Garden Variety: Weather can affect fall color forecast

Mid-October is usually the peak of fall color for trees and landscape plants in and around the Lawrence area, and hopefully trees and plants will catch up with the calendar soon. September’s extra-warm temperatures slowed down the color development slightly, but recent rains and cooler weather should get plants back on track. Leaf color development also indicates good timing for fall garden cleanup.

Fall color refers to the red, orange, yellow, gold, and purple hues taken by otherwise green trees and landscape plants. Color development is favored by ample moisture, cool nights above freezing and sunny days.

Yellow, orange and gold pigments are present in leaves but hidden by green chlorophyll. When short days and cool temperatures arrive, plants are triggered to slow photosynthesis and chlorophyll production, making the hidden shades visible. Since day length is tied to the calendar, this trigger can usually accurately predict development of these colors regardless of weather conditions. What the weather might affect with these colors is their brightness and how long they stay attached to the tree or plant before dropping and fading to brown.

Red and purple pigments develop with increased sugars in the leaves and are especially favored by the cool night/sunny day combination. Without this particular weather pattern, reds and purples may fail to fully develop and/or fade quickly.

Ash, elm, hickory, cottonwood, birch, ginkgo, honeylocust, redbud and tulip poplar are examples of trees that typically display brilliant shades of yellow, orange and gold in fall. Maples, oaks, some ash cultivars, burning bush, sumac, sweetgum, and blackgum are examples of trees and landscape plants that typically display shades of red and purple.

Changing colors in trees and landscape plants is an indicator that other plants are also slowing chlorophyll production and preparing for winter dormancy. Many plants can take care of themselves, but some will benefit from fall care.

The most important things to take care of in the landscape in fall are plants with diseased or insect-infested foliage, tender perennials and shrubs and cool-season weeds that will only be bigger in the spring.

Disease-causing organisms and insect pests often overwinter in plant foliage and leaf litter. Look for plants that are obviously infested. A white dusting on leaves likely indicates powdery mildew, and brown spots on leaves could be may different fungal leaf spots. For trees, the leaf spots or mildew are less likely to affect the health of the plant. In shrubs and perennials, removal of infected leaves could benefit the plant.

For perennials such as peony and iris, simply cut the foliage back to the ground or near ground level as they begin to fade. For vegetable plantings such as tomatoes and peppers, remove entire plants after frost or final harvest.

For shrubs such as roses, imply remove the leaves at the end of the season.

Tender perennials such as cannas, elephant ears, and gladioli should be dug after freezing temperatures arrive. Clean soil from bulbs and store in newspaper or brown paper bags in a cool dry place.

Tender shrubs such as certain rose and hydrangea varieties can be mulched heavily after freezing temperatures arrive.

Cool-season weeds such as dandelions may go through dormancy and be bigger and stronger in spring like desirable perennials. Remove weeds while soil is moist to allow for better removal of the whole plant.

Fall is also a good opportunity to prepare new garden spaces and to incorporate compost/organic matter into existing garden spaces.

— Jennifer Smith is a former horticulture extension agent for K-State Research and Extension and horticulturist for Lawrence Parks and Recreation.