Basic Storage Principles

Storage of the seedlings can be broken into two main categories, unfrozen stock and frozen stock. Both have their own requirements that should be followed.

Unfrozen Stock

Unfrozen stock must be maintained below 35 °F, but not allowed to freeze. On occasion, we have frozen spring-lifted stock with no apparent damage. But some stock lifted in the spring has been severely damaged when frozen. The difference is due to the physiological condition of the stock at the time of freezing. Since you usually don't know what condition the stock is in, don't freeze the stock.

Stock kept cold after being lifted early in the spring should be dormant and relatively inactive physiologically. Stock lifted later in the spring, or stock exposed to higher temperatures during storage, may have begun breaking dormancy. Such stock has a high probability of being damaged if it is frozen. To keep the trees at 32 to 34 °F, the thermostat should be set as near to freezing as possible without freezing the trees. A slight crust of ice is permissible in the outer perimeter of packing media, but hard freezing can damage the stock, especially the roots.

Frozen Stock

Frozen stock is maintained at 26 to 28 °F. The stock should be thawed slowly over a period of 1 or 2 weeks. Thawing is fairly simple. Set the cooler at 36 to 40 °F, so the trees thaw slowly. When the stock reaches the desired 33 to 34 °F, reset the temperature controls to maintain 33 to 34 °F ambient temperature. ONCE THE STOCK THAWS, DO NOT REFREEZE IT. Most districts thaw their trees 10 to 14 days before beginning their planting program, or before the arrival of new freezer tree shipments, whichever comes first. The stock needs to be monitored carefully, because the thawing may vary by species or cooler location.

Rapid thaw, (thawing over 3 to 5 days) is fine provided the thawing temperature is not higher than 45 °F. Recool the storage unit to 34 °F after the stock has thawed. The advantage of rapid thawing is that the seedlings can remain in long-term storage longer, and the district does not have to wait as long before planting them.

If the cooler contains both frozen and thawed stock, the frozen stock should be removed by taking individual boxes or groups of boxes from the cooler. The stock should be taken to a relatively cool storage area and allowed to thaw slowly. Inside storage areas, such as sheds and warehouses, where temperatures generally range from 40 to 50 °F, are ideal. If outside areas are used, the trees should be covered with Space Blankets or tarps to protect them from low temperatures at night. Never allow the tree boxes to be exposed to direct sunlight. If temperatures inside the box rise above 35 °F for more than a few hours, molds may develop rapidly.