Green Scene

Green Scene: Invasive plants are becoming a larger problem

Snowdrops naturalize a little too easily.
Snowdrops naturalize a little too easily. (Sandra Leavitt Lerner For The Washington Post)
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By Joel M. Lerner
Saturday, November 21, 2009

We live surrounded by many trees, shrubs, perennials and annuals that aren't from this area. It may seem like they belong here, but they don't. They aren't the plants that Mother Nature originally intended for our region.

These invasive alien plants escape from gardens and crowd out less-vigorous native flora that wildlife depend on. This problem is getting worse and should be examined from three perspectives: How is a native plant defined? What makes non-natives undesirable? And, what will the next generation of invasive plants be?

Defining a native plant

One commonly accepted definition for native plants is those that were growing here before Europeans arrived. It is believed these are indigenous plants that evolved in this region together with native wildlife over thousands of years.

Many native plant aficionados hold that that plants are native only to their precise location in nature. For example, plants hardy to wetlands in mid-Atlantic states are only considered native to that region -- not native to dry land in the same area or to wetlands in Colorado, for example, unless they also naturally grow there. Other native plant purists consider hybrids of native plants, especially when crossed with exotic species, not to be true natives.

Although not all texts agree, the ranges and descriptions of plant habitats are fairly accurate. I am not a native plant purist but do agree that indigenous flora is the most logical choice to plant in any given area. To find out if vegetation is native to a particular region, use this database offered by the Department of Agriculture at http://plants.usda.gov. More information about native species can also be found at http://www.natureserve.org/index.htm.

Plants noxious to D.C. area

Black seeded fountain grass (Pennisetum alopecuroides 'Moudry') is one of the most invasive grasses I've planted. Its beautiful inflorescence in late summer lasts until seeds drop in fall. The low growing, 24 to 30 inch tall and wide grass provides two to three months of ornamental value. However this clump grass seeds itself into every lawn it's been planted near, continuing to spread itself.

Another grass that makes the invasive alien plant list is tall fescue. There isn't a better groundcover for recreational use than lawn. Nevertheless, tall fescue (Festuca arundinacea) is an invasive exotic, introduced to the United States from Europe in the early 19th century. Many gardeners are reluctant to eradicate it because of its recreational uses.

Snowdrops (Galanthus nivalis) are native across Europe, Asia, and Eastern Russia to northern Syria. They are valued bulbs because of their early blooms but naturalize a little too easily in meadows and woodlands. They are considered invasive and toxic and have colonized in many U.S. states where they are considered invasive alien flora.

Norway maple, English ivy, amur honeysuckle, winged euonymus, Japanese barberry, multiflora rose, mile-a-minute weed and purple loosestrife are plants that thrive in this region and are also not indigenous. They've been growing here long enough that we know their tendency is invasive to the point of harming the environment.

Purple loosestrife (Lythrum salicaria) is a striking, dependable, flowering perennial with pinkish-purple masses that blooms from June to September and produces thousands of seeds per plant. It dominates habitats, creating monocultures across wetlands from Maine to California. Many states have laws prohibiting the propagation, planting and sale of this plant. Native to Europe and Asia, it was introduced in the Northeastern United States as an ornamental and medicinal plant in the 1800s. Its naturalization across the United States and Canada has robbed wildlife of habitats.

Winged euonymus is admired for its magnificent red fall color, earning it the common name "burning bush." The problem is that it competes vigorously and successfully with native plants like spicebushes, arrowwood viburnums, summersweets and Virginia sweetspires.


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