The start of a nutsedge problem begins with one tuber in the middle of a field. The next year it will be a clump 40- feet wide. By the third year, nutsedge will be all over the field. That's how fast purple nutsedge can take over a field if left uncontrolled, according to Steve Fennimore, University of California weed control specialist based in Salinas.

Fennimore is in the second year of a study designed to control nutsedge in the vegetable fields of the Coachella Valley, where herbicides alone have not provided the weed control necessary to economically grow crops like peppers. Economics is a key factor when controlling weeds in peppers. “Nutsedge can affect yields and severe infestations can cost more to hand weed than the crop is worth,” Fennimore says.

“This is a serious weed and one best managed by constant management. A good strategy is to use chemical and cultural control methods to deplete the nutsedge population to manageable levels.”

Purple and yellow

Two species of nutsedge inhabit fields of California — purple and yellow. Purple is found from central California south to the tropics. It does not tolerate soil temperatures much below freezing. Yellow nutsedge is found in a much wider range, mostly because it can tolerate soil temperatures down to zero degrees.

“The best way to tell the difference between the two species is to dig them up with a shovel,” Fennimore says. “Purple nutsedge has football-shaped tubers shooting out of the rhizome of a mother plant. As it grows, it continues to add tubers. Yellow nutsedge puts out only one tuber for each rhizome.

“The continuous tuber production of purple nutsedge makes it more aggressive and helps it spread rapidly. The tubers have a lot more energy than a seed, which means when they come out of the ground, they are ready to grow and ready to do damage. It's that early vigor that can cause so much competition for young plantings.”

Part of what makes nutsedge so difficult to control is the depth at which some of its tubers germinate. “Eighty percent of the nutsedge tubers are in the top six inches of soil,” Fennimore says. “That's the easiest place to kill them with a fumigant. Ninety-five percent are in the top 18 inches. That leaves 5 percent that are very deep. It's those deep tubers that can come up and cause you to lose control of a field again if not managed somehow.”

There are herbicides to control nutsedge, but they have drawbacks. The Eptam herbicide/fallow program can be somewhat effective if done carefully. Eptam is labeled for fallow land and has a 90-day plant back to cole crops, lettuce and some other vegetable crops.

Herbicide controls

“Eptam kills nutsedge shoots as they grow through the treated layer of soil, but is has no effect on the tubers themselves,” Fennimore says. “Roundup also can be applied to fallow land, but is not thoroughly translocated in the nutsedge plant and provides only partial control. Lastly, Sandea is labeled for in-crop use and has good activity. However, lengthy plant back restrictions — such as 18 months for lettuce and 24 months for spinach — limit its use in cool-season vegetable fields.”

In Florida, where much of the state's tomato and pepper crops are grown on plastic, researchers continue to seek alternatives to methyl bromide. Florida growers have traditionally used methyl bromide to control nematodes, soil-borne diseases and weeds, especially nutsedge. Research has shown that the leading alternative for nematode and disease management has been a combination of Telone soil fumigant and chloropicrin. That combination, however, lacks the weed activity of methyl bromide so a herbicide, such as Tillam, is typically applied to compliment Telone/chloropicrin for nutsedge control.

Fallow field study

Fennimore thought a similar herbicide/fumigant program could work in California vegetables.

In June 2003 in a fallow field to be planted to lettuce, Fennimore began his study. Telone C-35 was custom applied by Western Farm Service at a rate of 28 gallons per acre and at a depth of 18 inches. Additional treatments in the study included Eptam at 7 pints per acre alone and water-run metam sodium at 40 gallons per acre followed by Eptam at 7 pints. Eptam was applied over the metam sodium and Telone C-35 plots 12 days after fumigant application. “We're applying these materials far in advance of the planting date because Eptam has a 90-day plant-back in lettuce,” Fennimore says. “Include the re-entry interval for Telone and you'll see that this program needs to happen almost 100 days before planting.”

Lettuce was planted Oct. 17 with nutsedge control evaluated before thinning on Nov. 2. “Both purple and yellow nutsedge were found in the field,” Fennimore says. “Weed pressure was light to moderate and the Telone C-35 plus Eptam treatment did a really good job.”

The plot treated with Eptam had 1,287 nutsedge plants per acre. The metam sodium/Eptam plot has 1,023 nutsedge plants per acre and the Telone C-35/Eptam plot had 46 nutsedge plants per acre.

“From work done in strawberries, I knew that chloropicrin by itself would provide a good level of weed control,” Fennimore says. “The addition of Telone (to chloropicrin) creates a synergism that provides quite a bit of weed control. When Telone C-35 is applied deep, it moves throughout the soil profile and does a thorough job of killing nutsedge tubers.

“Knocking the weed population down and keeping the seed bank low will be necessary if a field is to be farmed long-term. A nutsedge-infested field cannot be managed with one year in mind. Growers need to make an investment in the field.”

Fennimore's experiment is being repeated this year in another lettuce field near Indio.