Planting Acorns to Grow Oak Trees


10/10/14, University Park, PA -- Central Pennsylvania is having a really good acorn crop this year. If you want to grow oak seedlings, now is the time to collect seed before the birds, squirrels, insects, and deer get them all. No one really knows why oaks produce better in some years than others; it likely it has to do with the weather conditions, nutrient availability, and acorn-feeding insects.

Why grow your own oak seedlings? It is difficult to naturally regenerate oak because many wildlife species relish acorns, and deer, especially, preferentially browse year round on oak leaves, buds and twigs. When oak seedlings germinate, they often spend years growing roots rather than stems. This investment in developing roots can give oaks a distinct advantage over other species in forests disturbed by logging or fire. If well-established oak seedlings, called advanced oak regeneration, are lacking in the forest understory when disturbance happens, other species like black birch and red maple can quickly occupy the site. These species quickly outgrow oak seedlings just emerging from acorns. If you grow your own oak seedlings they can become enrichment planting stock to reintroduce the species into a forest recently logged or disturbed in another way, such as gypsy moth defoliation.

There are two oak groups: red and white. Using leaves it is easy to separate the groups. The lobes or projections along the leaf edge of white oaks are rounded without bristle-tips; red oaks typically have bristle-tips on the lobes. White oak acorns develop in one growing season and germinate in the fall soon after hitting the ground. Red oak acorns take two years to develop and do not germinate until spring after falling from the tree.

Guidelines for Successful Acorn Collection and Planting:

Collect acorns when the most of them are falling. Ripening dates vary by year and  location and can vary by as much as three to four weeks. This makes it difficult to set dates to determine maturity. Acorns are mature when green, plump, and the cap comes off easily. Acorns are easy to collect from lawns, woods roads, field edges, or paved areas. Take time to identify the source tree by species and mark the bag or bucket so you know what you collected. Collect two to three times as many acorns as the number of seedlings you want to plant. This allows you to remove bad ones and still ensure enough seedlings, even with low germination rates.

Discard acorns that show rot, mold, or small holes that may indicate insect damage. Use the float test to identify good acorns. To do this, after collecting drop them into a bucket of water. If the acorn floats, it is no good, as this is an indication that the embryo is not fully developed or is damaged and the seed is hollow. Soaking also provides moisture to any acorns that may have dried out some during collection. Do not permit the acorns to dry or heat -- they quickly lose their ability to germinate. Keep acorns shaded and spray with water to avoid moisture loss. If you cannot plant them right away, refrigerate them in polyethylene plastic bags with damp peat moss or sawdust. Do not freeze acorns.

Seed Dormancy and Stratification

Seed dormancy is different for red and white oaks; therefore, guidelines for storage and sowing differ. Again, red oaks germinate in the spring  and white oaks germinate in the fall.

Red Oak Acorns
Red oak acorns require stratification before they will germinate in the spring. Stratification breaks down the heavy seed coat to allow sprouting. Red oak acorns need about 4 to 8 weeks of cold stratification. To store red oak acorns, place moist acorns in plastic bags (4 to 10 mil thickness), which can be sealed or left partially open, and put in a refrigerator. Do not use airtight bags as that can kill acorns. Keep the acorns moist by adding peat moss or sawdust. Every 2 to 3 weeks examine acorns for fungus or mold growth and dry them by opening the bag, which will also release any gas buildup. Because of seed predation recommendations are to not sow red oak acorns outdoors until spring, March or April.

White Oak Group Acorns
White oak acorns have no seed dormancy. As a result, in the fall they are often seen on the ground with the root radicle protruding from the seed. Either plant white oak acorns immediately or store them for spring planting. . For spring planting, store them in moist sand refrigerated to 34-40 °F. Do not store white oak acorns for more than 3 or 4 months.

Planting Acorns

Both red and white oak acorns can be planted in seedbeds. An outdoor seedbed will produce large numbers of seedlings. Prepare the seedbed as you would a garden. Plant acorns on their side at a density of five per square foot and about an inch deep. When germinated, remove suppressed seedlings to allow room for the remaining seedlings to grow and develop. Water and remove grass and other weed competition as needed. Grow seedlings in seedbeds for one year before digging and outplanting before they start to grow the following spring. It may be necessary to place wire cages or fences over seedlings to protect them from deer browsing.

Acorns can also be planted in pots that are at least a foot deep (1 gallon size or deeper) to accommodate the tap root. Fill the container with a mixture of potting soil and top soil. Place single or multiple acorns in each pot. Again, plant acorns on their side an inch deep. Once germination occurs remove smaller weaker seedlings leaving one tree in each pot. Place pots off the ground in a sunny location and water as needed. By placing pots off the ground, roots that emerge from drainage holes will be air pruned. Transplant seedlings as soon as the first leaves open and become firm but before extensive root development occurs. Be sure to protect from deer browsing with wire cages or fences.

Acorns planted directly in the forest must be protected from small mammals and deer. Plastic tree shelters or tubes are effective protection. Lay an acorn in its side an inch deep in the forest where you intend to plant it. Place a tree shelter over the acorn and gently tap it down until it is an inch or two into the soil. Stake the shelter in place. If deer browse pressure is not a concern then short tubes (16-18 inches) are sufficient; however, if browsing is a concern a 4-5 foot shelter is necessary to protect growing seedling.

Oak species grow at different rates. Growth is dependent on several factors including soils, water, nutrient availability, and the amount of sunlight. Once established, it is not uncommon to achieve 1 to 2 feet of height growth annually. For wildlife and acorn production, plant seedlings widely apart (20-30 feet) as an open grown tree will begin producing acorns at an earlier age. For timber production plant trees closer together to force them to self-prune lower limbs and grow straight and tall.

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Reference:
Rousseau, R., A. B. Self, and D. Beliech. 2014. Growing Your Own Oak Seedlings, Mississippi State University Extension Service.

The Pennsylvania Forest Stewardship Program provides publications on a variety of topics related to woodland management. For a list of free publications, call 800 235 9473 (toll free), send an email to RNRext@psu.edu, or write to Forest Stewardship Program, Natural Resources Extension, The Pennsylvania State University, 416 Forest Resources Building, University Park, PA 16802. The Pennsylvania DCNR Bureau of Forestry and USDA Forest Service, in Partnership with Penn State's Department of Ecosystem Science and Management, sponsor the Forest Stewardship Program in Pennsylvania.