Since 1958, the National Oceanic and Atmospheric Administration (NOAA) Climate Monitoring Laboratory at the Mauna Loa Observatory in Hawaii has measured the amount of carbon dioxide (CO2) and other greenhouse gases in the air. They tell us that carbon dioxide in our atmosphere has been increasing. There is a lot of debate about what the global climatic effects of more greenhouse gases will be. But we do know that when atmospheric CO2 is increased, plant growth rates increase. More and bigger plants fertilized by increased CO2 sounds good. I can imagine raising a 500-pound pumpkin, entering it in the county fair and winning the blue ribbon. As those CO2 rates continue to climb, crop yields should go up. That means enough corn for human consumption, as well as to make ethanol for fuel and to feed beef cattle. I like corn on the cob, thick steaks on the grill and Sunday drives through the countryside.

Thinking about it a bit more brings the realization that not all plants benefit mankind. Weeds are plants, too. Weeds are already beginning to make news because they've been growing bigger from CO2 fertilization during the past decade. Their toxic effects are getting worse, too.

Careful yard and garden cleanup is more important now than it ever was. Many weeds are species which have nothing inherently wrong with them but grow in the wrong place — wild violets in lawns or oxalis in flowerbeds, for example. They fit the definition of a weed as "a plant out of place." These plants are unwanted because they take moisture, nutrients, sunlight and landscape space that should be going to desirable plants. It will take more effort to get rid of weeds that are larger and more numerous in the future. Once "super-sized" weeds pumped up by more atmospheric CO2 start multiplying throughout your landscape, they'll be harder to control.

Homeowners also need to help knock out invasive species by eliminating any growing on our own properties. Invasive weeds make trouble by winning the competitive battle for available resources over native plants and wildflowers. As a result, desirable plants die and biological diversity is destroyed at sites where these weeds take over.

Crown vetch (Coronilla varia) and kudzu are invasive species that were imported for erosion control but got out of hand in the southern states. Crown vetch spreads by both airborne seed and by vigorously growing stems and roots. Dense vetch foliage can prevent other plants from taking root and overgrow nearby plants, shrubs and even small trees — killing them by blocking sunlight the desirable plants need to survive. Indiana has a good fact sheet on crown vetch with detailed pictures available for download to your home computer from the Web site www.invasivespecies.IN.gov.

Another group of weeds causing more severe problems to humans in recent years are those which introduce pollen allergens into the air, such as ragweed (Ambrosia artemisiifolia L.) or that produce skin irritants on contact like poison ivy (Toxicodendron radicans). Ragweed releases lots of pollen that causes allergic reactions or asthma attacks during late summer and autumn (August through November). Other weeds that contribute to the pollen count at this time of year are curly dock, lambs quarters, pigweed, plantain and sheep sorrel.

Chop down or bush hog these bad actors if you're trying to create better air quality around your home, but leave the golden rod. Golden rod has been falsely accused of stimulating allergic reactions. It has showy yellow flowers at the same time annual ragweed is in bloom but does not produce the airborne pollen that irritates your eyes and makes your nose run — ragweed is to blame for that. Find out more from the National Institute of Environmental Health Sciences page www.niehs.nih.gov.

Plateau Gardening is written by Tennessee Master Gardeners about home landscapes and gardening in our state's Upper Cumberland Region. For answers to specific yard and garden questions or to learn how to become a Master Gardener contact UT Extension Cumberland County, P.O. Box 483, Crossville, TN 38557 (phone 484-6743). E-mail inquiries to mgardener@volfirst.net. E-mail may be answered either individually or in future newspaper articles. Visit the companion Web site web.infoave.net/~mgardener to view photos and seasonal tips or to see Cumberland, Putnam and Smith County growing conditions.