Thursday, January 14, 2010

LOOK FOR: Hemlock Trees (While You Still Can)

Eastern hemlock, tsuga canadensis
Photo credit: lumierefl
Eastern Hemlock (Tsuga canadensis) is a majestic tree sometimes called the "redwood of the East." They can form tranquil evergreen groves in the understory, but really, they're just waiting for their big break. When a tree falls and opens a spot in the canopy, hemlocks will shoot up -- eventually reaching heights of 80 or even 100 feet over their 350 year lifespan. In the second-growth forests of the DC area, though, you'll see only scattered trees, and they're rarely taller than 15 or 20 feet tall.

Because they're evergreen, this is a great time of year to look for hemlocks, when other trees have lost their leaves. The needles of hemlock will help you distinguish this tree from other evergreens in our area: they are flat, about 3/4 of an inch long, and grow in a plane off the twigs. The overall form of hemlocks can also be distinctive:  branches grow horizontally from the trunk, but are floppy on the ends as the twigs haven't hardened up yet. Up close, you may notice very small cones on the trees -- they look like pine cones, but much smaller.

cones on eastern hemlock (Tsuga canadensis)
Photo credit: DaveSF
The genus name, Tsuga, is a Japanese word supposedly meaning "mother tree." Unfortunately the relationship between North American and Japanese hemlocks is not so nurturing. Around 1911, some imported hemlocks bound for a Japanese garden in Richmond came bearing an insect known as the hemlock woolly adelgid (uh-DELL-jid). This relative of the aphid is just a minor pest in Japan, where it's got several predators and the trees have some natural resistance. For our local Tsuga canadensis, though, infestation by the woolly adelgid is usually fatal within 4 to 10 years.

It wasn't until the 1980's that people started noticing that the adelgid had spread from ornamental plantings in Richmond to native trees in York River State Park and Shenandoah National Park. By the mid-90's, the adelgid had spread to Connecticut and Massachusetts, and it is now considered established throughout the hemlock's eastern range, from Maine to Alabama. Experiments are being done with the release of adelgid-eating beetles from Japan, but results don't seem very promising, and many stands of hemlock have already been lost. (To see some of the destruction to old growth hemlock groves, see this video from the Charlotte Observer.)

eastern hemlock (Tsuga canadensis) at Scott's Run Nature Preserve
Hemlock at Scott's Run by the Natural Capital
There are two morals to this story. First, garden with native plants, so that you don't unwittingly import a pest that will devastate an entire species. Second, get out there and appreciate what we've got, while you can.

In the wild: We're not aware of any really large hemlocks in the DC area (do let us know!) but there are still some small hemlocks scattered in our local forests. The most we've noticed are at Scott's Run Nature Preserve in McLean -- they grow right along several of the trails there (like the one shown here).

In your yard: It's probably better not to bother planting hemlocks unless and until someone figures out how to control the adelgids. Too bad, because they're beautiful trees.

8 comments:

Natalie @ 220 said...

This is really sad. I hate that these will probably be non-existant in the DC area in the next few years.

Elizabeth said...

Some days I have visions of Rock Creek park as a monoculture of Norway maples, with a carpet of lesser celandine underneath (both invasive plants that are making a good attempt at taking over). But it's not here yet...

Anonymous said...

Theres a huge Hemlock tree along the Bull Run River in Clifton. It's about two miles down river from Hemlock overlook but on the Prince William side of the river. Its on a steep hill by the gun club. There's alot of smaller ones ive seen around the Hemlock Overlook park area.

Elizabeth | The Natural Capital said...

Thanks -- will have to check them out!

Elizabeth | The Natural Capital said...

Just noticed there's a nice big hemlock in Sligo Creek park by the parking lot at Dale and Hartford Ave, in Takoma Park. Surely planted, but a great specimen, with a much nicer form than most of the struggling hemlocks at Scott's Run. Look for it sometime if you're in the area.

muzammal khan said...

Some days I have visions of Rock Creek park as a monoculture of Norway maples, with a carpet of lesser celandine. t-shirt personnalisé

muzammal khan said...

Just noticed there's a nice big hemlock in Sligo Creek park by the parking lot at Dale and Hartford Ave, in Takoma Park. Surely planted, but a great specimen, with a much nicer form than. assurance chine

Jay said...

I haven't been there since the 1990s, but I remember there being locally large stands of Eastern Hemlock along Occoquon Creek and its tributaries in Fairfax and Prince William Counties, Neabsco, Powells, Quantico and Chapowomsic Creeks in Prince William County, and Austin Run in Stafford County. Some of these may have been affected by the adelgid or development, but I suspect some of them are still there.

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