For deciduous trees and shrubs, the warmth and abundant sunshine of summer is the time to be productive. Leaves come out in the spring with the job of absorbing sunlight. They use the sun’s energy to produce food, glucose sugar. But these leaves are relatively fragile. They could never withstand the below-freezing temperatures of winter, so the plant “knows” to shed them in the fall. (Evergreens, on the other hand, have stronger leaves, even with their own antifreeze inside, so they can withstand the winter cold.)
The green we see in leaves is chlorophyll, the pigment that absorbs sunlight. But there are other pigments in leaves, with different colors. Carotenoids are always present in plant leaves, but their colors are masked by the chlorophyll green. When chlorophyll production slows and eventually stops in autumn, the yellow, orange or brown of the carotenoids can shine through. Anthocyanin is another pigment, one that is primarily produced in the fall by only some plants. Those that have it display brilliant red and purple leaves.
Pigmentation varies by species, as does the timing of the color change, thus we can get a varying panorama of colors in the autumn.
Timing Fall's Foliage Changes
Deciduous trees and shrubs have two cues they use to stop producing chlorophyll and shed their leaves. When the days become shorter and the nights longer in fall, it's a sure sign of colder temperatures to come. This is the prime cue plants use, because it is consistent year after year. The other signal is the actual decrease in overnight temperatures. This is a secondary cue, since temperature is dependent on weather patterns that can vary.
If temperatures are abnormally cold in the early fall, leaves will change sooner than normal. In a warm fall, foliage changes will be delayed, but they will still eventually occur, regardless of temperature.
Weather's Effects on Fall Color
Just as the onset of fall colors can vary, so can the vibrancy of the colors. And this is weather dependent. Prior to the fall, overall growing conditions are important. Moderate temperatures and regular rainfall are best. Heat waves and droughts stress all plants.
In the fall itself, warm days and cool but above-freezing nights are best for color. Anthocyanin production is maximized, leading to more vivid reds and purples. Warm nights not only slow the process but diminish color. Cloudy conditions limit sunlight, lessening anthocyanin amounts and dulling colors. Drought conditions can even cause premature leaf drop, as can strong winds. Put these factors together and you’ll understand how each fall’s colors can be quite different.
To tie in temperature effects and vegetation, we can go to the familiar USDA Plant Hardiness Zones (http://planthardiness.ars.usda.gov/PHZMWeb/). Fall colors will start in Alaska (USDA Zones 1 and 2) during August, with the exception of the warmer southern coast. Zone 3 areas, such as northern Minnesota and northern New England, will see color developing in September. Most of the lower 48 (Zones 4–7) will see fall foliage in October. The Deep South (Zones 8 and 9), the furthest south that leaves change, may not get color until November.
Air temperature usually decreases as elevation increases. Hopkins Bioclimatic Law relates these elevation-related temperature changes with biological occurrences. For fall foliage, the general relationship is such that for each 100 foot gain in elevation, fall colors will arrive a day earlier. In the Colorado Rockies, this means fall color in late September; in the southern Appalachians, early to mid-October. There are numerous mountain roads, especially through national forest areas, that afford great views of those fall colors.
Leaf Peeping Near and Far
You can travel to see the best colors in your area, or you can plan a longer trip to a fall destination. “Leaf peeping” is a big tourist attraction in New England and the mountainous terrain of the Appalachians in the East and the Rockies and other mountain ranges in the West.
Or you can just stay at home and enjoy the local color display, which can clue you in on plants that will add fall brilliance to your own garden. Your local plants are the ones that have adapted to your climate, and if you’re planting for fall color, you need plants that thrive in your area. For example, birch and aspen can survive even in the frigid cold of USDA Zone 1. Their yellow leaves in fall often stand in brilliant contrast to surrounding evergreens. Maples are known for their colorful leaves in fall. Red maple leaves (Acer rubrum) can turn a deep red in fall, while sugar maples (A. saccharum) change to yellow, orange or red. Maples can thrive in most areas of the United States, from Zone 3 to 8 or 9. In urban settings in these same zones, the well-adapted Ginkgo biloba can be found producing bright yellow leaves in fall. More warmth-loving trees for Zone 4 or 5 down to the Deep South’s Zone 9 include black tupelo (Nyssa sylvatica), which can show a spectrum of colors from yellow to scarlet, and sourwood (Oxydendrum arboreum), with red, yellow or purple leaves.
There are many more seasonally vibrant plants out there that will be suited to your area. Check with local experts or tour a nearby arboretum this autumn, and get planting for fall color.