The hemlock plays an important ecological role in our eastern forests. It is considered a foundational species which means a hemlock forest creates and defines an entire ecological community within itself. They are known as the ‘redwood of the east’ and like a redwood forest, a hemlock forest has an ecological system which is different from a hardwood forest. These forests serve as crucial habitat with a high wildlife value that benefits over 120 species of mammals and birds. The low, sweeping branches provide shelter year round but is particularly important in winter. During the summer months, a hemlock forest is cooler than other woodland types. The seeds offer food for over 90 bird species. Some birds such as the black-throated green warbler are considered hemlock obligates, meaning they exclusively use hemlock trees for nesting sites.
Hemlock woolly adelgid (HWA)
In the 1920’s, the Hemlock woolly adelgid (HWA), a tiny sap sucking member of the aphid was accidentally introduced to the United States from Asia. This insect has since become a serious threat to the survival of native hemlocks in the wild. HWA were first observed in the forests of Virginia in the 1950s and have since spread from to the southern and middle Appalachians. HWA has killed 95% of the hemlocks in Shenandoah National Parkas well as most of the old growth hemlocks in the Great Smoky Mountains National Park. Inability to survive cold winters have limited HWA’s ability to spread as far north as Massachusetts, however as winter temperatures continue to moderate, expansion into New England is predicted. HWA was discovered in Pennsylvania’s Allegheny National Park in November of 2013. Treatment of HWA is available using pesticides containing imidacloprid or dinotefuran.
HWA Treatment for Homeowners
HWA is not a death sentence for a hemlock tree in your landscape. Infestations are not as likely in a cultivated tree as they are in wild specimens due to being in isolated locations instead of large, forested groups. A leyland cypress carries more diseases than a hemlock tree. A hemlock is still a desirable tree for a landscape design and should not be ruled out due to HWA. Treatment can be a simple method of applying an inexpensive, systemic insecticide to the soil around the tree trunk base. The insecticide will be absorbed by roots of the tree so it may spread through branches and needles. A treatment can protect a tree for up to five years.
If you think your hemlock trees are infected or would like to learn more about treatment prior to planting, please see:
How to treat hemlock trees for hemlock woolly adelgid – Micigan State University Extension
HWA Control Options Info for Landowners – from The Hemlock Restoration Initiative
Hemlock Trees as a Privacy Screen
If you are considering planting a privacy hedge using eastern hemlock, we advice alternating hemlocks with another compatible tree species, This could be cedars, spruce, cryptomeria, or some cypress varieties. For example, every other tree could be a hemlock alternating with a cryptomeria. That way should one species become infected with a pest or disease, treatment is minimal and should the worst happen you will not lose your entire hedge, We recommend this system for any privacy hedge, not just hemlocks.
Further Reading:
The Hemlock Restoration Initiative – The objective of the Hemlock Restoration Initiative (HRI) is to restore eastern and Carolina hemlocks to their native habitats throughout North Carolina and to mitigate damage to hemlocks caused by infestation of the hemlock woolly adelgid (HWA).
Lumpkin Coalition – With an unprecedented degree of collaboration, citizen groups, government agencies, scientists and Georgia’s conservation community are working together to combat the Hemlock woolly adelgid and hopefully check its populations before it is too late for these beautiful evergreens.
Can We Save the Mighty Hemlock? – Informatics article from The Appalachian Voice. As the threat posed by the hemlock woolly adelgid grows, so do efforts to save this magnificent species.
TreeSnap App – Help our nation’s trees! Invasive diseases and pests threaten the health of America’s forests. Scientists are working to understand what allows some individual trees to survive, but they need to find healthy, resilient trees to study. You can help. Tag trees you find in your community, on your property, or out in the wild using TreeSnap! Scientists will use the data you collect to locate trees for research projects to build better tree breeding programs.
We Can Help
If you are in north Georgia and are considering a hemlock tree for your landscape or think yours may need treatment, Art of Stone Gardening can help. Please contact us at: Art of Stone Gardening.
NOTE: Carolina hemlock Tsuga caroliniana also applies to this article. Carolina hemlock is a species of hemlock native to the Appalachian Mountains in extreme northeast Georgia, western North Carolina, northwest South Carolina, eastern Tennessee and southwest Virginia. It prefers cooler temperatures and moist areas. The Carolina hemlock is smaller than eastern hemlock Tsuga canadensis and not commonly found in plant miniseries.
