Kristine Hanson of The Dirt on Gardening talks with Robert Norris, professor emeritus of plant sciences at University of California Davis, about weeds and invasive plants.
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Here are just a few examples of plants that have been deliberately introduced, but have outgrown their welcome on a huge scale:
Water hyacinth (Eichhornia crassipes), widely sold as an aquatic plant for backyard water features, first became identified as a problem in Florida, but has now found its way into water ecosystems across the southern US. The plant is capable of extremely rapid growth and has overtaken streams, lakes and parts of rivers, virtually clogged commercial waterways, actually halted boat traffic in some areas and generally caused a host of environmental problems. According to Norris, water hyacinth is now considered one of the most serious aquatic weed problems on a worldwide basis. If you have water hyacinth growing in a backyard pond or water feature, never dispose of it by simply dumping it out where it might inadvertently be introduced into nearby bodies of water. Make sure that it is thoroughly dried out and completely dead before disposing of it. Or you can take Dr. Norris's advice: "I suggest cremating it!"
Kudzu (Pueraria montana), native to China and Japan, was introduced to the US in the early 1900s as a forage plant for livestock, a soil stabilizer and ornamental vine. It is a trailing perennial with lush foliage, purple flowers and is sometimes employed for erosion control. But as anyone living in the southeastern US knows, it has a growth rate that is astonishingly rapid (more than a foot a day in peak growing season!), and gives new meaning to the term "biomass." It's dense growth pattern can mean tens of thousands of plants occupying a single acre of land, and its root systems may extend to a depth of twelve feet, making it particularly difficult to eradicate once established. The species has become hugely problematic for a variety of reasons -- able to crop up on roadsides, hillsides, empty lots and nearly any plot of disturbed soil; quickly covering wide expanses of acreage with its dense growth; choking out virtually all varieties of plant life in its path; killing even mature trees by growing over them and blocking out sunlight, and even covering over and growing into buildings. Some small towns in the rural South have virtually been "swallowed up" by kudzu.