Storage by Stock Type

Bareroot stock is usually planted in the spring. Container trees can be planted during spring, summer, and fall. Container stock is a little more forgiving in its temperature requirements during storage than bareroot stock. Some agencies can run a container reforestation system with no field cooler storage facilities.

• Bareroot Stock
• Container Stock

Bareroot Stock for Spring Planting

Successful bareroot planting depends on lifting dormant trees from nursery beds and keeping them dormant until they are planted in the spring. The nurseries must wait until the trees are dormant in the fall before lifting, or must lift the trees as soon as possible in the spring before the trees break dormancy.

Bareroot Trees Lifted in the Fall

In the fall, trees are lifted when they have reached a level of dormancy that allows them to be stored frozen. The inside box temperature of the trees is maintained at 26 to 30 °F. The shoots of many conifers become cold hardy even though their roots do not. To avoid damaging roots during cold storage, never let the trees get colder than 26 °F. Usually coolers set at 28 °F will provide good frozen storage.

Fall-lifted trees may be shipped frozen to the districts as early as February or as late as May just before planting. The primary storage requirement for frozen trees is simply to keep the trees frozen at 26 to 30 °F. The trees are packaged in bags inside boxes to prevent freezer drying. No humidity adjustments are required. If you turn on your humidifier, the evaporators will ice up. Thawing trees before planting will be covered later. Maintain airflow between boxes of trees even when the trees are frozen.

Bareroot Trees Lifted During the Spring

Spring-lifted trees are shipped to districts a few weeks after lifting. They are not frozen in storage at the nursery and are delivered unfrozen to the districts. When these trees are lifted, they are beginning physiological activity. It is critical to keep the trees as cold as possible. Temperatures inside the tree box must be maintained between 32 and 36 °F. Temperatures closer to 32 °F are better for pine seedlings. These storage temperatures may result in some ice crystals in the box, but as long as trees are not frozen below 30 F°, they will not be damaged. If lodgepole and ponderosa pine are stored at 36 °F or higher for very long, they may begin respiring actively, heating the boxes. This heating will stimulate fungal growth and production of toxic gases, spoiling the trees. This is a classic problem with pines, but can occur in all species if the conditions are bad enough.

Long-term storage above freezing requires maintaining the humidity in the cooler at 95 percent. However, if the trees are well packed and the packages are undamaged, humidity should be adequate in the bags or boxes without using humidifiers, which put a heavy load on the cooling system.

Sometimes it is necessary to mix fall-lifted frozen trees and spring-lifted trees in the same cooler. Spring-lifted trees cannot be frozen, nor can they be subjected to higher cooler temperatures sometimes used to thaw frozen stock. If the frozen stock is not thawed before the spring-lifted trees arrive, the fall-lifted trees will have to be thawed outside the cooler. (top)

Container Stock

Storage requirements of container stock depends on the planting season. There are three seasons where container stock is planted:

• Spring Planting
• Summer Planting
• Fall Planting

Container Stock for Spring Planting

The procedures for spring-planted container stock are those that apply to container-grown stock that has been pulled and packed. Storage requirements are the same as for spring bareroot stock.

Stock that is kept in the original container can be stored in a cooler or outdoors with occasional watering, but do not let the rootballs freeze.

Container Stock for Summer Planting in June or July

Stock grown for summer planting should be planted after as little storage as possible. If trees are held more than 7 to 10 days, the initial growth benefit of summer planting may be reduced.

Trees planted during summer are not conditioned for long storage periods. These trees are very active physiologically and the roots are growing. They should have active white root tips when they are received. These trees are susceptible to damage when frozen, especially the white root tips. The trees can be stored in district coolers or at the planting sites.

District Coolers

Cooler temperatures should be set to maintain the temperatures inside tree boxes at 34 to 38 °F. Temperatures above 38 °F will stimulate respiration; mold and gas will build up inside the closed tree boxes. The trees can probably take temperatures below 35 °F, but it is prudent not to allow temperatures to fall too close to freezing because the new root tips are easily damaged by freezing. If the temperature in the cooler is properly monitored, the risk of freezing is low. The box, plastic baggies, and the root plug itself buffer rapid temperature changes. Humidity in the coolers should not be a problem because the trees will not be stored for long.

Planting Site Storage

Trees are delivered to the planting site and placed in sheltered areas in the shade. The boxes are opened so that the trees are exposed to the ambient air, and the trees are planted as promptly as possible. Trees in the open boxes can be watered if necessary. They should be checked every day for drying. Normally, watering should not be required for 3 or 4 days. Trees should be kept in a sheltered area where they are not exposed directly to the sun and wind. They must also be protected from nighttime freezing that may occur at high elevations even during midsummer. The new white root tips are easily damaged by freezing. Cover the trees with Space Blankets, shiny side down. Be alert for possible animal damage to the trees. Deer, elk, and moose have gotten into tree caches in the past. However, these caches were left unattended for longer than would generally be expected for summer planting. Animal damage is another good reason to cover the trees at night when you are leaving the planting area.

Container Stock for Fall Planting

Trees are generally stored just for a short period during fall planting, but the trees can be stored for longer than would be normal during the summer. The stock is best ordered for planting when it has completed most of its seasonal growth and is entering the fall hardening-off phase. Some root growth may occur if the stock is planted early enough. If the trees are planted late in the fall, they are essentially stored on site. Growth will resume during the next spring.

Trees planted in the fall can be stored for 10 to 20 days in coolers with little danger. Temperatures in the box must be kept between 31 and 34 °F to prevent mold from developing. Botrytis will develop rapidly in western larch when the trees are stored at temperatures above 34 °F for any length of time. The emphasis should be to plant the trees as soon as possible.