HO-203
$1.00
Effects of Cold Weather on Horticultural Plants in Indiana
COMMERCIAL HORTICULTURE • DEPARTMENT OF HORTICULTURE _
PURDUE UNIVERSITY COOPERATIVE EXTENSION SERVICE • WEST LAFAYETTE, IN
Larry A. Caplan
Vanderburgh County
Horticulture Agent

Introduction
Every fall, winter, and spring, many fruit and vegetable crops, as well as ornamental landscape plants, run the risk of injury caused by cold weather. Depending upon the plant species, damage can be caused by anything from a light, overnight frost to a prolonged period of freezing temperatures.
While cold damage is hard to predict, you can count on the fact that it will occur sooner or later. It can range from the loss of a few early blossoms in a low lying field to the complete loss of hundreds of acres over several counties; from barely visible leaf burn on early spring vegetables and flowers, to the death of above and below ground plant tissues.
Depending on the crop and location, some economic loss from cold injury can occur every year. Losses can result directly from damaged or killed plants, and indirectly from reduced quality or delayed maturation. The severe freeze in December 1983 killed many fruit and landscape plants. Of the plants which survived, many were seriously weakened, making them more susceptible to borers, cankers, and other problems over the next several years.
The objective of this publication is to identify the types of injury caused by cold weather, the factors that influence the degree of injury, and ways to prevent or reduce injury. This information should help in planning a defense against cold injury, and explain past failures in freeze prevention.

Climatology and Site Selection
There are a number of factors that play a part in determining the risk of frost and freezing injury to horticultural crops. The regional and local climates are important, as is the area’s topography and the conditions of the specific site. The actual hardiness of the plant also helps
determine the risk of injury; this will be covered later.
The climate of a region, sometimes called the macroclimate, is affected by land and water masses (i.e., "lake effect"), prevailing wind patterns, and the latitude. The macroclimate of the Midwest is typified by cold winters, with large and rapid swings in air temperature caused by alternating warm and cool air masses. Compare this to the Southern states, which have cool winters with occasional cold snaps. Crops and landscape plants that thrive under one set of climatic conditions may not perform reliably in another. This is the reasoning behind the different plant hardiness zones found on the USDA Plant Hardiness Map (see Figure 1).
Other regional factors, including terrain and elevation, can cause differences in the climate. The terrain in Indiana varies greatly, and includes hilly areas, broad flat plains, and both broad and narrow river valleys. Air temperatures, especially the daily minimums, can differ widely over these varying land forms. It is generally cooler at higher elevations than at neighboring lower locations; the growing season is also shorter.
On clear, windless nights, temperature inversions can cause cold air to pool in low areas, called "frost pockets." An inversion exists when the temperature is colder closer to the ground than it is higher up. As the sun sets, surface temperatures drop, and the air directly above the ground becomes cooler. Since cold air is heavier than warm air, it will form a layer above the ground. The cold air flows downhill and settles in valleys and low area, much like water. Often, the air in these frost pockets can be as much as 15°F cooler than that of the surrounding high ground. The Kankakee River Valley is an example of an area affected this way.
Another terrain effect that influences the climate is the presence of large bodies of water, such as the Great Lakes. Within several miles of the lake shore, the warming influence of Lake