Urban and Community Trees: Black Gum

Planting for success in the urban environment is never an easy task. The national average life expectancy for street trees planted in tree pits, which some arborists refer to as planting tombs, is less than 12 years.  Urban trees face shortened life expectancy for several reasons:

• limited growing space
• compacted soils
• construction and mechanical damage
• pollution and soil contaminants
• excessive heat
• vandalism
• poor selection of nursery stock
• substandard planting and pruning techniques
• lack of water
• irregular maintenance practices. 

When trees are able and encouraged to grow to their maximum potential they can provide a tremendous amount of economic, environmental, and social benefits to our communities. An essential first step that allows urban trees to grow to their maximum potential is to select the right tree for the appropriate site. New Hampshire’s (NH) native Nyssa sylvatica, commonly referred to as black gum, is one example of a native species that can be successfully incorporated into the urban landscape.

Black gum, which happens to be the species of the oldest recorded tree in NH, is one of my favorite native species that I feel should be planted more often in the urban landscape. It is a deciduous hardwood tree, most commonly known for its early display of beautiful yellow, red and purple colored leaves in the fall. To view a good example of this tree’s fall display, visit NH’s Pisgah, Fox Forest, and Rhododendron state parks with populations growing throughout in the tree’s favored acidic wetland habitat.

Black gum grows successfully in hardiness zones 4 to 9, which makes it suitable for the majority of NH. It is considered a slow to medium growing tree, which may contribute to its longevity. The excurrent or pyramidal growth habit, similar to the form of Quercus palustris and several coniferous tree species, is a favored attribute for trees in the urban environment. To learn more about identifying black gum trees visit Virginia dendrology, or reference the Manual of Woody Landscapes by Michael Dirr, Peterson’s Eastern Trees Field Guide, or other tree ID guides.

All tree species have their pros and cons, especially in the built environment. Birds and squirrels favor the fruit produced by black gum, which can be beneficial for a community if objectives are to increase wildlife activity. However, one minor issue reported by pedestrians and municipal employees is the fruit litter and animal droppings produced underneath the tree. Consequently, it may be unwise to plant black gum in a densely populated area, such as next to heavily used sidewalks due to a potential slipping hazard. Another option is to plant cultivars such as Nyssa sylvatica ‘Firestarter’ or Nyssa sylvatica ‘Red Rage’ that do not produce fruit.

Under optimum conditions, black gum has the genetic ability to reach a height of 75 feet. However, due to the unnatural conditions in the urban landscape, it typically only reaches a height of 30 to 50 feet with a crown spread of 20 to 30 feet. It does not make sense to plant this particular species in areas with height restrictions, such as under utility wires. When possible, black gum should be planted in a site that provides adequate soil space for its root system. Black gum has performed well in wide tree lawns, medium and large parking lot islands, parks, and even as a street tree in suburban areas.

One of the most common issues trees face in the urban environment is compacted soils. Urban soils are frequently composed of clay, which is highly compacted during construction in order to meet engineers’ load-bearing specifications that support vehicles and pedestrians. When soils are compacted, their pores, which are responsible for drainage, are crushed. Therefore, it is difficult for trees species that require good drainage to survive in compacted soils. Since black gum naturally grows in an environment that is somewhat similar to these conditions in the urban environment, it is able to tolerate compacted soils better than most other species.

Another reason why black gum is an appropriate choice in the urban landscape is due to its habit or growth form. As previously mentioned, black gum’s excurrent form usually contains a centralized leader with branches growing at wide angles horizontally along the trunk that are usually considerably smaller than the trunk. This excurrent growth habit is rare for most hardwoods in our region. Many hardwood species in our region have a decurrent form, such as many of our oaks and maples, which develop several leaders nearly equal in size, referred to as co-dominant stems.

Research has proven that when a branch grows larger than half the diameter of the trunk, the connection at the union becomes weaker. The smaller the diameter of the branch is to the trunk, the stronger the union is at the junction. Species such as elms that have an upright growth habit develop leaders with narrow crotches that are much more likely to develop included bark between the union when compared to a branch with a wide angle of attachment to the trunk. Included bark between a co-dominant stem results in an extremely weak connection. Large leaders often tear off at the union and cause large wounds on the trunk, which may ultimately result in the removal of the tree. Decurrent trees require more pruning to improve structure, have higher maintenance costs, and a higher chance for litigations when compared to a species with an excurrent form such as black gum.  Due to black gum’s slow growth rate, mechanically strong architecture, and genetic ability to grow a deeper root system, this species makes an excellent choice for a wind tunnel.

Lastly, it has become apparent that our climate is changing.  In recent years, NH has experienced severe storms, droughts, and invasive pests more regularly. Although black gum often grows in swamps, it has been found growing well on dry sites and has shown the ability to tolerate droughts, which may be attributed to its genetic ability to grow a deep root system. Black gum has also shown tolerance to soil salt, which is frequently used as a de-icer in NH to keep pedestrians safe during the winter months. If you live in Concord, you have seen the damage the emerald ash borer (EAB) has done to the ash population. If you traveled through Worcester, Massachusetts, you have seen the damage that the Asian long horned beetle (ALB) has done to the maples. Fortunately, at this time there are no serious pests or diseases that attack black gum in NH. Nonetheless, that doesn’t mean there isn’t possibility for a detrimental pest or disease in the future for this species. Therefore, it is crucial to avoid planting a monoculture of any one species in the urban environment.