Most gardeners in the Northeast remember the drenching that Tropical Storm Irene gave the region late last summer, and her effects are still being felt, sometimes in unexpected ways. Flooding was especially widespread in Vermont, and it's there researchers are finding that, along with a lot of debris that was spread over the landscape by the raging waters, came an invasive species of plant as well.
Japanese knotweed (Polygonum cuspidatum), an Asian plant originally grown as an ornamental, chokes out native vegetation along waterways and roadsides in many parts of the country. It relishes disturbed soil. Portions of the stems and woody rhizomes carried by floodwaters or moved around in clean-up operations have found fertile ground in the bare soil left along flood-ravaged streams and rivers. According to Brian Colleran, the coordinator of Vermont's knotweed control program, Irene created ideal conditions for the spread of this invasive plant.
In other parts of the country, flooding has also helped to spread invasive purple loosestrife along the Missouri River. And plants aren't the only invasive species that get spread around when lakes and rivers jump their banks. Asian carp have also ridden floodwaters to find their way into new bodies of water.
An unexpected twist in the battle to control invasive species comes in the efforts to control another Asian import, mile-a-minute-weed (Persicaria perfoliata). Researchers have released a weed-eating weevil, itself an Asian species, in areas of New England and the mid-Atlantic states where mile-a-minute-weed runs rampant. The weevils have spread fairly quickly, do not seem to have an adverse effect on any native plants, and in some places have largely eliminated the weed. So what's the problem? Instead of native vegetation moving in to fill in the vacated territory, other invasive plant species are moving in instead! So there is still much work to be done in figuring out how to return areas affected by invasive plants back to their natural vegetation.