Planting Acorns to Grow Oak Trees

I noticed we are having what looks to be a bumper crop of acorns here in central Pennsylvania. The red oaks are loaded and I even saw a white oak in my yard that appears to have a good acorn crop. I wanted to provide a few pointers about how to grow an oak from seed. It is easy and you can begin collecting acorns now, before the birds, squirrels, insects, and deer get them all. The number of acorns produced by both oak groups varies greatly from year to year. Scientists have been studying this for years but are unable to explain it.  More than likely it has to do with the weather conditions, nutrient availability, and acorn-feeding insects.

There are lots of reasons to grow your own oak seedlings. Oak is one species that has a difficult time regenerating in the forest. Not only will deer and other wildlife species eat the acorns but deer also browse heavily on the twigs, leaves, and buds year round as it is one of the most preferred species. Once an acorn germinates in the forest it spends a lot of time and energy growing roots rather than stems. This means it may sit in the understory for years, maybe even decades growing roots. This gives oak a great advantage when the forest is disturbed by logging or fire. But, if there are no well-established oak seedlings, called advanced oak regeneration, present in the forest understory when a disturbance happens then species like black birch and red maple are quick to take over the site. These species will quickly outgrow an oak that is just starting from an acorn. Enrichment plantings are used to re-introduce oak in a forest that was recently logged or disturbed in another way, such as gypsy moth defoliation.

White Oak Leaf

It is important to be able to tell the difference between red and white oaks,

Red Oak Leaf

which are the two broad categories of oaks. They can easily be distinguished by examining the leaves. The lobes, which are the projections along the edge of the leaves, of white oak leaves are rounded without bristle-tips; red oaks typically have bristle-tips on the lobes. Another important difference is that white oak acorns take just one year to develop on the tree and will germinate in the fall soon after hitting the ground. While red oak acorns take two years to develop on the tree and will not germinate until spring.

Guidelines for Successful Acorn Collection and Planting:

·         Time your acorn collection until the majority of acorns are falling. Ripening dates vary from year to year and from state to state by as much as three to four weeks, making it difficult to use actual dates to determine maturity. The acorn is perfect when green, plump, and the cap is easily removed.

·         Lawns, woods roads, field edges, or paved areas help in collecting acorns. Be sure to identify the species of tree and mark the bag or bucket so you know the species collected.

·         Collect two to three times as many acorns as the number of seedlings you want to plant.  This will allow you to remove bad ones and still ensure enough seedlings even with low germination rates.

·         Discard acorns that show any rot, mold, or small holes that may indicate insect damage.

·         It is critical that acorns are not allowed to dry out or heat up.  They can lose their ability to germinate very quickly.  Keep acorns shaded and spray with water to avoid moisture loss. If not planting them right away place them in polyethylene plastic bags with damp peat moss or sawdust and put them in the refrigerator. Do not freeze acorns.

·         After collecting the acorns drop them into a bucket of water. If the acorn floats it is no good, as this is an indication that the embryo has 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.

Seed Dormancy and Stratification

Because of differences in seed dormancy between red and white oaks, the process of storage and sowing differs. White oaks germinate in the fall, and red oaks germinate in the spring.

Red Oak Acorns

Red oak, Treetopics.com

Red oak acorns must go through a process known as stratification before they will germinate in the spring. Stratification breaks down the heavy seed coat allowing the acorn to sprout. Red oak acorns need about 4-8 weeks of cold stratification. When storing, place moist acorns in plastic bags (4 to 10 mil thickness), which can either be sealed or partially left open, and put in a refrigerator. Do not place in airtight bags as that can kill acorns. Keep the acorns moist by adding peat moss or sawdust. Every 2 to 3 weeks visually examine acorns for fungus or mold growth and dry by opening the bag, which will also release any gas buildup. Because of the risk of seed predation it is not recommended to sow red oak acorns outdoors until spring, March or April.

White Oak Group Acorns

White oak, Ecoaddendum.org

White oak acorns have no seed dormancy. As a result, white oak acorns can be seen on the ground in the fall with the root protruding from the seed. They can be planted immediately or stored and planted in the spring. If sowing in the spring, they need to be stored by placing them in refrigeration at 34–40°F in moist sand. Do not store white oak acorns for more than 3 or 4 months.

Planting Acorns

Both white oak and red oak acorns can be planted outside in a seedbed, in containers/pots, or in the forest protected in tree shelters. An outdoor seedbed will produce large numbers of seedlings at once. Prepare the seedbed as you would a garden. Acorns can be planted at a density of 5 acorns per square foot and about an inch deep with the acorn on its side. Once emerged, remove the suppressed seedlings to allow more room for the other seedlings to grow and develop. Be sure to water and remove grass and other weed competition as needed. Seedlings should be left in beds until the following spring when they can be dug and planted when dormant as bare root seedlings. 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. Multiple acorns can be placed in each pot. Again, plant acorns an inch deep and oriented lengthwise. Once germination occurs weed out the smaller weaker seedlings leaving one tree in each pot. Place pots off 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. Seedlings should be transplanted 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 can be planted directly in the forest but must be protected from small mammals and deer.  Plastic tree shelters or tubes are effective at protecting the acorn while allowing seedling growth. Lay an acorn on its side an inch deep in the forest soil where you intend to plant it. Place a tree shelter over the acorn and gently tap it down until it sits approximately and inch or two in 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 shelter 4-5 feet will be necessary to protect growing seedling.

Different species of oaks grow at different rates. Growth is dependent on a number of factors including soils, water, nutrient availability, and the amount of sunlight.  Once established it is not uncommon to see height growth of 1-2 feet per year or more. If you are growing oaks for wildlife and acorn production then planting them wide apart is preferred. A more open grown tree will begin to produce acorns at an earlier age. This can mean planting trees as much as 20-30 feet apart. For timber production plant trees closer together to force trees to self-prune lower limbs and grow straight and tall.

Reference:

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