Garden Variety: Start of fall brings acorns
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Acorns are beginning to shed from oak trees with fall’s arrival in the Midwest. To some, the acorns are a welcome sign of cooler days to come and can even provide entertainment by attracting wildlife. To others, acorns are a nuisance as they fall on roofs, fill gutters, cover the lawn and create a slipping hazard on sidewalks and driveways.
Most arborists, landscapers and horticulturists will tell you that the good outweighs the bad with oaks and acorns, but proper placement and species selection make all the difference.
To understand the nuances between oaks, knowing a little about their biology is helpful. All oaks are grouped into two major families, collectively referred to as white oaks and red oaks. There are two major distinguishing differences: First, white oak acorns mature in a single season and red oak acorns take two years to mature. Second, the leaves of species within the white oak family have lobed, rounded edges. Leaves of species within the red oak family may be lobed or serrated and have distinctive bristles at the tip of each leaf and the point of each lobe or serration.
The white oak family includes the species white oak as well as bur, chinkapin, dwarf chinkapin, post and swamp white oaks that are native to eastern Kansas. The U.S. Forest Service lists 19 additional white oak species native to other regions of the U.S. There are also natural hybrids and introduced species, such as English oak, that are widely planted in the landscape.
The red oak family includes black, blackjack, northern red, pin, shingle and Shumard oaks that are native to eastern Kansas. The U.S. Forest Service lists 19 additional species in the red oak family that are native to other regions of the U.S. Sawtooth oak is an example of an introduced oak species that has become popular in the landscape industry in the last few decades.
Placement of any species of oak in a yard should be well thought out because of acorn production. Know the expected mature size of the tree and consider what the tree might hang over at maturity. Avoid planting where an oak will hang over gutters, sidewalks and driveways if possible.
If space is a factor, limit planting to oaks in the white oak group as they have larger acorns that are more quickly consumed by wildlife.
Acorn size is important because of the safety factor. Think about pin oak acorns that are rounded and about the size of marbles. Pin oaks are widely planted, so these acorns are the ones with which you are probably most familiar. In contrast, bur oak produces acorns that are 1-2 inches long, oblong in shape and mostly covered by a scaly cup that looks like a little hat over half of the shell. Larger acorns are more easily kicked or swept out of the way or picked up than small acorns, and they are harder to slip on because they are harder to step on.
White oak acorns — all of which are bigger than red oak acorns for the mentioned Kansas natives — are also the first choice of wildlife. Scientists believe this is due to the high tannin content in red oak acorns, which adversely affects taste and digestibility. When species in both families are in an area, white oak acorns are consumed before wildlife moves to red oak acorns in late winter or early spring.
Acorns provide food for squirrels, chipmunks, rabbits, quail, turkeys, wood ducks, deer, raccoons, opossums and other wildlife.
If acorns are problematic, use a leaf rake and/or leaf blower to gather and move them to alternate locations in the landscape. Then squirrels and other wildlife can continue to feed on them through the winter.
There are a couple of garden myths about acorns floating around. The first is that acorns acidify the soil. The claim probably stems from another misleading claim about oak tree leaves acidifying the soil. If either of these things were true, there would be fewer chlorotic pin oaks around. Pin oaks and other red oaks prefer slightly acidic soil and soils in this area tend to be alkaline (the opposite). The yellow or pale green tint you sometimes see in pin and red oaks is the result of alkaline soil limiting nutrient uptake.
The other myth about acorns is that they kill the lawn. In cases where acorns are so thick that they completely cover the grass, they could deplete light enough to kill it in the same way that laying a tarp on the grass would. But in cases where the grass is thin under the canopy of a tree, the culprit is more likely root competition. Pin and red oaks especially tend to have very dense root systems at or right below the soil surface, making it difficult for turf grasses to compete without supplemental watering.
— Jennifer Smith is a former horticulture extension agent for K-State Research and Extension and horticulturist for Lawrence Parks and Recreation.