Extension Educator, Horticulture
Gardeners don't stand erect until autumn. It takes trees we barely noticed all summer to grab us by the nose in autumn and say "hey look at me!" In fall the green chlorophyll in leaves steps aside to let the yellows and reds take center stage. Unlike the vivid colors of flowers that attract pollinators, changes in leaf color appear to have no biological importance. It's just a happy by-product of seasonal shift. I think it's a consolation prize for us plant geeks who are reluctant to say goodbye to another growing season.
People, also known as "leaf peepers," plan vacations around the best fall color. Luckily we don't have to travel far for a good show. Fall color does vary depending on plant genetics, growing season, weather, and changes in daylight. The sun and the earth have taken care of the day length issue. Each year after June 22 the length of daylight grows progressively shorter no matter when we set our clocks back. Plants respond to certain daylight lengths, but not all species and not even all individuals in the same species will respond exactly the same.
A plant's genetics trumps all the other factors. Some trees are very predictable. Ginkgos, redbud, larch, tulip poplars, hickory, and birch are always some shade of yellow. No matter what the weather may be, yellow doesn't vary much from year to year. The yellow, orange, and brown colors of the carotenoid pigments are already present in the leaves, but go unnoticed until the green chlorophyll is no longer produced. Some plants such as red maple and sweetgum are much less predictable.
Our late season warm weather and dry conditions has delayed fall color for many trees this year. Unfortunately a severe cold snap may kill the leaves before they have a chance to turn colors. The weather while the leaves are changing also affects fall color.
Dry sunny days and cool nights intensify red colors. Bright sunlight is crucial for production of red pigments. Often this creates a two-layer effect where the outer or upper leaves are red and lower, less exposed leaves are yellow.
If you want a plant with good fall color, purchase the plant in the fall when the plants are showing off. Great variability exists in many plants traditionally known for fall color. For instance red maples may be a brilliant clear yellow or multi shades of red and yellow. Burning bush may be pink or red. Or select named cultivars. Look for names such as 'Autumn Blaze', or 'Red Sunset' red maples.
Our native white oak is a gem with its maroon color of fine aged wine. Ginkgo leaves turn a clear yellow then hold hands and dive off the tree all at once. Blackgum is one of the best and most reliable fall performers with its leaves of yellow, orange, scarlet, and purple in a tangled collage of color.
For red/purple/orange fall color - White Ash (some cultivars); Sugar Maple (variable); Black, Red, White, and Scarlet Oaks; Dogwoods; Hawthorns; Ornamental Pears; Sassafras (variable); Serviceberry; and Sweetgum.
For yellow fall color – Beech; Birch; Green Ash; White Ash; Magnolia; Norway Maple; Red Maple (variable); Sugar Maple(variable); Silver Maple; Buckeyes; Catalpa; Hackberry; Persimmon; Kentucky Coffeetree; Willow; Linden; Elms; Honeylocust; Tuliptree; Bur Oak; Sassafras (variable). Perennial flowers of willow leaf amsonia and balloon flower.
If a pile of colorful leaves produces only visions of work for you, than you need to take a walk. Stop by our office at 801 North Country Fair Drive in Champaign for the State Street Tree Trail Guide in Urbana and City of Champaign Tree Walk Guide on University Avenue. Spend a pleasant fall day looking at leaves in other people's yards.