The acorn. Both a fruit and a seed. Produced by oak trees, they have a distinctive shape, a hard shell, and a scaly cap called a cupule. This nut-like fruit is critical for oak tree production and contains one or two large seeds inside this tough outer shell. They are food for many varieties of animals, including squirrels and birds, particularly in winter.
They take five to 24 months to mature depending on the specific species of oak. There are 38 acorn types, so the species of oak that will grow can be identified by the outer characteristics—its color, shape, type of cupule, and size—of the acorn.
The Cupule
The cup of the acorn is either scaly, smooth, or hairy. The cupule of native North American acorns have overlapping scales.
Shape
The typical shape of an acorn is round or oblong, and some have pointed tips while others have smooth-rounded tops.
Color
Mature acorns are either purplish-red, reddish-brown, light or dark brown, or black. Green acorns are immature.
Size
Acorns are typically one to six centimeters long. The small acorn nut is about 0.5 of an inch. The largest acorn from the burr oak can be 2-3 inches in diameter.
Acorns are edible and can be roasted and ground into flour. They can also be used to make a caffeine-free, coffee-like drink. Raw acorns contain bitter-tasting tannin. Tannin can be toxic to humans. The acorn should be leached to remove the tannin to improve taste, and safe for human consumption.
The sweetest acorns, and those containing the lowest levels of tannin are found in the pin, burr, east coast white, and cork oaks. The largest acorns are produced by the east coast white, burr, and California black oaks. The fattiest acorns, used to make acorn oil, are from the eastern red and black oaks. The smallest acorns come from the chinquapin and cherry bark oaks.
How to Plant an Acorn
Remember that acorns come in a variety and different acorns will produce different oak trees based on the criteria above. Collect your acorns for planting after they have fallen from the tree (avoiding green acorns). Wait until the second drop, as the acorns in the first drop often are of poor quality. Gather your acorns promptly as they dry out quickly and will be inviable.
All types of acorns should be planted in the fall. Gather twice as many acorns as the number of seedlings you want to grow. Not all will germinate. Discard the acorns if the cups are attached, are damaged in any way, or are moldy or rotted. Plant the seeds right away. If you can’t plant right away, spray them with water to keep them moist, but not wet, in a ventilated plastic bag, stored in a cool place.
Initially, soak the acorns in cold water. Viable acorns will sink and remain at the bottom of the bowl. Floating acorns are no good for planting and should be discarded. Plant the acorns in commercial potting mix based on peat moss. This is the safest way to start healthy seedlings. Use small pots, planting two acorns sideways in a pot. Water them until the water runs out of the drainage holes. Cover the pots with a screen or hardware cloth. Keep the seeds moist until the onset of winter, and keep it weed and pest-free.
Once the seedlings appear, cut off, do not pull out, the weaker of the two about two weeks after they emerge. Plant the stronger seedling in a two-quart nursery pot with large drain holes. The soil should be a half-and-half mixture of potting soil and garden soil, adding one teaspoon of slow-release fertilizer in the soil.
Once the seedling has developed a strong root system, it’s time to plant in its permanent location. Dig a hole three times the diameter of the container at the same depth and place the sapling in the hole and fill in. Water the sapling and spread a thick layer of mulch around the tree, leaving a two-inch space between the mulch and the tree trunk.
Sources: biologyinsights.com/are-acorn-trees-and-oak-trees-the-same; leafyplace.com/types-of-acorns; thespruce.com/plant-acorns-grow-oaks-5203188
Did You Know?
Acorns are also known as oaknuts. Only oak trees can produce acorns.
Leaching acorns to remove the tannin is accomplished by soaking them in water, draining, and repeating until the water runs clear.
Oak saplings are a favorite food for deer and other wildlife. Place a mesh tree guard around the tree for at least three years to protect it.