National Invasive Species Week is February 26 - March 3, 2012 (my first reaction was "who thinks of these things?" but my second reactions was "gotta love it!)
What is an invasive species? According to the USDA: "Invasive plants are introduced species that can thrive in areas beyond their natural range of dispersal. These plants are characteristically adaptable, aggressive, and have a high reproductive capacity. Their vigor combined with a lack of natural enemies often leads to outbreak populations." See http://www.invasivespeciesinfo.gov/plants/main.shtml
Invasive species are weeds, but more so. They can tolerate a range of conditions and kill other plants by smothering (through the shear bulk of their leaves, stems and roots), out growing them or through allelopathy, that is, the production of substances that limit the growth of other plants (though not an invasive, the walnut tree is a well-known, allelopathic plant, producing juglone, a chemical that inhibits growth of many other plants - look online for listing of plants not disturbed by juglone). Many common garden plants are invasive (English ivy, obedient plant, purple loosestrife, and some ornamental grasses like river oats). The VA Department of Conservation and Recreation publishes a list of invasive exotics at: http://www.dcr.virginia.gov/natural_heritage/documents/invlist.pdf
What to do? Remove invasive plants from your yard and garden and don't buy them in the first place (Remember the 11th commandment: "Know thy plants"). Help groups like The Nature Conservancy and parks and conservation groups on work days where they remove invasive plants...and...spread the word!
Happy non-invasive gardening!