Proceedings of a Symposium on Oak Woodlands: Ecology, Management, and Urban Interface Issues
March 19-22, 1996, San Luis Obispo, California
USDA Forest Service
Pacific Southwest Research Station
Acorn Collection, Storage, Sorting, and Planting For the Establishment of Native Oaks Without Supplemental Irrigation
Although the most common method of planting oaks is from containerized seedlings, direct seeding of acorns into the landscape produces clearly superior trees. The root system develops naturally without the twisting, girdling, or spiraling that often occurs in traditional containerized seedlings. Most important, the taproot grows at its natural rate without any premature termination or damage. The taproot is able to penetrate deeply to the water source while also providing a firm anchor to the seedling and food storage to help ensure survival.
The taproot grows from the end (apical meristem), and once it is exposed to air or damaged, as is inevitable in traditional containers, it never regenerates (although multiple replacement roots usually form at the point of injury.) A seedling without a taproot will have less chance of survival without supplemental irrigation since it will never develop the root system nature intends for it to successfully adapt to the planting site.
Direct seeding of acorns is often discouraged because growers expect poor germination rates and a high loss of planted acorns to rodents. These problems are eliminated with careful selection and storage of acorns and the use of newly available low-cost tree shelters to protect the seed and growing seedling in the ground. The seed-handling method described below has been shown in numerous settings to produce germination rates >90 percent. When used with the appropriate tree shelters, a high rate of healthy vigorous saplings with strong natural root systems will result. While other planting strategies may be effective in some circumstances, results will be less predictable, and more follow-up and maintenance may be required.
Yearly weather patterns and geography affect when acorns are ripe for harvest. Acorns are ready when the caps are removed easily without damage to the acorns. Usually when acorns start dropping to the ground, most of the acorns remaining on the tree are ripe. Acorns may be picked directly from the tree when they are ripe. The freshest seeds are collected this way. Seeds may also be gathered from the ground. Choose the acorns that are green or dark brown. Light brown color usually indicates that the acorns have been on the ground longer and are more likely to have become dehydrated. Select the largest acorns, and avoid those with obvious cracks, holes, or damage from rodents or worms, and those that feel unusually light or hollow.
Collect acorns as close to the proposed planting site as possible, preferably within 25 miles, and within 500 feet of the planting site's elevation. Maintain a broad genetic base by choosing individual trees sufficiently separated to avoid collection from closely-related individuals. (Lippit 1991) Keep a record of the specific trees or groves from which the acorns were collected. Note the date and exact location and species of the trees. These notes, perhaps supplemented with photos made in the field, can help locate the trees in future years. Avoid purchasing seeds when the specific collection site is not known.
The harvest time can vary, plus or minus 1 to 4 weeks, from year to year, based on yearly weather changes. Oak trees in the wild can have unpredictable reproduction patterns, with some trees producing an acorn crop only once every 3 to 5 years and others producing only a handful of acorns. However, trees in parks or trees that have received supplemental watering can produce a crop each year. These trees often produce the largest acorns, with the most predictable crop.
Controlled studies have shown that larger acorns produce bigger seedlings faster. (Tecklin J. and McCreary D. 1990) A larger seed has more cotyledon to feed the radicle and rapidly growing root system. The seed is still supporting root growth long after the root has started to branch. Plants increase on the basis of their present size because of the geometric rate of cell division, and therefore a larger seed produces more cells faster. Larger acorns produce better seedlings both in the nursery setting and with direct seeding into the landscape, but using the largest available acorns is especially crucial in wildland planting where growing conditions will be less controlled. The early advantage of more stored food for the emerging seedling may be critical to early seedling survival.
Direct seeding from the tree into the landscape is the best planting method. But where this is not practical, because the planting site is remote from the source of acorns, or there is insufficient ground moisture to ensure successful germination at the time of harvesting, storage of the acorns is necessary. The primary goal of storage is to reduce the metabolic activity (i.e., keep the seed "dormant") and maintain the health and vigor of the acorn until planting time. Proper storage technique is essential to maintain metabolic inactivity. It is preferable to keep the radicle from emerging, even inside the shell, until the acorn is planted in the soil. If the radicle emerges during storage, the roots will continue to twist as the acorn is repositioned in storage and in planting. The acorn is perishable, and the other goals of storage are to prevent the acorns from drying out, becoming moldy, or freezing. Some oak species require a cold wet storage period (stratifying) to simulate winter conditions and allow germination.
Do not wash or soak acorns before storage, as the water and room temperature will start the germination process. Freshly harvested acorns should be stored at 33-41°F as soon as possible. A home refrigerator is adequate; however, the temperature will vary greatly within each appliance. Use a thermometer to check for the coldest spot. The temperature should not reach freezing. For larger quantities, commercial cold storage facilities are preferred, since the temperature will be maintained continuously within 1-2° of the ideal. Longer storage of 3 to 4 months can be successfully achieved this way, but temperature closer to 33°F is important for long-term storage.
The easiest way to store acorns is in 1-gallon zip-lock-type plastic bags. Fill them only half full with acorns. Add a handful of dry peat moss. Peat moss is slightly acidic, which inhibits bacterial growth, and it absorbs excess moisture given off by the acorns, which helps prevent mold growth. Do not seal the bags. Leave them completely open, and lay them on their sides to allow air circulation so the acorns do not become moldy.
Planting of the acorns should be scheduled once the ground has been saturated with substantial rain. Acorns will thus be stored for 1 to 2 months at most.
Once the radicle has emerged, the acorn is already past the optimum opportunity for successful planting, since the tap root, which grows from the end (apical meristem), may be damaged when exposed to air. If the acorn is to be planted at this point, the radicle must be kept moist and the acorn planted as soon as possible. However, if the tip of the radicle is discolored or damaged, the acorn should be discarded.
Seeds should be sorted after an initial storage period and immediately before planting. Seeds can be sorted for size by eye or mechanically, by weight or by screen.
No more than a few days before scheduled planting, remove the desired quantity from cold storage and place them in a plastic bucket filled with cold water. Soak the acorns for a few hours. The unhealthy seeds will float, and the solid seeds will sink to the bottom. Discard the "floaters." Drain the remaining healthy acorns, and dry them on newspaper about 1 hour at room temperature before replacing them in the bags. Place a handful of new peat moss in the bag with them, and store as described above, but this time for no more than a few days.
If the available crop of healthy acorns is inadequate, an alternative soaking method may rehydrate some "floaters" that would otherwise be considered inferior. After the soaking and separation described above, re-soak the "floaters", changing the water every 12 hours. Retrieve the acorns that sink, and continue soaking the remaining "floaters" until no more acorns sink. Drain and store any salvaged acorns as described above. Sometimes even seeds with obvious damage from insects or rodents can be salvaged, but it is important that the apex of the acorn (i.e., the end opposite from the cap) not be damaged.
Remove from storage only enough acorns for a day's planting. Maintain the acorns at a cold temperature at all possible times. Keep the acorns cool while transporting them to the planting site, for example, by using an ice chest. Never leave the acorns unrefrigerated for more than a few hours. Any acorns that are not planted that day should be refrigerated again until the next planting time.
Once autumn rain has fallen and the ground moisture is sufficient, time is of the essence. Sowing the acorns as early as possible is extremely important. (McCreary D. 1990) The tap root must penetrate to levels where moisture will be present the following summer. Plant only 1 acorn per hole, no more than 1 inch deep with 1 inch of soil covering the seed above ground level. Planting acorns too deep in soil with poor drainage may result in the newly-emerged radicle being flooded or deprived of oxygen and may make it difficult for the shoot to grow through the soil. The use of a low-cost tree shelter is recommended, for protection and enhanced growth.
If the first winter's rainfall totals have been below normal, partial top-pruning of the seedlings may be beneficial. (McCreary D and Tecklin J. 1993) This should be done before summer approaches to decrease transpiration of moisture through the leaves and conserve the limited available ground moisture.
Where feasible because the extent of the planting is limited and the site is accessible, supplemental watering will accelerate growth. However, if acorns are stored properly and the above procedures are followed, establishment of trees from acorns is extremely successful even without supplemental watering.
Lippitt L. Producing containerized seedlings. Paper presented at Intermountain Nurserymen's Association meeting, August 1991.
McCreary D. Acorn sowing date affects field performance. USDA Tree Planters Notes, Vol. 41-2 (1990), p. 6-9.
McCreary D and Tecklin J. Top pruning improves field performance of blue oak seedlings. USDA Tree Planters Notes, Vol. 44-2 (1993), p. 73-77.
Tecklin J. and McCreary D. Acorn size as a factor in early seedling growth of blue oaks. Paper presented at the Symposium on Oak Woodlands and Hardwood Rangeland Management, October 31-November 2, 1990, Davis, California.