The Dirty Dozen

April showers bring May flowers, but  these are increasingly turning out to be the unwelcome blooms of unwanted plants.   

Thousands of organisms have been introduced to North America since European colonization. The vast majority of these nonnative species do not threaten native flora and fauna. Those wild apple trees blooming in old fields are a net ecological gain, and the black vultures that prefer to roost in the Norway spruce in my yard make good use of a tree that does not overwhelm native conifers in natural settings.

Nonetheless, a small but significant percentage — perhaps 10 percent  of all  introduced species on this continent — do disrupt native habitats, dislodge native species and have the potential to dramatically impact the diversity of species and the quality of our woodlands and waterways.  

They seem to be everywhere

The  most agressive of these nonnative invasive species are highly successful plants, pests and pathogens that have no natural predators in these environments, are able to disperse across great distances through multiple vectors and may have special adaptations that allow them to exploit a wide range of native habitats at the expense of species with restricted niches.

In early spring, before the forest canopy leafs out, the woodlands in this region typically host a dazzling display of ephemeral wildflowers that bloom in the dappled sunlight. Where shade-tolerant Japanese barberry (Berberis thungbergii) occupies the shrub layer, however, such infestations can progressively become so massive that they shade out the native wildflowers.

Garlic mustard (Alliaria petiolata) will start to displace native species when it represents only 2 percent of the biomass, and each plant produces thousands of windborne seeds that remain viable for years.

Once you become aware of invasive plants, they become all that you see. Virtually every roadside shrub is an invasive Asian honeysuckle, buckthorn or multiflora rose. In some wetlands, the dominant — even defining — species are common reed (Phragmites australis) and purple loosestrife (Lythrum salicaria L. — Lythraceae).  

It has gotten so I cannot look at a native tree species without thinking about the invasive pests and pathogens that threaten its very viability, ranging from those that are on the landscape now such as Dutch elm disease and hemlock woolly adelgid, to those on the threshold such as emerald ash borer and Asian longhorned beetle. Frankly, it can get depressing.

But it is a mistake to assume that there is nothing to be done.

It can be stopped

It may be too late to eradicate many invasive species entirely from our natural areas. The opportunity costs go up exponentially the longer an infestation is established and allowed to expand.  

One of the best approaches is to learn to identify the most threatening invasive species and strive to detect new incursions early and respond promptly with appropriate management.

It is also vital to be managing for something, not just to control invasives.  As troublesome as they can be, they may also provide some positive benefits for things we care about that need to be accounted for as part of control efforts.

When the entire riparian buffer of a stream is nonnative, simply removing the plants does not address issues of soil retention, water quality, shade and cover in its absence.

On May 22, the Salisbury Association will host an exhibit and talk on invasive species, emphasizing a Dirty Dozen of the worst offenders, starting at 10 a.m. at the Academy Building in Salisbury, across the street from Town Hall.  This is a good opportunity to learn more about these invasive plants, their impacts and methods of control.   

There are certain things all of us can do to reduce the likelihood of unintentional introductions of unwanted invasives, including how we screen fill at construction sites, clean our boats and trailers when moving between water bodies, and monitor disturbed areas.

Unintended consequences

Many plants that have proven invasive were introduced because of other desirable qualities.  Perhaps as many as two-thirds of these were horticultural introductions, while others were promoted by state and federal agencies to provide wildlife cover and as erosion control.

We’ve been dealing with the unintended consequences of these introductions for a long time.  

Back in 1775, the citizens of Ridgefield, Conn., set out to eradicate another species of introduced barberry that proved to harbor a wheat-blast that threatened their food supply, and many other communities followed suit, even imposing fines on those who did not destroy the plants on their property.  

Common barberry was aggressively controlled and has not been a significant presence on the landscape since that time.

That is an encouraging thought.

Tim Abbott is program director of Housatonic Valley Association’s Litchfield Hills Greenprint. His blog is at greensleeves.typepad.com.