Urban and Community Trees: Swamp White Oak

One New Hampshire native tree species that I hadn't seen before my experience in 2014 (which involved trudging through the swamps of central Massachusetts in search of the Asian Longhorned beetle) was the swamp white oak (Quercus bicolor). My first look at the leaves made me think of white oak due to its rounded lobes. While questioning whether it was white oak or not due to its shallower sinuses in the leaf, deep fissures in the bark, and ability to thrive in standing water, my co-worker chimed in and informed me it was. Since my first encounter with the species, swamp white oak has been my favorite oak species due to its rarity in the natural environment and its ability to be used successfully in the urban landscape.

It's quite apparent why this species of oak has the common name swamp white oak. In the natural landscape, it is found growing in water-saturated areas with acidic soils. Therefore, it is able to tolerate soils with low oxygen levels, frequently found in the built environment where soils are heavily compacted. One would think this species requires saturated conditions to survive, but surprisingly it has been able to tolerate high temperatures and drought conditions.  

One concern for this species in the built environment is its low tolerance for alkaline soils. The dark green leaves can start to yellow when pH levels are high. Try to avoid planting this species near concrete or limestone structures that may cause pH levels to rise. If unable to do so, provide the soil with protection from surface water runoff by installing planters or mounding the soil.             

Swamp white oak is a relatively slow but large growing tree and should be planted in areas that provide adequate room for it to reach its mature size. In the open landscape, this species reaches a mature height and crown spread of about 50 to 60 feet. Although, when conditions are optimum, this species has obtained a height of 85 ft in New Hampshire. New Hampshire’s Big Tree Program registered that the swamp white oak state champion resides in Swanzey, and has a recorded height of 67 feet, an average crown spread of 75 ft, and a circumference of 192 inches. As you can see, it would be an unwise decision to plant this species underneath utility wires or in a small planting pit. When allowed to reach its mature size, the swamp white oak can provide significant environmental, social, and economic benefits to our communities. 

When provided with the proper space and soil volume, it can be used successfully as a street tree. However, it is essential to note that this species does not form a central leader naturally, and the branches tend to droop as the trunk and branches grow in size. Therefore, it is imperative to structurally prune the tree to a central leader when it is young and to prune the lower limbs to avoid violating line of sight requirements. Local ordinances regarding line of sight regulations may vary by community. Typically, the lower crown should be raised to a height of 8 feet along sidewalks and at least 14 feet adjacent to streets.

In the urban community, vehicles, buildings, and people occupy areas where trees occur and can become potential targets if the tree or tree parts were to fail. Therefore, not only is it important to plant the right tree in the right location to maximize environmental and social benefits, it is also crucial to plant trees appropriately to reduce the likelihood of failure or level of risk within the community. In New Hampshire, trees within the Quercus genus are proficient at compartmentalizing wounds caused by pruning, equipment damage, vandalism, and other harmful acts that frequently occur in the urban environment. Being able to compartmentalize wounds effectively allows this species to reduce the spread of decay. Not only is it effective at compartmentalizing wounds, it is also very strong. According to the wood database, it has a Jenka hardness rating of 1600 pounds-force. This rating means it required 1600 pounds-force to imbed half the diameter of a .444" diameter steel ball into the wood. This Jenka hardness rating of 1600 pounds-force is significantly higher than all of the other native oaks in New Hampshire. Due to this species’ strength and ability to compartmentalize wounds, I would expect this species to have a lower probability of mechanical failure when compared to a weak wooded species such as red maple. Genetically, trees within the Quercus genus tend to have a deep growing root system when environmental conditions allow. Therefore, this species has rarely been reported to cause significant conflicts regarding sidewalk damage that may eventually become a tripping hazard. 

Although swamp white oak is considered to be an outstanding tree for the urban environment, it is crucial not to plant a monoculture of any one species. Due to the destructive exotic pest known as the emerald ash borer, New Hampshire is currently experiencing the consequences of planting a monoculture of ash trees in the urban environment. Currently, there is no pest or disease in New Hampshire that is destructively killing oak trees. However, a tree-killing disease known as oak wilt has been found recently in several locations in New York. Fortunately, trees within the white oak group, such as swamp white oak, have shown more resistance to the disease when compared to the red oak group. For more information regarding oak wilt, please visit www.nhbugs.org.

Cory Keeffe is the Community Forester for the State of NH’s Division of Forests and Lands. His duties include providing technical assistance to communities to support the development and maintenance of healthy street trees and community forests. He received his Associate’s and Bachelor’s degree in Forestry from the University of New Hampshire. Cory is currently an International, New Hampshire, and Massachusetts certified arborist.