It's a beautiful green water frond with delicate petals. Pleasant enough to look at with names like "Water Thyme" and "Star Vine."
But don't let its looks fool you. Hydrilla verticillata, which grows up to several feet thick and chokes the life out of lakes and ponds, has been dubbed the "killer" weed by those in the know.
"It's just a thug," says Leslie Mehrhoff, director of the Invasive Plant Atlas of New England.
Once a problem mostly in Southern states, hydrilla has become an aquatic scourge in a third of US states. Researchers blame the weed's spread not only on unwitting aquarium hobbyists and motor-boat propellers, but also the Internet.
Indeed, online sales of such noxious weeds - some of them illegal - have flourished so much in the past few years that the federal government is preparing a high-tech crackdown.
"We've seen a link between growing Internet sales, the mail system, and the spread of these plants," says Larry Fowler, a botanist with the US Department of Agriculture's Animal and Plant Health Inspection Service (APHIS). "We realize many people are simply unaware of our laws, but that still doesn't make it right. And there's still a segment that is quite aware of the law and is still selling."
The hydrilla - named after the hydra-headed monster of Greek mythology - is on the federal govern- ment's short list of "noxious weeds," making it illegal to buy or sell it nationwide.
Nevertheless, an online search leads within minutes to the website of a New York pet store offering not only hydrilla, but also another federally banned plant - Hygrophila polysperma, known as "Indian Water Star."
"We know our products," the website boasts. "We're proud to have the most knowledgeable staff in the industry."
Clearly, federal law is not the company's strong point.
Businesses selling banned plants can be fined up to $250,000 under federal law, while smuggling them into the US can bring criminal penalties.
Nearly three years ago, Mr. Fowler saw the danger of Internet sales of invasive species. Miscreants were selling with impunity invasive plants like hydrilla. Next-day courier services made it possible to ship them coast-to-coast or even from abroad with little scrutiny.