In most years, oak trees produce scant acorn crops and finding enough to use for decorative mulch would be difficult. In a "mast year," oaks produce unusually robust crops of acorns, enough to blanket the ground beneath the trees. The acorns could linger for weeks undisturbed or vanish overnight, depending on the variety's flavor. Nearly all become forage for birds and animals. As a landscaping mulch, acorns could cause problems.
Spreading acorns beneath trees and shrubs near the home compares well to spreading a layer of corn kernels as mulch. The high nutritional value of acorns, a rich source of carbohydrates and fats, make this nut a staple food of many types of wildlife. Large animals such as deer and black bear fatten up for winter on a heavy crop of acorns and both squirrels and acorn woodpeckers store the nuts for winter food. An acorn mulch attracts wildlife and could make the yard a memorable foraging area. Later in the winter, hungry wild animals shift their attention to bark and twigs, damaging fruit trees and ornamental shrubs.
Rats and mice also find acorns a tempting food source. Leaving large numbers of acorns on the ground attracts rodents. As temperatures drop, the mice and rats seek winter shelter and storage spaces. Buildings attract more rodents if the animals find plentiful food nearby. Keeping a yard clear of fallen fruit and acorns and covering garbage containers makes the area less hospitable. Large acorn crops increase both deer and mouse populations. Both animals host ticks carrying Lyme disease and cause higher rates of infection in human populations following heavy mast years.
Humans find acorns inedible without special processing, due to the high tannic acid content. Game animals and birds tolerate the bitter acids in the nuts, but feed first on the nuts with the least tannin and the best flavor. Deer prefer white oak acorns but won't graze exclusively on more bitter red oak acorns. White oak leaves show rounded lobes and produce large sweet acorns, while red oaks grow leaves with sharp lobe tips and yield smaller bitter acorns. The most bitter varieties could lie on the ground untouched until spring.
Any acorn with a damaged shell leaches tannic acid into the ground as the nut decays. Crushing or grinding acorns creates a mulch less attractive to wildlife but releases a heavy dose of tannins which could cause problems for some plants. A crushed acorn mulch could benefit other plants such as fruit trees and blueberries. Many fruit trees prefer acid soil with a pH between 5.5 and 6.5. Blueberries need even more acidic conditions and grow well in highly acid soil with a pH between 4.09 and 5.0, according to the U.S. Highbush Blueberry Council's website.