HO-157
YARD AND GARDEN
HORTICULTURE DEPARTMENT • COOPERATIVE EXTENSION SERVICE ' PURDUE UNIVERSITY • WEST LAFAYETTE, INDIANA 47907
OVERWINTERING OF NURSERY PLANTS
by Mark A. Hawley, Landscape Horticulture Technician, and David F. Hamilton, Assistant Professor of Horticulture, Purdue University
Introduction
Proper winter storage of container-grown ornamentals is as important to the success of their production as watering, fertilizing, and pruning. All efforts and expenses to properly grow and train plants are wasted if plants die from having been improperly stored during the winter months. The roots of ornamental plants growing in containers are more susceptible to injury from cold than similar plants growing in the nursery row or in the landscape.

The Importance of Root Hardiness
Although winter storage of container-grown plants is costly, loss of plants from low winter temperatures can be more costly than providing them with adequate winter protection. Plants which have suffered extensive root damage from cold generally die in early spring shortly after growth starts. Plants which have suffered limited root damage frequently exhibit dieback of the branches and are slow to initiate new growth in the spring. However, plants which have overwintered without root damage initiate new growth as soon as conditions are favorable. The differences in top growth between properly stored plants and improperly overwintered plants appear to be in proportion to the number of roots that have been injured. In other words, as the number of injured roots increase, there is a decrease in the top growth produced during the following season.

Considerations
There is an immense range of diversity in the cold hardiness of landscape plants. Plant hardiness zone maps serve as a basis for the selection of landscape plants for a given locality based on temperature minimum. However, these hardiness ratings are based on the low-temperature tolerance of the shoots and disregard the effect temperatures have on roots. The criticalness of this effect on roots is shown by the fact that 40-45% of landscape plants are grown in containers with root systems aboveground.
Hardiness of shoots relative to roots within a single plant can differ by as much as 28°C (50° F). Pyracantha coccinea ‘Lalandei’ stems can survive- 15°C (5°F) when fully acclimated. However, mature roots are generally killed at -17°C (2°F), and young roots cannot survive temperatures less than -4° C (22° F). This trait is typical of many broad- and narrow-leaved evergreen plants, but root hardiness of only a few species has been determined (Table 1). Root hardiness has not been examined to any extent in deciduous plants. Hardiness of young roots varies between -12° and 4°C (10-25°F), while hardiness of mature roots varies between -23° and +5°C (-10 and +23° F). This difference in hardiness does not pose a problem to the winter survival of field-grown stock, but it is a major concern for winter survival of container-grown stock since subfreezing root-zone temperatures may kill the roots and cause "winter burning" of leaves and stems. Winter burn is especially apparent following large fluctuations between night and day temperatures.