AMES, Iowa – Fall is here, and so are acorns, falling from oak trees into yards everywhere. Viable acorns can be grown into oak trees, if properly handled. How is this done?
Here are some tips from Iowa State University Extension and Outreach horticulturists on how to best handle, germinate and plant acorns.
My oak tree produced just a few acorns this year. Why?
It’s common for the acorn crop on oak trees to vary from year to year. Most oak species produce a good crop of acorns once every two or three years. However, the white oak (Quercus alba) tends to produce a good acorn crop once every four to six years.
Weather and other factors can affect flowering and fruiting. For example, freezing temperatures in spring (when trees are flowering) can damage or destroy the flowers, drastically reducing the fruit crop.
The acorns of white oak, swamp white oak (Quercus bicolor), and bur oak (Quercus macrocarpa) mature in one year. Red oak (Quercus rubra) and pin oak (Quercus palustris) acorns mature in two years.
How do I germinate acorns?
Acorns should be collected as soon as they fall to the ground. Sound, viable acorns can be separated from damaged or unfilled acorns by placing them in water. Sound acorns sink. Most floating acorns are not viable and can be discarded.
The acorns of white oak and swamp white oak should be planted in fall. They will germinate immediately after sowing.
Acorns of bur oak , pin oak, and red oak will not germinate until they have been exposed to cool temperatures and moist conditions for several weeks. Winter weather in Iowa normally provides the necessary conditions to break dormancy. The cold-moist requirement can also be accomplished through a process called stratification. Acorns can be stratified by placing the seeds in a moist mixture of sand and peat moss and then storing them in a cool location.
Suitable containers include coffee cans, plastic buckets and food storage bags. The refrigerator is a good storage location. (Stratification temperatures should be 32 to 41 degrees Fahrenheit.) Acorns of the bur oak require a 30 to 60 day stratification period, while red and pin oak acorns require 30 to 45 days. Acorns of bur, pin and red oaks can be planted in fall or stratified seed can be sown in spring.
When planting acorns, place the seeds one-half to one inch deep. Choose a planting site where the oak seedlings can receive good care for one to two years before they are transplanted to their permanent locations.
To prevent squirrels and other animals from digging up and eating fall planted acorns, cover the area with chicken wire or hardware cloth fencing after planting. Promptly remove the fencing material in spring when the acorns begin to germinate.
There are small, round holes in many of the acorns on the ground. What made the holes?
The small, round holes on the sides of the acorns were likely caused by the larvae of the acorn weevil.
The adult acorn weevil is a brown beetle about three-eight inch in length and has a long, thin snout. Adult females lay their eggs inside developing acorns on trees in mid-summer. The eggs hatch into creamy white, grub-like larvae that feed inside the acorns until fall. In fall when the acorns have fallen to the ground, the fully grown grub chews a round one-eighth inch hole in the side of the acorn, exits the acorn and tunnels into the soil to complete its development.
Squirrels and other wildlife eat or stash away the good acorns, leaving the “holey” (destroyed) acorns on the ground.