Through all of the nastiness of this winter, is there an upside for gardeners?
Unfortunately, not really, said Susan Littlefield, horticultural editor at the National Gardening Association in Williston, Vermont. Gardeners will get little help from the bitterly cold temperatures that have gripped much of the nation this winter in the two areas where she said they might expect it the most: a decrease in insect pests and plant diseases.
And here's why.
If you think a bitterly cold winter will knock back insect pests and reduce the damage they'll do to your spring and summer gardens, you'll be disappointed. That's unless the groundhog was wrong and we really don't have six more weeks of winter.
What impacts insect populations is not how cold it gets in the winter but when spring arrives, said Paul Guillebeau, a professor of entomology at the University of Georgia. "Insects survive the winter as eggs, pupae, larvae or, in some cases, as adults in tiny micro habits in leaf litter, the ground, bark on trees or even in your house," he explained. "When the temperature is at 40 degrees [Fahrenheit] or lower, they can't move. At 45 degrees, they begin moving, but only slowly. If the temperature gets to 70 degrees in mid-March or early April, insects get a fast start and quickly produce multiple generations that can quickly soar to hundreds of thousands. If, however, cold temperatures extend into April or even May, insects will miss one or more of their population cycles."
As far as weather goes, said Guillebeau, temperature is not as important to early insect populations as the amount of moisture. Very dry conditions are especially harmful to insects and will depress their populations more than cold temperatures. "Many different insects live in the soil, and they depend on soil moisture to survive," he said. "Additionally, a drought will reduce the amount of plant biomass available as food for herbivorous insects."
Too much water, on the other hand, can be a mixed bag. It will be beneficial to mosquito larvae that need the water to survive, but it will take a negative toll on fire ant populations. "Fire ants go underground when cold weather arrives," Guillebeau said. The colder it gets, the deeper the ants go to escape the cold. If there is a lot of snow or rain, though, and the water table is wet, the ants will head back up toward the ground's surface to escape the moisture. When they do this, they may well be killed by the cold. Should they befall this fate, the surviving ants will carry the dead ants out of the mound. If Southerners see dead ants around fire ant mounds after winter snows have thawed, this is what is happening, explained Guillebeau.
Plant pathogens also have a way of surviving bitter winter temperatures. Fungi and other plant pathogens tend to live inside perennial plant stems and buds and in decaying matter on the ground such as twigs and last year's leaves and are dormant this time of year, said a UGA colleague of Guillebeau's, Jean Williams-Woodward, an associate professor of plant pathology.
"Because the pathogens are inside the plant tissues, they are protected from winter freezes," she said. "Even pathogens on the outside of plants are dormant and are not affected by temperature extremes."
"We call this a disease triangle," she continued. The three sides to the triangle are a host, a pathogen and the environment. When dormant pathogens have a protective host, all they need is warmth of spring and the usual spring rains to make them active again.
Think of tomatoes, she said. If you plant tomatoes in the same place year after year and leave bits of stem or leaves in the ground, the disease spores also stay in the ground and will infect tomato seedlings you plant in the spring. The same is true with roses and black spot disease. The fungus that causes black spot can survive the winter in infected canes and leaves.
The best way to help reduce pathogens in vegetable and ornamental gardens is to get rid of last year's residue," she said. "Don't roto-till it into the ground!"
Of course, she advised, you have to be careful in trimming back some plants in the fall. Most azaleas and hydrangea cultivars, she pointed out, set next year's flowering buds right after they finish blooming. If you cut them back too severely in September and October, you'll be cutting off spring flowers.
Cold weather not only doesn't kill plant diseases, it can actually contribute to them, she added. Freezing temperatures, ice, and snow can split bark and cause branches to break off, both of which create open wounds that make plants vulnerable to diseases. Severely low winter temperatures can also kill cold-hardy weather vegetables and leave the plant tissues susceptible to diseases and infections.
Some good news
But all is not gloom and doom. Winter temperatures will help give plants that need cold weather to maximize flowering enough chilling hours to help their buds fully develop. For those brave enough to get outdoors and take a walk in the woods, a backdrop of snow can bring a new appreciation to foliage on evergreens such as pipsissewa, hollies, Christmas fern, and the common native orchid rattlesnake plantain. Of course, if you prefer to stay indoors, it's a good time to catch up on spring catalogs. And some, especially the native plant devotees, can always hope (if perhaps not realistically,) that the cold, snow and ice will kill off a few of the ubiquitous trees such as Bradford pears that are so often seen in mall parking lots, industrial parks and along city streets!