As I’m writing this, on Tuesday (Sept. 11), rain is falling once again, flash-flood warnings are out and Hurricane Florence is heading our way with the promise of even more rain — lots of it. While some rain during the growing season is great for plants, too much can be disastrous.
After the drought last year, which started in late summer and lasted until this spring, native plants benefited from finally getting rain. Now they are facing threats from the continual deluges. According to the Weather Channel, by August 15 Richmond and Luray were having their rainiest year on record, and the rest of Virginia wasn’t far behind them. The impact of the heavy rains is not only important to plants and the animals that rely on them for food but also to human safety and the economic health of the commonwealth.
Around my yard, I’ve been noting leaves with spots, or that are drooping or curling, then turning brown and dropping off. In economic terms, the rain has been affecting field and orchard crops, but native plants are also getting hit by the onslaught, and this may affect another driver of Virginia’s economy — the brilliant fall foliage that draws visitors from around the world. For the most brilliant fall foliage color, trees need adequate moisture during the growing season, followed by an autumn that is dry, cool and sunny, with warm days and cool but frostless nights. Too much rain during the growing season (spring and summer), with few dry breaks, can severely damage leaves or limit the intensity of their color.
Going beyond the fall-foliage show, continual heavy rains can cause more important damage to plants. The rains can wash nutrients out of the soil, starving plants, and conditions in saturated soil can cause a host of problems.
Photosynthesis — the process in which chlorophylls in leaves convert sunlight into food for the plant —shuts down within five hours of the soil becoming saturated, according to the University of Florida’s Institute of Food and Agricultural Sciences (UF/IFAS.) Water fills in air spaces, lowering the levels of oxygen plants need to take up, and without oxygen, plants also stop taking up the water and nutrients that fuel growth and other processes. The aerobic bacteria that convert nitrogen into forms that are available to plants are replaced by anaerobic organisms, which do not perform this important function. When soil is saturated, its pH also rises, making manganese, iron and sulfur unavailable, and the rate of decomposition of organic material in the soil changes. All these conditions can damage or destroy plants, from trees to herbaceous wildflowers.
Another big threat from sustained damp weather during the warm season is the rise in fungal diseases that affect roots, including phytophthora root rot. The loss of root mass from decay and fungal attack leave the tree prone to damage from droughts that may follow. “After only two weeks of saturated soil conditions the root crown area can have so many problems that decline and even death are imminent,” according to UF/IFAS. Heavy winds accompanying some rain storms can deliver the coup de gras to trees, bringing them crashing down onto roads and structures, as we’ve experienced in the biggest rain storms this year.
Above ground, fungal diseases can damage leaves and interfere with photosynthesis. Among these diseases are powdery and downy mildew, leaf spot and gray mold (botrytis blight), which particularly affects flowering plants. Fungi can also cause cankers — dead sections of bark on branches or trunks of trees — providing a pathway for invading insects, such as the emerald ash borer, that can further interfere with the tree’s food conveyor belt, damaging or killing the tree.
In wet weather, foliar nematodes and bacteria thrive. The nematodes are microscopic worm-like organisms that can stunt new growth and cause small, discolored blotches on leaves. Fire blight, caused by bacteria, targets native and nonnative plants in the large rose family. It turns blossoms, leaves and branches of plants dark brown to black, giving them a scorched look.
Continual rain can also prevent pollinators from doing their work, leading to fewer flowers being pollinated and thus fewer fruits and seeds that develop from them. Heavy rains can also knock down flowers and fruit prematurely while fungi may attack what remains on the trees or falls to the ground. This means less food is available in late summer and fall for animals trying to layer on fat for migration, hibernation or just making it through winter, when food options are limited for many species.
Warmer, drier weather is forecast to come our way eventually this fall, but it may be too late to save some plants. We’ll know soon enough about the fall foliage show, which should peak in mid-October.
© 2018 Pam Owen
Why leaves change color in fall
Leaves are a plant’s food factory, and when the leaves of deciduous trees turn bright colors in autumn, they are signaling that the factory is shutting down for the year. Trees draw water up from the ground and mix it with carbon dioxide from the air in chlorophylls, which give leaves their green pigment. Sunlight triggers a chemical reaction, photosynthesis, that turns these ingredients into oxygen and carbohydrates (starch and sugar) needed for the tree’s growth and reproduction.
As winter approaches, light wanes and water becomes scarcer. Trees shut down their food factory in preparation, drawing water and carbohydrates into their roots, which enables trees to survive during the winter in a state of relative dormancy.
As chlorophylls shut down, they fade, revealing the bright yellow and orange pigments of carotenoids underneath that were obscured during the growing season. Carotenoids absorb light energy used in photosynthesis and protect chlorophyll from damage from sunlight. The red and purple pigments — anthocyanins — develop in late summer from the breakdown of sugar in bright light and from the reduction of phosphate. Traveling in sap that runs throughout the tree during the growing season, these pigments lend color to flowers to attract pollinators, and to fruits to attract animals that will disperse the plant’s seeds.
Mixtures of the various pigments can produce a variety of colors, even in the same tree. The hue and intensity of color depends not only on the species of tree and the differing amounts of pigment within it but also on temperature, sunlight and the amount of water available. Oaks, maples, sumacs and dogwoods are among the stars of the autumn leaf-turning show.