Across much of America, farmers have already noted that spring is coming earlier and growing degree days are adding up faster. Weeds have sensed it, too, according to a U.S. Department of Agriculture study noted this week by the Weed Science Society of America.
One look at tropical agriculture will tell you that it's "a jungle out there." And the warming trends recorded in the last 50 years, teamed with rising amounts of carbon dioxide in the atmosphere makes everything grow more quickly – weeds included.
The impact of rising carbon dioxide levels on weeds can be striking. A study conducted by USDA Ag Research Service Agronomist Lewis Ziska in the Baltimore-Washington area confirms that weeds grown under urban conditions of warmer temperatures and more carbon dioxide – conditions anticipated for the rest of the world in 50 years – grew to four times the height of those in a country plot 40 miles away.
"Weeds are survivors," say Lee Van Wychen, director of science policy for the Weed Science Society of America. "They can fill various niches and thrive under a wide range of conditions.
'While we have about 45 major crops in the U.S., there are more than 400 species of different weeds associated with those crops. There's always another weed species ready to become a major competitor with a crop if growing conditions change, such as an increase in carbon dioxide levels."
So what if there are a few more weeds?
Ziska's research shows that common ragweed plants exposed to higher levels of carbon dioxide dramatically increased the amount of pollen produced. A doubling in carbon dioxide led to a quadrupling of pollen.
So maybe it's more than a coincidence that, as the U.S. Centers for Disease Control reports, the numbers of people suffering from "hay fever" are fast-rising. People who are allergic to ragweed pollen and other plant pollens suffer through the season with sneezing and watery eyes.
Ziska's work also suggests that recent increases in carbon dioxide during the last 50 years may have led to bigger poison ivy plants. And, these plants produce a more virulent form of the oil that causes people to break out in rashes.
"As the climate and carbon dioxide levels change, we can no longer assume the weed control strategies used in the past will continue to work," Ziska contends. "Not only are some of the nation's most invasive weeds spreading; they're becoming more difficult and costly to control. As researchers explore new approaches, we'll need to mix and match the strategies currently available."
A bigger 'so what' for farmers
Actually, there is a bigger "so what", reports Bill Curran, Penn State Extension weed specialist. "Ziska's study included lambsquarter, with similar growth results. When he treated the plants with Roundup, they metabolized it faster and were more tolerant of it."
And, what happens in non-crop areas can impact cropping systems, he adds. Years ago, kudzu, the predominantly southern invasive weed was not known in Pennsylvania. When it turned up in along the Susquehanna River, it was thought to be non-reproductive.
"That's not the case today," says Curran. "And kudzu is a prime winter host for Asian soybean rust."
That means continued vigilance to new or more tolerant weed species invading your cropping system and non-cropping environments. It means rotation of crops and crop protectants. With the rise of no-till, it means even more judicious management.