In northern climates, field grown nursery plants that remain unsold after harvest must be prepared to survive the winter if they are to be suitable for sale the following spring. The greatest threat to their survival is the vulnerability of their root systems to freezing that results from their being above ground. The same danger exists for containerized plants. Neither root system has the insulation protection that plants have while growing in the field.

To protect the root systems of container plants and b&b stock, nurs­ery growers use the following techniques:

• Heeling-in: The plants are set into bins or piles of loose material, such as wood shavings or saw dust, deep enough to cover the root ball or containers to a depth that affords the needed insulation.

• Over-wintering houses: The plants are placed into white polyethylene covered houses, made as animal proof as possible. There they are protected from temperature extremes and drying winds. On abnormally warm days, the houses must be vented so that the plants do not break dormancy. In high snow areas, the houses must be strong enough to support the predictable snow loads (Figure 21-16).

figure 21-16. A typical white polyethylene house for overwintering nursery plants (Delmar/Cengage Learning. Photo by Jack Ingels.)

• Bordering pots: Container plants are grouped as close together as possible, then surrounded by one or more rows of unplanted containers filled with soil. The border of pots serves as a buffer against the wind. Along with the close spacing, they help to insulate against the temperature extremes.

• Pot-in-pot production: Large, empty pots are buried in the ground up to their lips. Container plants, in slightly smaller pots, are set inside the larger ones. This technique affords the root systems the same protection as field grown plants (Figure 21-17). If kept in the field pots during the summer, the roots may suffer from excessive heat, so this is not suitable for year-round production in every region of the country.