When a Wisconsin Department of Natural Resources employee discovered water hyacinth and water lettuce growing in a Mississippi backwater near Winona last year, state and federal workers removed about 1,500 plants by hand.
The removal and the winter would take care of the invasion, they thought.
But the exotic plants not only survived the winter, the population exploded this summer to about 10,000 plants in the same area.
"It is a bit of a surprise that water lettuce and water hyacinth was able to overwinter," said Doug Jensen, aquatic invasive species program coordinator at Minnesota Sea Grant, a Duluth-based research organization.
The incident is an example of how the complexity of fighting invasive plant species in Minnesota waters is increasing.
The latest complication arrived this summer when scientists found another exotic species, a Brazilian milfoil called parrot feather in the same area of the Mississippi.
In this case, officials think the plants likely came from a backyard pond or an aquarium. They say it's very difficult to regulate the exotic plants grown that way, and they are trying to kill the new arrivals with herbicides.
The Minnesota DNR regulates aquatic plants, prohibiting people from possessing or selling some exotic plants and regulating others, like parrot feather.
That means people can grow them, but cannot release them into natural settings.
The DNR tries to encourage the use of native plants in backyard ponds. But Chip Welling, coordinator of the management of aquatic invasive species, said, "At the same time we recognize people can and do buy non-native plants, especially from vendors outside the state. But then we ask them to take personal responsibility for this choice and ensure that those plants are not able to get into our streams, rivers and lakes."
No one tracks the number of backyard ponds and water gardens, but experts say they're growing in popularity.
Currently the DNR says invasive plant species are found in more than 1,000 Minnesota lakes and streams.
The most common aquatic invasive plants in the state are Eurasian watermilfoil, curly-leaf pondweed and flowering rush.
It might be time to re-evaluate the risk some exotic plants pose, Jensen said. The apparent success of plants like water lettuce and water hyacinth, which no one thought would survive Minnesota winters, raises questions about the risk of many new invasive species.
Jensen says the unusually mild winter conditions likely allowed the plants to survive.
"They had overwintered and obviously had produced some seed and now have the ability to spread," Jensen said. " So some of those species we thought were at low or no risk, I guess are putting themselves on the map."
"I think that a risk assessment might be in order to determine whether or not they could become widely established in the state and therefore could warrant additional regulations."
In the case of the water lettuce and water hyacinth, state and federal officials say it's still not clear the exotic plants found in the Mississippi are established, or if their survival was a fluke caused by the unusually warm winter.
Welling said those plants have been found before in Minnesota and did not survive long term.
He said the state is constantly re-assessing the regulation of exotic plants, but he says regulation has limited effect, and must be balanced with personal responsibility.
"The balance is pretty heavy on the personal responsibility side," Welling said. "Certainly we do track to some extent the plants that are available for sale at facilities in Minnesota. But the Internet is difficult to regulate."
A survey done by Minnesota Sea Grant found most people with backyard ponds are well aware of the dangers posed by releasing exotic plants. But experts say it only takes one uninformed or careless person to introduce a new invasive species to Minnesota waters.