Weed Control: Case Studies in Flowers and Strawberries

Weed control in farm situations is a vexing problem. Until now, methyl bromide has handled the job admirably, but with the mandatory 50 percent reduction next year in the chemical's availability, alternatives must be found and tested. Whereas methyl bromide was the fumigant for any number of crops and situations, life without methyl bromide will require different approaches for different crops to get effective weed control.

Soil solarization can be used as a preplant treatment for weed control and soil-borne pathogens, but must have the proper conditions, such as climate and high solar radiation. Some crops, such as high-quality flowers, are grown in climates not conducive to soil solarization, but could greatly benefit from its use.

Field-Grown Flowers

In an effort to increase effectiveness of solarization in the coastal regions of California, Clyde E. Elmore, extension weed specialist, Weed Science Program, University of California, Davis, conducted field studies to determine the effect of additives such as metham, ammonia, and some organic materials to the soil solarization process, as well as to examine the effectiveness of various types of coverings to improve solarization in the growing areas.

The use of metham in conjunction with soil solarization, at varying rates of 76 to 153 L/ha, increased weed control over soil solarization alone. The solution was applied using drip injection beneath the plastic tarp. This combination increased pest control over the same rates of metham alone. Ammonia at 459 L/ha injected into holes on 30-cm centers was not as effective, liter for liter, as metham. Organic materials may offer some increased effectiveness as an additive to soil solarization. Studies show that composted chicken manure at 7,250 kg/ha or 2,240 kg/ha of chopped broccoli covered with polyethylene increased control of some weeds over soil solarization alone in coastal sites.

While the studies show that chemical or organic additives to the soil solarization process increase weed control, these combinations are still not as effective for broad-spectrum weed control as methyl bromide.

Strawberries

One strategy to obtain effective weed control in strawberry crops is to enhance the efficacy of alternative fumigants to methyl bromide with the use of herbicides, in order to bolster the fumigants' weed control capacity.

S.A. Fennimore, extension vegetable weed control specialist, and S.J. Richard, both at the University of California, Davis, investigated several candidate herbicides at various planting stages on Camarosa and Selva strawberries. The site, near Salinas, California, was fumigated with 125 lb/acre of chloropicrin. This was followed 2 weeks later with pretransplant treatments of corn gluten meal at 300 lb/acre and 400 lb/acre and DCPA at 9 lb/acre. Two days later, the strawberry cultivars were transplanted in a 52-inch bed with two rows per bed—one row of each variety.

The corn gluten meal and DCPA produced little or no crop injury and had no adverse effects on plant diameters or crop biomass. Corn gluten meal, at the 400 lb/acre concentration, reduced annual bluegrass by 54 percent but provided no control of shepherdspurse, clover, or corn spurry. DCPA provided 100 percent control of annual bluegrass and corn spurry, but had no effect on shepherdspurse or clover.

In evaluating candidate herbicides for post-transplant use, two rates of each of the following were tested: carfentrazone (0.075 and 0.15 lb/acre), cloransulam (0.0156 and 0.0313 lb/acre), dimethenamid (0.94 and 1.2 lb/acre), flumioxazin (0.063 and 0.125 lb/acre), halosulfuron (0.032 and 0.047 lb/acre), isoxaben (0.5 and 1.0 lb/acre), rimsulfuron (0.0156 and 0.0313 lb/acre), and sulfentrazone (0.175 and 0.25 lb/acre). Napropamide was tested at 4 lb/acre only.

The following treatments resulted in acceptable crop injury and had no effect on plant diameters or biomass: carfentrazone at 0.075 lb/acre, sulfentrazone at 0.175 lb/acre and 0.25 lb/acre, and isoxaben at 0.5 lb/acre. Marginally acceptable crop injury was shown with the use of carfentrazone at 0.15 lb/acre, napropamide at 4 lb/acre, and flumioxazin at 0.063 lb/acre. Due to unacceptable crop injury, cloransulam, dimethenamid, halosulfuron, and rimsulfuron were eliminated as candidates.

Of the five candidate herbicides with marginal or acceptable tolerance by strawberries, carfentrazone at 0.075 lb/acre provided 100 percent control of shepherdspurse, 40–77 percent control of annual bluegrass and corn spurry, and no control of clover; napropamide at 4 lb/acre and sulfentrazone at 0.25 lb/acre provided 100 percent control of annual bluegrass and corn spurry, 73 percent control of clover, and 67 percent control of shepherdspurse; isoxaben at 0.5 lb/acre provided 100 percent control of clover, corn spurry, and shepherdspurse and 31 percent control of annual bluegrass, flumioxazin at 0.063 lb/acre provided 100 percent control of annual bluegrass, clover, and corn spurry and 73 percent control of shepherdspurse.

Delayed post-transplant applications of isoxaben at 0.25 lb/acre and triflusulfuron at 0.0156 and 0.0313 lb/acre were made 3 weeks after transplanting with little or no crop injury. All other treatments resulted in unacceptable crop injury or significant reduction in plant diameter or biomass. Triflusulfuron at 0.0313 lb/acre provided 100 percent control of annual bluegrass and shepherdspurse and 70 percent control of clover and corn spurry. Isoxaben at 0.25 lb/acre resulted in 100 percent control of annual bluegrass, clover, and corn spurry and 73 percent of shepherdspurse.

According to Fennimore, weed control with these compounds is not as reliable as methyl bromide. "Currently registered herbicides are not adequate as replacements for methyl bromide," he says. "There is a need to identify new herbicides."

Of the registered herbicides examined in his study, Fennimore feels sulfentrazone works best as a pretransplant and immediate post-transplant herbicide, with little negative effect on plant stand, and no negative effect on plant diameter or biomass. Triflusulfuron, according to Fennimore, has the greatest potential for use as a delayed post-transplant herbicide.

More studies are needed to evaluate the merits of carfentrazone, flumioxazin, isoxaben, and sulfentrazone as pretransplant or immediate post-transplant herbicides, and triflusulfuron as a delayed post-transplant herbicide in strawberries.

Integrated pest management, at least for now, will need to be employed by some growers to create an effective alternative.

Last Updated: February 24, 2000