Garden Variety: How bitter cold affects plants — and pests

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Whether plants can withstand cold temperatures depends on their hardiness and whether they were healthy going into the winter.

The bitter cold that accompanied recent winter storms in the Lawrence area will likely have long-lasting effects on certain trees and landscape plantings.

And while many gardeners think the silver lining of the cold is a decrease in insect and mite populations, the actual effect is difficult to predict because of these creatures’ adeptness at survival.

Before thinking too much about long-term effects on plants, keep in mind that the Lawrence area was fortunate to have storms that lacked ice or heavy winds. Either factor could cause significant tree damage, and the combination of the two is catastrophic. In such an event, trees would have suffered breakage that could have further interrupted power and damaged structures or other objects. This is a good reminder to have mature trees examined and pruned periodically by certified arborists to reduce damage in such events.

The health and cold-hardiness of plants are the factors that create issues when severe cold arrives.

Trees, shrubs and other plants that were under stress going into the winter could be pushed over the edge by cold temperatures. There are many stress factors that affect plants, including planting depth and techniques, soil type, root competition, mechanical damage (such as from mowers), insects and diseases and too much or too little moisture. This past fall and early winter were dry, causing plant roots and evergreens to desiccate and be more susceptible to the cold.

For hardiness, the USDA Agricultural Research Service has a map based on decades of weather data that separates the U.S. into zones based on periodic winter lows. All plants are categorized according to these USDA Hardiness Zones.

Lawrence is in Zone 6a. Plants that are identified as being hardy to Zone 6a can survive temperatures of minus 5 degrees to minus 10 degrees. Plants labeled as Zones 1a to 5b can withstand even colder temperatures; those labeled as Zones 6b to 13b cannot withstand these temperatures.

Native plants, natural areas and healthy landscape plantings that are known to be cold-hardy in the Lawrence area should withstand the cold without damage.

Some plants, like crape myrtle, Japanese maple, Southern magnolia, mimosa, hardy fig, hardy banana and others, are marginally hardy in Zone 6a. That means they are not hardy to the temperatures identified by the zone but will survive milder winters here. Some plants, like certain species of crape myrtle and hardy fig, die back to the ground most winters. The roots survive and the plants rejuvenate, so some gardeners choose to grow them anyway.

Hardiness is also related to plant health and is very site-dependent. Healthy plants can withstand lower temperatures than stressed ones. Protected locations and southern exposures with radiant heat can create microclimates that allow southern species better survival rates. Mulch insulates the soil, providing greater protection to plant roots. Species and cultivar selection are also very important, as some crape myrtles, Japanese maples, etc. are hardier than others.

Insects and mites

Insects and mites have various mechanisms to survive the winter. Japanese beetle grubs, for example, overwinter deep in the soil, and emerald ash borer larvae are deep in the wood of trees right now. Squash bugs and cucumber beetles keep warm in plant debris left on the garden, near the soil surface or in nearby pastures and wooded areas.

Protected locations, southern exposures, mulch and plant debris all allow insects better rates of survival. Snow also provides insulation for insects that overwinter on or near the soil surface. Although some insects die even in a mild winter, those that find good overwintering sites are likely to withstand even extreme cold such as what Lawrence experienced recently.

Ticks also bring up a lot of discussion with cold temperature extremes. Ticks’ overwintering habits vary with species, but they typically survive in leaf litter or by attaching to a host.

In studies, ticks have shown mortality rates as high as 70% to 80% from extreme cold temperatures. However, a 2012 study published in the Journal of Medical Entomology showed a survival rate of greater than 80% of blacklegged ticks when overwintering in a more natural setting than what most other studies provided. This study was conducted in two locations in upstate New York with temperatures reaching well below zero. Researchers attributed the high survival rate to the ticks’ ability to shelter in the soil surface. Snow cover may have also played a role.

— Jennifer Smith works in regulatory horticulture and has worked as a horticulturist for various government entities. She has experience in landscape design and maintenance and as an educator.


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