This week we look at what some very durable choices for street trees. Oaks! There are some great choices that lend themselves to be used as street trees. There are also some Oaks you may wish to avoid, but if using appropriate cultural practices in pruning, you can minimize much of the risks.
Overall, addition of Oaks to be used in an urban setting will help increase diversity in the landscape. Several aspects to consider are the production of acorns, leaf litter, and the threat of Oak Wilt and Sudden Oak Death. The Oaks in the White Oak category (Quercus alba, Q. bicolor, Q. macrocarpa, Q. robur) and some related hybrids, tend to be more resistant to Oak Wilt than those in the Red Oak category (Quercus coccinea, Q. palustris, Q. rubra).
We will start with the White Oak group with cultivars. Quercus alba, White Oak is pyramidal when young and becomes upright with a round head and wide spreading at maturity.
Quercus bicolor, Swamp White Oak
is a good native tree that is easy to transplant and will grow in poorly drained soils as well as upland areas.
Quercus macrocarpa, Bur Oak
is a native oak that will form a broad crown. Most important, Bur Oak is adaptive to a broad range of soils from acid to alkaline.
Quercus robur, English Oak
and its cultivars can be good choices for lawns, parks or street trees. Quercus robur ‘Fastigiata’, Columnar English Oak
can be a good choice for a street tree but can show signs of powdery mildew in summer/fall.
Quercus x waryi ‘Kindred Spirit’™, Kindred Spirit Oak
is a cross between Q. robur ‘Fastigiata and Q. bicolor. The tree has a columnar growth habit and is resistant to powdery mildew. The narrow habit makes it suitable to be used as a street tree.
The Red Oak group includes Quercus rubra, Red Oak, which forms a large tree so requires room to spread.
Quercus coccinea, Scarlet Oak, is known for its deep red fall color and becomes a large tree.
Quercus palustris, Pin Oak, is a great tree but has the habit of lower branches growing low toward the ground which can create a maintenance problem for traffic and pedestrians.
Quercus palustris 'Pringreen" TM, Green Pillar Pin Oak is a fastigiate, upright grower so would be a better choice for a street tree.
Quercus imbricaria, Shingle Oak has willow-like foliage,
and Quercus muhlenbergii, Chinquapin Oak, has a yellow-brown fall color.
The Red Oaks as a group can succumb to Oak Wilt. This should not be the determining factor on whether to add them to your landscape. Caution needs to be used in pruning. Pruning only dormant trees greatly reduces the chance of infection.
Joseph OBrien, USDA Forest Service, Bugwood.org
The cultural practice that has shown highly effective is limiting the spread of the disease is to only trim oaks when dormant, Typically November 1st to April 1st. These oaks are still great trees to use in many park settings, urban landscapes and in some cases, as a street tree. Pruning and maintenance needs to be a consideration. For more information on Oak Wilt, see the link attached:
Plant some Oaks. Increase diversity. Happy New Year!