Oaks in Minnesota
Oaks are among the best deciduous trees for our climate. Different types of oaks can be found throughout Minnesota and most of them are native to our soils and climate. One of the keys to success with oaks in the landscape is to match the variety of oak to the specific soil type and other limitations of the site. Just because they are all hardy doesn't mean they are all interchangeable.
While oaks do have their problems, they are also plagued by misconceptions. Oak wilt is a problem; some varieties are much more susceptible than others. A diagnosis of oak wilt isn't an absolute death sentence since there is some very promising research being done in our area. It might also be another condition misdiagnosed as oak wilt. One common misconception about oaks is that all oaks are slow growing. Sited correctly, some oaks are actually classified as fast-growing. And not all varieties of oaks hold their leaves into the winter, a trait some homeowners find undesirable. Once you have done an evaluation of the site where you would like to plant an oak, the next step is to learn what each species prefers. Here is a listing of many varieties currently sold for landscape use in our area.
Varieties of Oaks
Bur Oak Quercus macrocarpa Bur oaks are large trees native tree to almost every corner of Minnesota. They have a broad, rounded crown and are often described as rugged in appearance. Bur oaks are valued for their winter silhouette. One of the largest native trees, bur oaks grow 60-80 feet tall and wide, making them suitable for large spaces only. A member of the white oak group of oaks, their leaves have rounded lobes and are typically 6-8 inches long. Fall leaf color ranges from yellow to dull brown. The dark green leaves are shiny on the surface and dusty-gray underneath. The acorns on bur oaks average about an inch long with a fringed cap, earning them an alternate common name of Mossycup Oak. Bur oaks are hardy throughout Zones 3 and 4. While they may be slow to become established and begin growing after transplant, once they have been in a few years they are considered average in growth rate. Their gray-brown bark develops attractive deep furrows as the plant matures. Bur oaks are tolerant of a wide range of soil types including sandy, clay, moist, slightly acid, neutral and slightly alkaline. They prefer full sun to very light shade. Bur oaks have a sensitive root system and will not tolerate much compaction or change of grade. They are fairly resistant to oak wilt and if they are infected, the disease progresses very slowly.
Pin Oak Quercus palustris A very popular variety of oak, this type of pin oak is also called Eastern pin oak and is valued for its attractive pyramidal shape. Classified as a red oak, pin oak leaves are 4-6 inches long, deeply lobed and the lobes are sharply pointed. Leaf color in summer is a dark, glossy green on the surface and a lighter green underneath. Fall leaf color is bronze-red. This type of pin oak will grow 50-70 feet tall and 40 feet wide, making it more suitable to average landscapes than some other types of oaks. It is only hardy to Zone 4 and is considered one of the fastest growing oaks. It has average size acorns and holds its acorns and leaves over winter. Eastern pin oaks have specific site requirements. They will not tolerate heavily compacted or alkaline soils. Pin oaks need full sun and the ideal soil conditions would be moist, well-drained, deep acid soil. When they are planted in unsuitable conditions, like much of our urban alkaline soils, they suffer dramatically from chlorosis. Research is currently being conducted to determine if anything can be done to alleviate the chlorosis long term. Eastern pin oaks are moderately susceptible to oak wilt.
Northern Pin Oak Quercus ellipsoidalis This variety is a much more suitable pin oak for our area. It is native to most of Minnesota including some large native stands just to the northwest of the metropolitan area. Although it looks very much like Eastern pin oak at first glance, Northern pin oak is distinctively different. Its leaves are not as deeply divided as its Eastern cousin. The leaves are a glossy dark green in season and turn a very attractive red in fall. Northern pin oaks aren't quite as pyramidal as Eastern and grow to about the same height. The most significant difference between the two is that Northern pin oak will tolerate a much wider range of soil pH and heavier soils. They have a moderate growth rate and are susceptible to oak wilt. Hardy to Zone 3b. In both types of pin oak, the bark is grayish brown developing shallow ridges as they age.
Red Oak Quercus rubra Also known as Northern red oak, this native oak is a fast-growing shade tree. As a young tree, it has a very pyramidal shape. As it matures, the habit becomes more rounded and symmetrical. Red oaks grow 60-80 feet tall and 40-50 feet wide. They are hardy to Zone 3b. As with all red oaks, the leaves are deeply lobed and the lobes are pointed. Leaves are 6-8 inches long, dark green on the surface and gray on the underside. Red oaks have variable fall color ranging from russet red to yellow. Red oak leaves are persistent into the winter. Under ideal conditions, red oaks have been known to grow as fast as almost any other shade tree. They prefer sandy loam soils that are well-drained and slightly acid. Plant them in the full sun or very light shade. Acorns are less than an inch on average, maturing and falling very early in the season. The bark of red oaks is dark gray when the tree is young and darkens almost to black with furrows as it matures. Red oaks are susceptible to oak wilt.
White Oak Quercus alba The White oak is another highly desirable native shade tree. As a young tree it is fairly upright but as it matures it develops a wide, spreading crown 50-60 feet tall and wide. The growth rate is moderate and they are hardy throughout Zone 4 and possibly into Zone 3b. White oaks have large 4-8 inch leaves with deep lobes that are rounded. The leaves are dark bluish green on the surface and very pale on the underside. In fall, white oak color varies from a rich brown to red. Color and leaves hold for a long time in fall. Bark is fairly smooth when the plant is young and develops scales as it ages. Acorns are about an inch long and oval in shape. White oaks should be sited in full sun. They are tolerant of most soils but ideally would prefer deep, moist, well-drained slightly acidic conditions. They don't seem to develop the chlorosis suffered by pin oaks in alkaline soils. Like bur oaks, they don't like the soil surrounding them to be disturbed. White oaks are resistant to oak wilt.
Swamp White Oak Quercus bicolor A native tree with a very unfortunate name, swamp white oaks are well suited to a wide range of conditions and make beautiful shade trees. They have a narrow, pyramidal habit when they are young and a round, broad crown at maturity. Swamp white oaks mature at about 45-50 feet tall and wide. Leaves have rounded lobes like other white oaks but they aren't very deeply dissected. During the growing season, the leaves have almost a two-tone effect, dark green on the surface and almost white and velvety underneath. The bark is light gray when the trees are young and develops large, flaky scales as they age. Their mature branching habit is very crooked and coarse, making it distinctively attractive in winter. While they are often found in moist soil conditions, they also do well in drier areas and once established, are considered very drought tolerant. They prefer neutral to slightly acid soils but don't seem to suffer if it is a little alkaline. Swamp white oaks are resistant to oak wilt.