Acorn Storage

Most seed managers do not like to store acorns over long periods, because their size requires large refrigerated storage space and because viability declines each year. Acorns of most red oaks can be stored for 3 years without critical losses in viability (Bonner 1973), while most white oaks can be stored only 6 months without complete loss of viability. One solution to the problem is to plant acorns in the fall immediately after collection and avoid storage. For some nurseries and conditions, this option is a good one for next years crop. Short-term storage under good conditions between collection and sowing is essential to maintain good acorn quality, however, and many managers would like to store extra acorns for use 1 or 2 years later. Since good storage practices for both purposes require the same facilities and procedures, the recommendations are the same. (Bonner, 1993)

Acorns of red oak species should be stored with their moisture contents at 30 percent or higher in temperatures near, but above, freezing (34?F to 40?F). Air-tight storage is lethal, so containers must allow some gas exchange with the atmosphere while maintaining high acorn moisture levels (Bonner 1973). Polyethylene bags with a wall thickness of 4 to 10 mils are good. For large quantities of acorns, storage can be in drums, cans, or boxes with polyethylene bag liners. Container tops and liners should not be completely closed; this will allow sufficient gas exchange. If water collects in the bottoms of storage containers, it should be drained from acorns intended for storage longer than over winter. (Bonner, 1993) (Table:Germination and Moisture Contents of Cherrybark Oak Acorns)

With proper care, many southern red oaks should maintain good viability for at least 3 years. We have had good success in our laboratory with water, cherrybark, and Nuttall oaks (Q. nuttallii Palmer), but less success with Shumard and willow (Q. phellos L.) oaks. Similar methods were used by Farmer (1975) for successful storage of northern red and scarlet (Q. coccinea Muenchh.) oaks, and by Suszka and Tylkowski (1982) for northern red oak in Poland. (Bonner, 1993) (Table: Viability and Retention of Various Southern White Oak Acorns)

With few exceptions, white oak acorns cannot be stored longer than over winter (4 to 6 months) without complete loss of viability. For over-winter storage, the same methods outlined for red oak storage should generally be used. Thinner polyethylene (1.75 mil) or cloth bags may be advantageous for white oaks because of a need for greater aeration (Rink and Williams 1984). Schroeder and Walker (1987) reported excellent results in storage of bur oak (Q. macrocarpa Michx.) for 6 months at 34?F and 44 percent acorn moisture in sealed plastic bags. No information was given on the thickness of the bags. Any reduction in acorn moisture significantly decreased germination capacity and rate. Laboratory testshave provided some rare successes with storing white oaks species. (Bonner, 1993).

Moisture content remains a crucial factor throughout storage. With acorn moisture levels above 30 percent and temperatures above freezing, respiration proceeds at a rapid rate. This process gradually decreases acorn dry weight, causing small increases in the percentage of moisture over time ((Table:Germination and Moisture Contents of Cherrybark Oak Acorns)). Schroeder and Walker (1987) found no increase in bur oak moisture content over 6 months of storage, but Gosling (1989) reported that English oak acorn moisture contents increased as much as 5 percent over 6 months in storage. The loss in dry weight is why a static state of equilibrium between internal acorn moisture and the storage atmosphere, such as we find in orthodox species, is never reached for acorns. Approximate equilibrium moisture contents have been determined for a few species, but these probably change over long storage periods. Note that white oak has much higher equilibrium levels than the two red oak species. This is because starch, the major storage food in white oak, is more hygroscopic than lipid, the major storage food in red oaks. (Bonner, 1993) (Table:Equliibrium Moisture Contents of Acorn for Three Southern Oaks)

Between 70 and 75 percent of total acorn moisture in water and Shumard oaks is in the cotyledons and embryonic axes, while the comparable total for white oak is only 58 percent (Bonner 1974b). White oak pericarps are thicker than those of red oaks, and they retain more moisture. Cup scar vascular openings are major conduits for moisture uptake (Bonner 1968). Recent experiments in our laboratory on acorn desiccation indicate that these openings comprise the key pathway for moisture loss also. As acorns dry, the proximal end of the cotyledons (just beneath the cup scar) loses moisture first. Unless the pericarp splits, as it does at radicle emergence, the embryonic axis and the cotyledon tissue surrounding it (distal end) is the last area to be desiccated. (Bonner, 1993)

One method tested successfully for storing Nuttall oak acorns solved the drying problem nicely. Johnson (1979) stored Nuttall acorns for 6 months in drums of water maintained at 34?F to 40?F. Similar results were obtained for water and cherrybark oak acorns in our laboratory for 5 months, but longer storage periods of 17 and 29 months led to considerable loss in viability. (Bonner, 1993)

Germination during storage has always been a problem for acorns, although not as great a problem as some might think. Southern white oaks have so little dormancy that they will germinate on the tree in extremely wet falls, so it is no wonder that they germinate profusely in storage. There seems to be an inverse relationship between degree of dormancy and germination in storage among red oak species. The conditions recommended for storage are the same ones normally prescribed for pretreatment (stratification) to overcome dormancy. Epicotyls usually do not appear, but radicles emerge and can grow as much as 8 in. in storage. (Bonner, 1993)

Microorganisms kill many radicle tips in storage, and many more are broken when sowing takes place. Secondary radicle development occurs in oaks, however, and other radicles should develop. These secondary radicles may even form sort of a multiple taproot system. Broken radicles did not adversely affect seedling production in nursery beds of cherrybark and Shumard oaks in Mississippi (Bonner 1982). Barden and Bowersox (1990) obtained similar results with northern red oak in Pennsylvania, but there were strong family differences. The common and proper response by seed managers is to do nothing to prevent this early germination. Decreasing acorn moisture slightly during storage of California black oak (Q. kelloggii) is reported to reduce the sprouting, yet not harm acorn quality (Tim Plumb, personal comm.). This approach should be studied for southern oaks. (Bonner, 1993)