The branch of a hemlock tree hangs over brown leaves.
Hemlock Branch Hemlock woolly adelgid threatens Tenneessee's hemlocks. © The Nature Conservan

Stories in Tennessee

Hemlocks and How to Save Them

Our majestic trees are under attack from an invasive insect called the hemlock wooly adelgid. Find out how we can save them.

man applying hemlock chemical treatment
Hemlock treatment A National Park Service ranger gives a hemlock a protective chemical treatment using the soil-drench method. © National Park Service

At the South Cumberland State Park, some of Tennessee’s most formidable hemlock trees—some well over 200 years old—tower over the Fiery Gizzard Trail. One huge specimen near the northern trailhead is estimated to be more than 500 years old. Hemlocks also adorn portions of The Nature Conservancy’s Bridgestone Nature Reserve at Chestnut Mountain.  

Sometimes called the “redwood of the east,” hemlocks are beautiful, slow-growing and long-lived evergreens that can reach more than 150 feet tall. Eastern hemlocks (Tsuga Canadensis), and their close relative Carolina hemlocks (Tsuga caroliniana), represent a “keystone species” that play a critical role in an ecological community that is much larger than its abundance would indicate.

Usually located around streams and gorges, hemlock trees provide dense shade that helps to moderate air and water temperatures throughout much of eastern Tennessee, especially in the Great Smoky Mountains and on the Cumberland Plateau. This benefits forest dwelling animals as well as fish, insects and other aquatic species that depend on the presence of hemlocks for all or part of their life cycle. As part of a larger forested landscape, hemlock trees also contribute to filtering air and water, and storing carbon in the atmosphere. 

A map of hemlock woolly adelgid infestated counties in Tennessee.
Hemlock Infestation A map of hemlock woolly adelgid infestated counties in Tennessee. © TN.GOV

Hemlocks Under Attack

Hemlocks in Tennessee and throughout the eastern United States are under attack by an invasive insect known as the hemlock woolly adelgid (HWA). Native to Asia HWAs have no natural predators in the United States. This enables them to freely feed on and kill hemlocks in as few as three years.

First spotted in East Tennessee in 2002, HWAs have killed many hemlocks in the Great Smoky Mountains National Park and the Cherokee National Forest. As of early 2019, most counties with native hemlocks recorded the presence of HWAs. It is believed that birds are the primary source of their spread.

A large tree viewed from below.
Eastern Hemlock The evergreen branches of an Eastern hemlock tree reach up to a blue sky. © Kent Mason

Combatting Hemlock Woolly Adelgid

The most common method of combatting HWA involves chemical treatments that aid in stopping HWAs from harming the hemlocks. Applied manually, tree-by-tree, treatment periods can range from five years up to eight years or more. 

An alternative action against HWAs involves biological tools, such as predator beetles imported from Asia and the western U.S. After careful quarantine and studies, releasing these beetles shows signs of success in controlling HWAs.

Both types of treating HWAs are costly when implemented at large scales. 

A hemlock tree trunk emerges from a forest floor.
Hemlock Tree Trunk A hemlock trunk is fully rooted in a layer of talus. © Creative Commons/Nicholas A. Tonelli

TNC Takes Action

In 2005, the Tennessee Division of Forestry (TDF) led the process for creating a strategic plan for HWA.  Since public agencies did not have capacity to fulfill all of the assigned actions, The Nature Conservancy stepped in to fill gaps. This included organizing experts from government, academia and other land managers from the Great Smoky Mountains National Park into the Tennessee Hemlock Conservation Partnership (THCP) where partners shared best practices for chemical treatment, developed science-based planning for hemlock conservation areas and teamed up to seek funding to support these efforts.

In this role, TNC also sought and secured funding from the USDA Forest Service to support a TDF position charged with coordinating the THCP. TDF secured additional USDA Forest Service funds to create a HWA Strike Team. This seasonal team, which works with representatives from various agencies and private lands, convenes between October and May to implement an intensive and environmentally-conscious pest-management plan to prevent and combat HWA infestation throughout  the Cumberland Plateau and East Tennessee.

The THCP meets twice a year to share current information about HWA and hemlocks in order to identify priority areas where the HWA Strike Team should focus on in the following season. During the 2018 season, the HWA Strike Team treated 15,000 trees over 1,300 acres, released thousands of predatory beetles and conducted community workshops to train others to protect hemlock trees.