Invasive Species

Hemlock Woolly AdelgidHemlock Woolly Adelgid

Over the last twenty years, imported pests have damaged SNP: gypsy moth, woolly adelgid, with more possibly on the way.

Keeping Trails Worth Maintaining

In recent years, imported plants have become a threat to the park. Unlike gypsy moth and adelgids, which targeted only one species of tree, invasive plants can crowd out everything.

Controlling Invasive Plants

I believe volunteers, including trail maintainers, should try to stop invasive plants although what we can do is limited. Everyone, even those who never enter SNP, should be concerned about preservation of biodiversity. People who visit SNP probably want it to stay like it is, not be a place where all the grass is stilt grass, or garlic mustard, with clumps of Asiatic bittersweet, and the trees either dead or ailanthus. Even without those considerations, trail maintainers are better off without invasive plants. Bittersweet kills tree, so there are more blow downs, more open patches and summer growth. Who wants to smell ailanthus when pruning trees? And multiflora rose has thorns unlike mountain laurel.

Wavyleaf Basketgrass

wavyleaf grass patch

Wavyleaf Basketgrass is a perennial and the plants regrow from existing root systems as well as dispersal by seed. It was first found in Patapsco Valley State Park in Maryland around 1996 and has since spread throughout the understory of that park. It is spreading in Virginia and Pennsylvania. Wavyleaf Basketgrass thrives in shade (deep forest). This is of concern to anyone who uses the forest: photographers, birders, botanists, and hunters. If we locate colonies before they are established, we can eradicate them from that site.

Asiatic Bittersweet

oriental bittersweet

The native bittersweet is found in only a couple of spots in SNP. Native bittersweet has longer leaves, berries at ends of stems—Asiatic Bittersweet has berries between leaf and main stem.

  • You can pull it up, but do not pull all of a big patch—the open area can let in other bad things
  • Tamp down disturbed ground—pieces left in ground may sprout again (I have seen this)
  • Cutting big vines will give trees some time until someone can clear bittersweet around the tree
  • Repeated cutting may eventually stop it.

Stilt Grass

stilt grass

Japanese stilt grass is distinguished from other plants ts asymmetric leaves with silver streak in middle.

  • Cut to the ground at the end of summer before it forms seed
  • It may grow back if cut earlier
  • It spreads seeds that may germinate for years
  • PATC brush cutters chew it up into little pieces



Black walnut and sumac look similar to ailanthus.

  • The “Tree-Of-Heaven” has bumps near base of leaf (strictly leaflet)
    • Bark looks like cantaloupe skin
    • Broken ailanthus leaves stink
  • Cutting only encourages it—it may sprout from roots
  • Seedlings can be pulled up
  • Cutting trees may make them produce seed

Garlic Mustard

garlic mustard

You can pull up garlic mustard, but it has to be carried out of the area after it starts flowering. It has a two-year life cycle and spreads seed, so one pulling will not stop it. Eradication is a multi-year project.

Oriental Lady’s Thumb

oriental ladys thumb

This import has pink flowers (with little pink spheres). SNP staff say it behaves worse in SNP than anywhere else. It muscles into native vegetation areas—other places it just grows in disturbed areas e.g. trail edges.

Tufted Knotweed

Polygonum cespitosum is a “bad” relative can be distinguished from other Polygonum species by the presence of pink flowers/fruit, and combined long bristles on the sheath encircling the stem at the point where the leaf blade attaches.

Spotted Ladysthumb

Polygonum persicaria is another import that has pink flowers, but the bristles at the summit of the leaf sheath are much shorter than the bad one, and is much less invasive than P. caespitosum so there is not as much of it in SNP.

Pennsylvania Smartweed

It might look like the bad ones, but Polygonum pensylvanicum has pink flowers/fruit that are a bit larger than those of caespitosum, leaf sheaths with no hairs at the summit, belongs here.

Multiflora Rosa

multiflora rose

I have multiflora rosa on my trail section, but many trails do not. Pull it up or cut lots of times.

Control Is Difficult

It pulls up fairly easily, and in my yard after aggressively pulling it last year, I saw much less this year. But there is so much. I can think of people I would like to sentence to pulling an acre of oriental lady’s thumb. Once it flowers, you need to bag it—the flowers quickly turn to seed.

Although not tested in SNP, cutting it to the ground before flowering (which happens mid summer, earlier than stilt grass) should slow it, but another cutting may be needed later in the season.

Identifying Invasives

Things You Can Do

This is somewhat limited. Volunteers can pull and cut things. On a Park-wide scale, herbicides are probably needed, but volunteers are not allowed to use them. NPS does not want everything poisoned.


Shenandoah National Park has a program, but it is small. If NPS had what they need, would there be so many people scraping out waterbars with their AARP cards? (The district manager says: YES!) There is the Mid-Atlantic Exotic Plant Management program, headed by This email address is being protected from spambots. You need JavaScript enabled to view it., who oversees exotic plant (cheap and nasty imports) programs for 18 units of the National Park System.

The lead biologic science technician with Shenandoah National Park, Jake Hughes, knows from experience how to root out invasive plants and provides clear answers about how to control invasive plant. He is better than Google if you have questions—his answers are relevant, specific, and concise. This email address is being protected from spambots. You need JavaScript enabled to view it. or call him at 540.999.3492.

Both people are happy to see people interested in controlling exotics and willing to help if they can. Mr. Akerson has offered to meet with groups interested in learning more.


  • Thoroughly clearing some small patches (such as garlic mustard) is a better use of effort than a reduction of a big patch of invasives.
  • Seed banks in the ground mean some plants will grow again in cleared areas, even if seeds are not brought in from elsewhere.
  • Some plants sprout from pieces left in the ground, so repeated visits are needed.
  • Those trained to use herbicides don’t just spray everything, but also cut stems of some plants and apply concentrated herbicide to stop the remnants sprouting. Herbicides kill native plants, too; spraying big areas would damage the Park’s resources.


The hard question is, “What is the most important thing to do with your time on the trail?”

Most overseers signed up, expecting to cut blow downs, paint blazes, and clean out waterbars. Selectively taking out some plants makes it more complicated, and potentially even more time consuming. Clean out a waterbar or pull up a dozen bittersweet plants? Trim yards of mountain laurel or kill a few multiflora rosa?

These days, I lean towards thinking the most important things are preserving the tread with good drainage and protecting the forest by neutralizing some invasives.

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