Last year's hurricanes and flooding not only engulfed homes and carried away roads and bridges in hard-hit areas of the country, it dispersed aggressive invasive species as well.

In Vermont, the floodwaters from Tropical Storm Irene and work afterward to dredge rivers and remove debris spread fragments of Japanese knotweed, a plant that threatens to take over flood plains wiped clean by the August storm.

The overflowing Missouri and Mississippi rivers last year launched Asian carp into lakes and oxbows where the fish had not been seen before, from Louisiana to the Iowa Great Lakes. Flooding also increased the population along the Missouri River of purple loosestrife, a plant that suppresses native plants and alters wetlands.

"It's quite an extensive problem around the country and it's spreading," said Dr. Linda Nelson, aquatic invasive species expert with the U.S. Army Corps of Engineers. The agency's budget for controlling invasive aquatic plants has grown from $124 million in 2008 to $135 million for fiscal year 2012.

Dr. Al Cofrancesco, director of the Corps' Invasive Species Center in Vicksburg, Miss., said invasive species are not a problem when they're in their native range.

"There are things that keep them in natural balance. The problem occurs is when we move into areas where they don't have those natural controls or regulators and they expand very rapidly," he said.

In Vermont, floodwaters and repair work broke off portions of stems and woody rhizomes of the aggressive Japanese knotweed. The perennial, imported from Asia as an ornamental, was already a problem in Vermont and a dozen other states in the Northeast, the Pacific Northwest and the Midwest. It spreads quickly on riverbanks, floodplains and roadsides, choking out native plants, degrading habitats of fish, birds and insects and weakening stream banks

"The whole Irene event was ideal" for knotweed, said Brian Colleran, a coordinator for Vermont's knotweed program.

The plant, which resembles bamboo when mature, spreads quickly in disturbed soils. Just this week, new young plants were inching out of the silt on the banks of the Camp Brook, a tributary of the White River, where the land looks like a moonscape since floodwaters washed away trees, rocks and other native plants. Once these invasive plants take over, their root structure and a lack of groundcover and native plants and trees with deeper roots, weakens the stream banks, causing erosion, and flood damage.

"We'd like to get out the message that if there's ever a time to hand pull or mechanically control so we can avoid the use of herbicides, this is the one year where that's possible," said Sharon Plumb, invasive species coordinator, for the Vermont Chapter of The Nature Conservancy.

Once the knotweed becomes established, it's laborious to remove. The plant, which can grow 12 feet high, needs to be cut down four to five times a year for a number of years or chemicals or machinery will need to be used.

Efforts are under way to restore those bare banks with native trees and shrubs that will shade out knotweed.

This spring, the state of Vermont hired Colleran to scout out new infestations, educate river groups about the invader and to coordinate community efforts to remove the plants.

Another invasive species problem, Asian carp, was aggravated by the past year's flooding, which moved the fish from the Missouri and Mississippi rivers into isolated lakes and oxbows, said Duane Chapman, research fish biologist with the U.S. Geological Survey based in Columbia, Mo. Some carp were able to pass over dams during the floods, he said.

"Give these guys an opportunity and they'll take it," he said.

Invasives are also spread by repair and cleanup work after storms.

After Hurricane Katrina, Louisiana imposed a quarantine in a dozen parishes to restrict movement of wood and cellulose material that could be infested with Formosan subterranean termites. Millions of tons of wood debris left behind by Katrina and Rita was shredded into mulch and there were fears the material could spread the pests.

"Any time you move a potentially infested material, there's the potential to move an invasive species," said Alan Lax, the former research leader for the Formosan subterranean research unit for the U.S. Department of Agriculture.