The Crape Myrtle Debate: Is It Ever OK to Plant Non-Native Trees?
In a blog we previously published regarding the “Natchez” Crape Myrtle, we received some comments wondering why anyone would opt to plant a non-native tree. It’s a legitimate concern, especially when one realizes the threat invasive species pose to local ecosystems. But is the delineation between native and non-native that cut and dry when it comes to choosing which trees to plant, or is there more to this story?
There’s a lot to consider when deciding on a tree to plant, including the choice between a native species and something more exotic. We asked ISA Board Certified Master Arborist and NTCC board member Bo Arrington for his perspective on when exotic trees should be avoided and when they can be responsibly incorporated into Nashville’s landscape.
What Is a Native Tree?
Arrington points out the difficulty of defining what a native tree is in relation to a given location. The first question to ask is, “Native to where?” This could have several answers, including the state, region, ecosystem, hardiness zone, or continent. “Here in Nashville, we live in a basin, and if you were to drive 30 minutes in any direction, you would be in a completely different ecosystem. Everything changes: species, soil, climate. Furthermore, our state is divided into three distinct geographic regions with the east being mountainous, the middle being rolling hills, and the west being made of lowlands.”
He highlights that urban and neighborhood environments have different conditions than rural or wilderness locations, explaining that each property is, in essence, its own little ecosystem. His advice is that people consider the characteristics of their property when choosing trees and shrubs, since not all trees that grow in Tennessee’s wild will do well in a residential or garden setting.
For example, red spruces grow in the Smoky Mountains in some impressive old growth stands at higher elevations, but they won’t grow well in Nashville. Eastern hemlock grows an hour east of the city, but it does not do very well in Nashville.
Location Matters
Arrington continues, “Many of our ‘native’ species do not tolerate our urban environments because, let's face it, there is nothing native or natural about concrete and steel, so we need to get creative.” In many cases, exotics are planted because they can survive the conditions of urban environments, including the heat island effect, nutrient-depleted and compacted soils, disease, and insect pressures, among other stressors. Soil structure and pH, water, and light should be considered first and foremost when choosing a tree.
What Is an Invasive Tree?
There are a few reasons why some plants are more invasive than others. Certain plants are just extremely prolific; these are usually first successional (or “pioneer”) species and heavy seeders. Birds and squirrels will often feast on their fruits and then deposit their seeds elsewhere. Many of these species are fast growers, quickly taking up space and resources that can crowd out other types of plants and trees.
Invasiveness can also vary by region. Some species are highly invasive in some areas of the country and not in others due to environmental conditions. For example, the Norway maple is considered highly invasive in the northeast, but not in Tennessee because of our climate.
Arrington notes that native plants can also become invasive, and for this reason, “...perhaps the behavior of the plant is more important than the provenance.” Here in Nashville, hackberry dominates our canopy. It is the most common tree in our city at 10% of the canopy, and it’s often considered a nuisance tree because it grows everywhere, gets infested with the invasive (and messy) Asian hackberry woolly aphid, and is not the most structurally sound tree.
Our second most common tree is the white ash, which is also currently under attack by another invasive insect: the emerald ash borer. Unless the trees are chemically treated, they will die. With that being said, disease and insect resistance are other key reasons that we turn to exotics for biodiversity in the landscape.
To give an idea of the outsized impact of invasive species, Arrington shares a statistic: “Something like 99% of our exotic species are not invasive. However, the one percent that is [invasive] causes a significant amount of damage. One report indicates that the economic cost of invasive species to Americans is an estimated $137 billion every year (Pimentel et al. 2000).” This study includes aquatic organisms, fungi, insects, plants, and trees.
He continues, “I strongly believe that certain habitats need to be protected, especially unique habitats like our cedar glades in Middle Tennessee. However, this can also come at a cost to our ecosystems because the preferred method of invasive control is herbicide, and that is a different can of worms.”
For property owners in Middle Tennessee, the major invasives that should be removed are the Bradford pear, tree of heaven, Chinese and European privet, Japanese honeysuckle, and winter creeper. There are plenty more invasives in the Nashville area, but these are the most common (the Tennessee Invasive Plant Council has a more exhaustive list, as does the State of Tennessee).
Planting Non-Invasive Exotic Trees in Nashville
A rule of thumb in municipality landscape design is not to plant too many of the same species (no more than 10%), genus (no more than 20%), or family (no more than 30%). This is one area where non-invasive exotics can actually be extremely beneficial.
As an ISA Board Certified Master Arborist, Arrington gives his ultimate opinion: “I have no issues at all with exotic, non-invasive species. We should emphasize and incorporate native trees and shrubs in our landscapes, but I certainly don't think we should be limited to them. I love trees no matter where they are from, as long as they are able to grow well in that particular site. Exotic species can be extremely beautiful specimen trees, and I think that the way humans have augmented nature by going into the forests and bringing these plants into our gardens has been a wonderful way for us to appreciate and coexist with nature in a more meaningful way.”
Some non-invasive exotic (non-North American) trees that may thrive in a Nashville yard include:
Japanese maple
Ginkgo
When it comes to planting crape myrtle trees, however, Arrington does note that annual pesticide treatments for the common tree disease Crape myrtle bark scale are often necessary.
Some good native (southeastern American) tree options for Nashville in particular include:
Black walnut
Eastern redbud
Flowering dogwood
American holly
Choosing a Tree to Plant
Survivability despite the stressors of the built environment, natural pressures of a particular location (such as soil type, weather, and local disease and pests), and the need for biodiversity in the environment all factor into the choice of a tree for any given spot. The purpose and behavior of the tree are also very important aspects of that decision.
So, sometimes, a non-invasive exotic fits the bill for an interesting ornamental or hardy addition to a yard. Other times, a native tree might be a better choice if it’s being planted for native wildlife habitats or local tree biodiversity. In either case, the right tree should be a good fit for the place it’s being planted. Trees can be expensive to maintain, so picking a good one from the start can save a lot of time and money in the long run.
Embrace Non-Natives for Your Yard
Browse the Nashville Tree Conservation Corps’ tree sale for a wide range of native and non-invasive exotic species! You can also donate a tree to be planted or volunteer with us to help get those trees in the ground and maintain them. Another way to support NTCC’s activities is with a financial donation, which is welcomed and essential to funding our mission! Stay updated with information, news, and tips about Nashville’s canopy by subscribing to our email newsletter and checking out our Linktree.