The challenges of growing trees in urban areas are many: pollution, cramped spaces, and degraded soils make it tough for urban trees to reach maturity. And now, the effects of climate change call for trees that can handle increasing heat, more severe weather events, and longer periods of drought than in the past.
The Pennsylvania Horticultural Society suggests three broad types of tough urban trees to try, depending on priority: native species, a variety of oaks, and selected shade trees. Within these categories are seven species suggestions listed below that can handle 21st century abuse.
“These trees are truly the best of the best for tough urban conditions,” according to Andrew Bunting, PHS’s VP of Horticulture. “Most of them can grow in the toughest of soils and have proven they can thrive in culturally challenging environments.”
PHS recommends a couple of tough street trees that are native to the East Coast.
- Kentucky coffee tree. Gymnocladus dioicus, the Kentucky coffee tree, has become an increasingly valuable street tree, especially for urban environments. Male clones are preferred, as female trees will produce stout pods in the fall which can become messy. At maturity, the Kentucky coffee tree can reach over 80′ tall. The leaflets are small, and therefore the canopy creates dappled shade. Gymnocladus is native to many states in the Midwest and eastern U.S. It’s tolerant to a variety of soils including degraded urban soils, as well as soils with poor drainage. Zones 3-8.
2. Black tupleo. Nyssa sylvatica—commonly known as tupelo, black tupelo, black gum, or sour gum—is one of the most versatile of the tough urban trees. It is native throughout the eastern U.S. In its native habitat, it’s often found in creek bottoms, so it’s adaptable to growing in poorly drained soils. Black gums are characterized by exceptional, fire-engine red Fall color. Most of the selections have a strong pyramidal habit, which makes it a great candidate for both a street tree and a shade tree. Zones 4-9.
3. Willow oak. Many types of oak trees have been trialed and used over the years as urban trees, but the willow oak, Quercus phellos is definitely in the “best of the best” category according to PHS. The willow oak is native to many states in the South and has proven to be an adaptable tree exhibiting good drought and heat tolerance. As the name would imply, it has narrow, willow-like leaves that create a beautiful round canopy and filtered shade. It’s a popular choice for plazas where there is a need for a tough shade tree, but also a desire to have some sunlight, which shines through the narrow leaves. In the Fall, the leaves turn a bright golden yellow. Quercus phellos is adaptable to a myriad of soils including wet soils, which is unusual for an oak. Zones 5 – 9.
4. Swamp white oak. Quercus bicolor has also emerged as one of the best oaks for street trees in both city and suburban conditions. These upright oaks tower to nearly 100′ tall. Like the willow oak, and as the common name would imply, it can grow in lowlands and swamp-like conditions, making it adaptable to poorly drained soils. It’s also hardy in a large range of Zones, making it a great choice throughout most of the U.S. Quercus bicolor, as well as Nyssa Sylvatica have been designated by the PHS as Gold Medal Plants. Zones 3 – 8.
5. Heritage® oak. Quercus x macdanielii Heritage® is a hybrid of the English oak, Quercus robur, and the native bur oak, Quercus macrocarpa. Heritage is a tough, stalwart urban tree that has been used extensively as a street tree. While the English oak often suffers from powdery mildew, this clone is resistant. At maturity, it will reach 60′ – 80′ tall with a spread of 40′ – 50′. Zones 4 – 8.
6. London planetree. Platanus x acerifolia is a long-time staple in parks and plazas and along stately boulevards like the Benjamin Franklin Parkway in Philadelphia. It could be argued that it is the “toughest” of all the shade trees. It’s a hybrid between the native American sycamore, Platanus occidentalis, and Platanus orientalis, native to Europe and Asia. The American sycamore has great adaptability to all types of soils, including those that are wet or poorly drained. Platanus x acerifolia is fast-growing and extremely durable. It recovers quickly from pruning and can withstand a lot of structural damage. It’s upright and pyramidal as a mature tree, reaching 60′ tall. Zones 5 -9.
7. Silver linden. Tilia tomentosa, has multiple seasons of interest. This fast-growing shade or street tree has beautiful silver undersides to the leaves. Even with the gentlest breeze, the silvery leaves create gorgeous ripples throughout the growing season. At maturity, it has a rounded canopy reaching 70′ tall with a spread of 50′. It’s also adaptable well into the South, East, and Midwest. Additionally, in May, it produces an abundance of yellow-white flowers that are highly attractive to native bees and honeybees. Zones 4 – 8.
Street trees need to withstand tough urban conditions that are only increasing in severity in some areas. Reflected heat, pollutants, and soils that stay excessively wet or become dry in periods of drought will all be challenging to tree health. These species should prove more resilient than most.
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