Early Spring Aliens

At the end of February, technically still wintertime, I saw this carpet of white on the Cabin John Trail. It wasn’t spring beauties, and it wasn’t leftover snow, either.

This is Galanthus nivalis (snowdrops; Amaryllidaceae), a perennial native to Europe. It naturalizes well, meaning you can plant a few dozen bulbs and enjoy the show year after year as they spread through your garden. Unfortunately, it naturalizes a little too well, and so can be found in woodlands throughout the mid-Atlantic, as well as some other parts of the country.  The National Park Service doesn’t list the species in its manual Plant Invaders of Mid-Atlantic Natural Areas, but the University of Maryland Cooperative Extension considers it invasive.

What exactly does it mean to be “invasive”? I like the USDA Forest Service definition:

An “invasive species” is defined as a species that is

  1. Non-native (or alien) to the ecosystem under consideration; and,
    2. Whose introduction causes or is likely to cause economic or environmental harm or harm to human health.

Whether or not snowdrops cause environmental harm may be just a matter of degree, or time.

I didn’t know that snowdrops were a problem before researching them for this post. Now there’s another item on my garden to-do list: pull out the snowdrops.

When it comes to Vinca minor (periwinkle; Apocynaceae), though, there’s no doubt. At least twenty-two authorities consider it invasive. It’s easy to find at this time of year. If you follow some of the footpaths near Carderock to the base of the climbing wall, you’ll find vast slopes of the stuff, competing with the Dutchman’s breeches, squirrel corn, Virginia bluebells, and trout lilies that belong there. The plant roots easily at the stem nodes, and since the long stems trail along the ground, it spreads quickly. This characteristic, along with evergreen leaves, pretty purple flowers, and tolerance for a variety of growing conditions, makes it a popular groundcover in home gardens. Please don’t plant it.

next time: more aliens


A Beautiful, Useful Pest


yellow flag iris, pale yellow iris
Iris pseudacorus


In motor racing a yellow flag shown to competitors is a warning: there’s a potential hazard on or near the racing surface, something that could cause a serious problem.

“Yellow flag” is a good name, then, for this particular iris. Native to North Africa and Europe, it was introduced to the US and Canada in the early 1900s as a garden ornamental. As early as 1911, it was found in the wild in Newfoundland, and it took only a few decades to establish itself as a serious wetland pest, capable of outcompeting all other plants to form dense monocultural stands.

This is not merely an aesthetic problem for people like me who enjoy the beauty of native flora. Loss of floral diversity leads directly to the loss of faunal diversity, from insects to mammals. The ripple effects of a changing hydrology can be enormous:

The clonal nature of I. pseudacorus causes it to form dense stands and thick, submerged rhizome mats (Idaho Invasives 2007, Lui et al. 2010) that can prevent the germination and growth of native species (sedges, rushes, etc.) and eventually displace them entirely (Lui et al. 2010, MNDNR 2012, Noxious Weed Control Program 2009, ODA 2012). This vegetative growth can also trap sediment, raise local elevation of the ecosystem, and alter wetland hydrology (Noxious Weed Control Program 2009, Sarver et al. 2008). Populations of yellow iris create a positive feedback loop: once established, the roots trap sediment, which enables growth of new seedlings, which in turn trap more sediment (Jacobs et al. 2011). This increase in sedimentation also creates new habitat for shrubs and trees, thereby altering it to a drier ecosystem (Lui et al. 2010, Sarver et al. 2008). This alteration reduces the food supply and nesting habitat of many fish and waterfowl that depend on wetlands (Noxious Weed Control Program 2009, ODA 2012).*

Like many invasive species, yellow flag iris is highly adaptable to a variety of growing conditions. It prefers to grow in shallow water or wet soils, but once established it is surprisingly tolerant of lower moisture levels. It also tolerates low oxygen levels, high organics levels, and it will grow in a wide pH range, from highly acidic to slightly basic.

Despite all this bad news, if controlled yellow flag iris can be useful. It has been shown to significantly reduce levels of pathogens, including E. coli and Salmonella. It is known to be able to remove heavy metals from wastewater and has been used in sewage treatment systems in the Czech Republic and China.

It’s a stunningly showy plant, a perennial that stands up to five feet tall with dark green foliage and bright yellow blossoms. It’s now blooming in several parts of the recently de-watered parts of the C&O Canal near Carderock. Found in most of the US and Canada (except for 13 states and provinces), it’s reported as invasive in 12 states, including Maryland (where it’s been found in eight counties) and is listed as prohibited or noxious in six states.

Strangely despite the obvious problems it is still recommended by Missouri Botanical Garden because it’s easy to grow in problem (that is, wet) areas of the yard and has pretty flowers, but they also note “yellow flag should not be planted along streams or ponds or lakes where it can spread into natural waterways…”



USDA Natural Resources Conservation Service
Missouri Botanical Garden
Maryland Biodiversity Project
University of Florida, Institute of Food and Agricultural Services
*Great Lakes Aquatic Nonindigenous Species Information System

This is Why We Hate Invasives

Last summer I was tickled to discover eleven new (to me) species of wildflowers growing along the Potomac between Fletcher’s Cove and Chain Bridge. These weren’t little bitty plants, either – most were big and showy. So yesterday I went back and hiked the river trail from Fletcher’s to a point within view of Chain Bridge – about six-tenths of a mile.  And this is what I saw:


It’s forgivable if you find it pretty, but to a native plant enthusiast it’s a horror show, a textbook example of the danger of alien invasive species. It’s lesser celandine, aka fig buttercup (Ranunculus ficaria, previously Ficaria verna, Ranunculaceae), one of the worst invasives in the area. And you can see why from the picture. Strictly speaking, that isn’t a monoculture, as I did find other species in there: henbit and some purple deadnettle (also alien invasives), with a few spring beauties popping through. And, near Chain Bridge, I saw a single clump of field chickweed.

Try to imagine this. The area is a floodplain about one-tenth of a mile wide (from the river to the slope up to the canal towpath) by at least six-tenths of a mile long, and the whole thing is predominately one species.

This is why we hate invasives. Not because they’re alien per se, but because they can overcome everything. Imagine what this might have look liked a few decades ago: a carpet of Virginia Bluebells giving way up-slope to trout lilies, Dutchman’s breeches, spring beauties, and cut-leaved toothwort.

It makes me a little sick.