Tree Storage Coolers

• Maintaining and Monitoring Tree Coolers
• Temperature
• Humidity
• Hygrothermographs

Any long-term storage of tree seedlings requires refrigerated storage units that can maintain the trees at temperatures of 33 to 34 °F with relative humidities of 95 percent or higher. This normally requires cooler units specially designed for tree storage. The original tree storage coolers were made and sold by Bally Refrigeration Company. Several other companies now market similar units. These new units can also be used to store frozen trees.

The basic design of these coolers is fairly straightforward. The system has several evaporators with a hot gas bypass defrost. The evaporators are on different defrost cycles. When one evaporator is defrosting, its fan is off, and the hot gas melts the ice off the coils quickly. Meanwhile, another evaporator is operating, maintaining the temperature in the cooler.

Ordinary coolers are not acceptable for long-term storage of tree stock because of long defrost cycles. Refrigerated trucks, grocery meat coolers, or beer coolers can be used only for short-term storage. When such facilities are used, trees should be left at nursery until right before planting starts so they will be stored no more than 7 to 10 days.

Some older refrigeration units can be fixed to meet storage requirements by adjusting the defrost system (hot gas defrost) and providing humidity by various means. A few older units in the Northern Region have performed well after being modified in this manner. A competent refrigeration firm should make modifications.

Maintaining and Monitoring Tree Coolers

Refrigerated tree coolers represent a substantial investment. Proper maintenance is necessary to maintain the units' designed capability. Maintenance should be performed by qualified service representatives recommended by the manufacturer. Following manufacturer's recommendations for operation, startup, and shutdown is very important.

Most coolers have various types of sophisticated electrical monitoring devices. Warnings include noises, telephone calls, and flashing lights. Despite these devices, cooler failures have gone undetected for days. In such cases, trees have sometimes been damaged by excessive heat and sometimes been damaged by freezing. Check the coolers and the monitoring equipment every day. Assign someone to check the coolers in the morning when they arrive at work and in the afternoon when they leave. (top)

Temperature

The ambient temperature surrounding the packages of trees is important, but seedling temperature is critical. The most economical instrument for measuring seedling temperature is the Reotemp thermometer. It has a needle in a circular dial attached to a metal probe or stem. Use a thermometer with a pointed end, a 12-inch stem and a temperature range of 25 to 125 °F. The thermometer can be poked into the box to read the temperature inside. Digital thermometers are also available that can be used in the same fashion.

The thermomoter should be calibrated using a container of water packed with ice. The thermometer should read 32 °F. Most districts with large planting programs insert several dial thermometers in boxes at different locations in the cooler. In addition, a digital thermometer is used to check additional boxes and to check the cooler's ambient temperature or the temperature of trees as they come off the trucks. It is advisable to have several dial-type thermometers on hand as they do break occasionally. Some digital thermometers operate only within a certain temperature range and should not be left in the coolers. If the digital readout unit gets too cold, it will give false readings. Thermometers can always be checked with a container of ice and water.