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Why do Leaves Turn Colors in the Fall?
Hillary T
Why do Leaves Turn Colors in the Fall?

The Cutting Edge Plants' blog, The Hort Science Shelf (a nod to the practice of arranging bookshelves by subject), is launched with this blog post on fall color, a spectacle I am happily witnessing out my office window.

Autumn is a heady time of year for we tree lovers. The colors themselves are lovely, but I especially enjoy watching the layers of color appear in stages as each tree species turns color.

First the Dogwood with plenty of still-green trees behind it causing the red or burgundy fall color of the Dogwood to especially pop out of the scene. Then the interplay of oranges, reds, yellows, and purples as the entire tree canopy becomes a kaleidoscope. Finally, watching the last golden colors float to the ground as the tardy Ginkgo lets go of its leaves and joins the other leafless trees.

What are the colors? 

Leaves are green due to the famous pigment chlorophyll. Chlorophyll’s function is to absorb light and start the photosynthetic process, the process of making food (sugars) for the plant.

Equally famous is the pigment carotene (we all know that the carrots contain carotene and are good for you). Carotene is an orange-colored pigment. In the fall, certain events (discussed below) trigger the cessation of chlorophyll production and the breakdown of chlorophyll in leaves. When the chlorophyll is removed, the remaining pigments, such as carotene, become visible. The green chlorophyll masks the orange-colored pigment carotene; when the dominant green pigment is removed in the fall, the leaves then appear orange. A process we delighted humans enjoy when we look at a favorite tree and say things like, "that tree turned a nice orange this year."

When xanthophyll is the remaining pigment, leaves “turn” yellow and when anthocyanin is the remaining pigment, leaves turn red or purple.

What Triggers Color?

Regarding the events that trigger fall color, scientists agree that the shortening daylight hours and lengthening nights of autumn trigger the cessation of chlorophyll production and the breakdown of chlorophyll in leaves of deciduous trees, shrubs, and vines. All evergreens, such as camellias, firs, and pines, retain chlorophyll and thus express no fall color.

You’ve probably noticed that the quality of fall color varies from year to year and is certainly better in cold areas, such as the mountains, and is quite week in warm areas of The Deep South (even where there are plenty of deciduous trees). Scientists postulate that two events are key in triggering vibrant fall color: cold nights and a touch of dry weather preceding the onset of fall. If autumn’s nights are too warm, my observations tell me not to expect a spectacular display.

Even with these rules of thumb, it is still 100% impossible to predict when fall color is going to be vibrant. For instance, the fall nights may be plenty cold, but the summer was too dry and the leaves are already half crispy brown from drought and just can’t muster up a show.

Also, if it is too cold and the temperatures dip below 32 degrees, the leaves freeze and thus fall color is lost. Fall color is a balancing act and it is fun to watch and wait and anticipate the quality of each year’s show.

Trees for Fall Color

Trees are the unrivaled bearers of fall color. Even in the worst years for fall color the following trees will not disappoint: hickories (Carya species, yellow or oranges), sugar maple (Acer saccharum, orange or yellow), Japanese maple (Acer palmatum, yellow or red or orange), red maple (Acer rubrum, red or yellow), serviceberry (Amelanchier species, red or orange), dogwood (Cornus florida, red or burgundy), sweetgum (Liquidambar styraciflua, red or orange or yellow).

Fall color has been co-opted by the arboreal constituency. Trees, with their height, majesty, and imposing size, insist upon all the autumnal glory. However, there are shrubs, vines, a perennial, and two grasses that vie for autumnal leaf color recognition. I will introduce you to a few.

Shrubs for Fall Color (with diversion into Hydrangea, of course)

The best shrubs for fall color are: winged euonymus (Euonymus alatus, red - don't shoot the messenger, yes, I know this plant is invasive, therefore we won't be selling it, but I can't deny it has beautiful fall color), oakleaf hydrangea (Hydrangea quercifolia, red or burgundy), large fothergilla (Fothergilla major, yellow or orange or red), summersweet clethra (Clethra alnifolia, yellow or orange), Virginia sweetspire (Itea virginica, yellow or orange or red), and blueberries (Vaccinium species, red).

Since I have been studying Hydrangeas, it has come to my attention that there are several Hydrangea macrophylla cultivars that have good fall color. Personal favorites are Twist-N-Shout® and 'Lady in Red', both with reliable burgundy fall color, and I'm seeing our wee little 'Green Mantle' cuttings showing red (insert >clicking heals< emoticon here, if there were only such a graphic).

And from left field, Hydrangea serrata 'Diadem' is showing burgundy fall color in a 3-gallong right now! (November 30, 2017)

My favorite gardening forum is Garden Web and there is a Hydrangeas with Fall Color discussion where these are listed for fall color: Lets Dance Starlight, Endless Summer, Double Delights Expression. I cannot vouch for them myself.

Twist n Shout Bigleaf Hydrangea fall color

Vines for Fall Color

As for vines, poison ivy (Toxicodendron radicans, yellow or orange) is not welcomed in the garden, but look for its vibrant fall color and just try to convince yourself that it is not showy and even beautiful. Wild hyrangeavine (Decumaria barbara, yellow) and Virginia creeper (Parthenocissus quinquefolia, red) are other showy, more friendly choices.

Perennials and Grasses for Fall Color

Perennials and ornamental grasses (as opposed to turf grasses) are undoubtedly known and grown for their flowers and leaf colors and textures, however there is one perennial and two grasses that are grown for their flowers and their fall color. The perennial is Arkansas amsonia (Amsonia hubrectii, yellow) and the grasses are switch grass (Panicum virgatum ‘Shenandoah’, red and burgundy) and hakone grass (Hakonechloa macra turns copper or orangish and the yellow-leaved cultivar ‘Aureola’ turns pink).

As I write this piece during the dog days of late summer, I look forward with keen anticipation and curiosity in the quality of this autumn’s color.

Please comment with more examples of Hydrangea macrophylla fall color, we want to learn from you!


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