By Andrew Bunting, PHS VP of Horticulture
With an increase in extreme and changing climatic conditions, the need for tough and urban adaptable species in the landscape will become increasingly important. In particular, street trees will need to be able to withstand tough urban conditions impacted by reflected heat, pollutants, and soils that stay excessively wet or become extremely dry in periods of drought. Here are three types of trees that can thrive in even the toughest conditions.
Gymnocladus dioicus, the Kentucky coffee tree has become an increasingly valuable street tree, especially for urban environments. In its youth, this upright tree can have a somewhat gawky habit, but over time it develops into an upright, statuesque tree. If possible, it is desirable to get a male clone. Female trees will produce stout pods in the fall which can become messy. A good male introduction is Espresso™. At maturity, the Kentucky coffee tree can reach over 80 feet 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 is tolerant to a variety of soils including degraded urban soils, as well as soils with poor drainage.
The black tupelo, Nyssa sylvatica is one of the most versatile of the tough urban trees. It is native throughout the eastern U.S. In its native habitat, it is often found in creek bottoms, making it adaptable to growing in poorly drained soils. There are many excellent selections including ‘Wildfire,’ Red Rage® and Green Gable™ which was awarded the PHS Gold Medal Award distinction in 2023. The black gums are characterized by exceptional, fire-engine red fall color. The small black fruits, which are borne in the late summer to early fall, are a great food source for native songbirds. Most of the selections have a strong pyramidal habit, which makes it a great candidate for both a street tree and a shade tree.
Many types of oaks have been trialed and used over the years as urban trees with varying amounts of success. The willow oak, Quercus phellos is definitely in the “best of the best” category. The willow oak is native to many states in the South and, because of this, it has proven to be a very 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 is a popular choice for plazas where there is a need for a tough shade tree, but also a desire to have some sunlight which is facilitated by 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.
Quercus bicolor, the swamp white oak, has emerged as one of the best oaks to use for street trees in both city and suburban conditions. At the Scott Arboretum on the campus of Swarthmore College, there is a beautiful and majestic allée that was planted in 1881. Today, these upright oaks tower to nearly 100 feet 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 is hardy from zones 3-8, making it a great choice throughout most of the United States. Quercus bicolor, as well as Nyssa Sylvatica have been designated by the Pennsylvania Horticultural Society as Gold Medal Plants. American Dream® is a new cultivar that was selected for its vigor and resistance to anthracnose and powdery mildew. Beacon® is an upright selection that is perfect for tight spaces or can be used for creating formal avenues.
Quercus x macdanielii Heritage® is a hybrid between the English oak, Quercus robur, and a titan of the North American prairies, the bur oak, Quercus macrocarpa. Heritage® is a tough, stalwart urban tree that has been used extensively as a street tree. The English oak often suffers from powdery mildew, but this clone is resistant. At maturity, it will reach 60 - 80 feet tall with a spread of 40 - 50 feet. It is hardy to USDA zone 4.
There is no tree as iconic in the urban landscape as the London plane tree. Platanus x acerifolia, has been used extensively 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 is a hybrid between the native American sycamore, Platanus occidentalis and Platanus orientalis, which is native to southeastern Europe and Asian minor. The American sycamore is commonly found along streams and rivers, and this brings 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. The abundance of large leaves makes it one of the best shade trees. Exclamation!™ was selected by the Morton Arboretum and introduced by Chicagoland Grows®. It is very upright and pyramidal as a mature tree, reaching 60 feet tall. It is also a recipient of the Gold Medal Plant distinction by PHS.
The silver linden, Tilia tomentosa has multiple seasons of interest. This fast-growing shade or street tree has amazingly beautiful silver undersides to the leaf. Even with the gentlest breeze, the silvery leaves are exposed creating great ornament throughout the growing season. At maturity, it has a stately rounded canopy reaching 70 feet tall with a spread of 50 feet. It is hardy to USDA zones 4-8 making it adaptable well into the South, East, and Midwestern states. In May, it produces an abundance of yellow-white flowers that are highly attractive to a myriad of native bees and honeybees.
These trees are truly the “best of the best” for tough urban conditions. Most of them can grow in the toughest of soils and have proven they can thrive in culturally challenging environments.