Sure, the growing season is over and cooler weather is moving in, but don’t think the weeds in your lawn and garden are taking a little R & R. In fact, many of them, especially invasive weeds, are still vexing unsuspecting victims or are busy plotting their next move so they can come out swinging at the first hint of spring next year.

"Fall can be an important time of year for some native and exotic weeds," says Jeffrey Derr, Ph.D., Virginia Tech Weed Scientist and presidentelect of the Weed Science Society of America. "Plants like yellow nutsedge, bermudagrass and phragmites are building underground storage reserves before they go dormant. Early autumn gusts can be quite effective at dispersing seeds of summer-flowering plants like Tree of Heaven. Winter weeds, like annual bluegrass, common chickweed, and wild garlic are just starting to emerge, while rosettes of garlic mustard will be around in the fall and winter. That's why awareness of invasive plants shouldn't die out with the onset of cooler weather," warns Dr. Derr.

Some to watch out for in the fall:
  • Poison ivy (Toxicodendron radicans)
    Though the leaves may have fallen off this native species for the season, the vines of this woody perennial can still cause an allergic reaction. The sticky urushiol oil, which causes the irritating reaction, doesn't evaporate, so touching shoes or clothes that may have rubbed against a plant and are carrying the oil can cause an allergic response even hours later. So beware of petting any animals that may have been exposed. Never burn poison ivy since the toxin can be inhaled from smoke. Also, be careful when cutting firewood since you may not notice poison ivy vines that have grown up trees.
  • Common ragweed (Ambrosia artemisiifolia)
    This native summer-annual weed releases one billion pollen grains from each plant, beginning in August and continuing through October, triggering allergic reactions for nearly 36 million Americans each year, according to the American Academy of Allergy, Asthma and Immunology. Unfortunately, common ragweed grows almost anywhere in the U.S.
  • Purple loosestrife (Lythrum salicaria)
    Don't be fooled by those pretty purple flowers. This invasive, exotic perennial will flower from June through September. But once the flowers are gone, that's when the plant goes to work and releases millions of seeds which can be carried great distances. Seeds that don't germinate can remain dormant for long periods. That's why eradication should take place before seed production begins. This tenacious plant can even sprout from fragments. So don't mow over them because new plants can sprout from the cut pieces.
  • Creeping charlie/ground ivy (Glechoma hederacea)
    A non-native and cool season perennial weed, ground ivy grows best in spring and fall. Ripping it out of the ground or treating large patches with the appropriate herbicides now may help salvage your lawn for next spring. Maintaining a thick, healthy turf through reseeding and feeding, while paying particular attention to the bare spots, helps keep this unwelcome intruder from moving in and taking over.
  • Giant hogweed (Heracleum mantegazzianum) Once a prized, exotic plant in many home gardens, giant hogweed has held a place on the Federal Noxious Weed List since 1983 because of its potential harm to humans. The sap of the giant hogweed plant can cause severe blistering if the affected area is exposed to sunlight. As its scientific name suggests, this plant can reach Herculean heights of fifteen feet and can quickly dominate an area, crowding out native plant species. You must also be careful of the seeds it releases once it is done flowering because they also can cause irritation and blistering.
  • Garlic mustard (Alliaria petiolata )
    Garlic mustard, a cool season biennial that has settled in many parts of the country, has a garlic odor when its leaves or stems are crushed. It is shadetolerant and found in forests, near streambanks, and along roadsides and trails. Hundreds of seeds are produced from each plant and can remain viable for up to five years. In the first year, these plants remain green throughout the winter, making them easy to spot for control or removal actions.
The effects of weed species on the nation’s agriculture, water quality, wildlife and recreation have been estimated to cost the U.S. $34.7 billion annually, according to a recent Cornell University report. "Invasive exotic plants and problematic native weeds are present in every region of the country," says Dr. Derr "It is not an isolated problem. So everyone must play a part in helping control these troublesome invaders."

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About the Weed Science Society of America:
The Weed Science Society of America, a nonprofit professional society, was founded in 1956 to encourage and promote the development of knowledge concerning weeds and their impact on the environment. The Weed Science Society of America promotes research, education and extension outreach activities related to weeds, provides science-based information to the public and policy makers, and fosters awareness of weeds and their impacts on managed and natural ecosystems. For more information, visit http://www.wssa.net.