With an ability to survive in harsh conditions, the noxious weed yellow nutsedge could be botany's equivalent of the cockroach.
“If the world comes to an end due to air pollution, we'll still have nutsedge,” joked a University of California weed expert at a recent field day with Merced and Madera cotton producers.
Yellow nutsedge, which looks like an herbaceous grass growing in home landscapes, gardens and agricultural fields, produces numerous underground tubers that make it one of the toughest weeds to control. In fact, preliminary research results are showing that yellow nutsedge is mostly unfazed by ozone, an air pollutant that, at current levels in the San Joaquin Valley, is robbing cotton growers of one-fifth of their crop.
The research indicates that, if valley ozone levels rise, nutsedge may become an even more fierce competitor with cotton than it is currently, according to Anil Shrestha, UC integrated pest management weed science advisor, and David Grantz, UC air quality effects specialist, both based at the UC Kearney Research and Extension Center near Parlier.
Shrestha and Grantz conducted experiments with nutsedge in large open-topped chambers at the Kearney Research and Extension Center. Cotton and nutsedge were planted in pots together and separately and grown in the chambers with three different air pollution levels: no ozone, ozone at approximately current ambient levels, and ozone double current ambient levels.
“Cotton is harmed by ozone, and with nutsedge, it fares even worse,” Shrestha said.
Grantz has found previously and in this recent study that ozone damage is evident not only in the part of the cotton plant visible above the soil, but also by how much less was growing below ground.
“The cotton plants were being choked by ozone and strangled by nutsedge,” Shrestha said.
Ozone consists of three oxygen atoms. It is created when volatile organic compounds react with nitrogen oxides — present in the air mainly from car and truck emissions — and sunlight. In the stratosphere, ozone protects Earth from harmful ultraviolet rays. At ground level, however, ozone is a powerful oxidizing agent that reacts with all organic matter — harming human beings, agricultural crops and weeds — but some species are more tolerant than others.
According to UC Davis air pollution research scientist Thomas Cahill, ozone in the San Joaquin Valley is worse now than in the Los Angeles area, where air pollution control efforts have kept ozone levels in steady decline over the past 30 years. In the valley, average ozone levels have stayed nearly the same for decades.
“Geography, topography and meteorology make the Central Valley and foothills an ozone machine,” Cahill said.
Nevertheless, the San Joaquin Valley is California's dominant agricultural area. Crops in the valley are being grown in complex agro-ecological systems, particularly with rising interest in integrated pest management. A critical component of IPM is reducing crop competition with weeds. Weeds cause considerable crop loss, despite extensive use of herbicides, other control technologies and human labor.
Shrestha and Grantz found that cotton in their experiment couldn't tolerate nutsedge in the “medium” treatment chamber as well as it did under no ozone. The plants in the medium chamber are exposed to air with up to 80 parts per billion of ozone, a treatment similar to concentrations experienced in the field on polluted days. Peak ozone concentrations of 130 ppb have been observed this year in the San Joaquin Valley.
“What's worse,” Shrestha said, “is that in the chambers where nutsedge was exposed to ozone, the weed is putting most of its resources into producing the underground tubers. These tubers help in producing new plants that could create problems for subsequent crops. Our study underscores the importance of controlling nutsedge very early in the season.”
If ozone in the valley increases, the weed threshold in cotton will be lower, he said. In addition, because cotton's root mass was dramatically affected by ozone, cotton could require more frequent irrigation in a higher-ozone environment.
Shrestha and Grantz also learned the prognosis is not quite so dire for all California crops. Tomatoes are relatively tolerant of ozone, so being exposed to high levels of the air pollutant doesn't give nutsedge an edge. The scientists are continuing their studies on ozone and crop-weed competition to assess the implications of this interaction on integrated pest management.