisture and we provide additional landscape watering. Mountain soils generally are drier and well-drained. Metro soils are alkaline, mountain soils generally are acidic, which makes iron more available to plants.

Many homeowners want plants with reddish or purple fall color for landscaping.

Following is a list of some suitable plants with red or purple coloring:

'Autumn Purple' Ash (tree)

Cotoneaster (shrub)

'Autumn Blaze' Red maple (tree, prefers acidic soil)


Nannyberry Viburnum (shrub)

'Cimarron' Ash (tree)


'Redspire,' 'Stone Hill' or 'Aristocrat' flowering pear (tree)

'Red Sunset' Red maple (tree, prefers acidic soil)


Purpleleaf Wintercreeper (groundcover)

Burning Bush Euonymus (shrub)

Sumac (shrub)

Amur Maple (shrub or tree, prefers acidic soil)

Red oak (tree, prefers acidic soil)

'Flame' Amur maple (tree, prefers acidic soil)

Serviceberry (tree or shrub)

Remember, most metro soils are alkaline and are difficult to change to a more acidic soil.

Photo: Judy Sedbrook

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Date last revised: 01/05/2010