Weeds, Weeds, Weeds

Weeds. These unwanted plants can often bring a farm to its knees. One of the worst is nutsedge. Of the various types of weed, perennials such as nutsedge pose a significant problem because of their multiple reproductive systems.

Perennial weeds live for more than 2 years, regrowing each year from roots or rhizomes—tubular extensions below the soil surface—or from seed.

Methyl bromide is fairly effective in managing nutsedge, but alternatives are limited. "Though Tillam and Devrinol provide some nutsedge control, they are not nearly as effective as methyl bromide" says ARS weed scientist Erin Rosskopf of the Horticultural Research Laboratory in Fort Pierce, Florida. "Integrated weed management is necessary."

One part of integrated weed management that is under consideration is biological control—using living organisms to control pest organisms. Rosskopf's lab, in a cooperative project with R. Charudattan at the University of Florida, is evaluating a fungus as a biological control for purple and yellow nutsedge. The isolate was highly pathogenic to purple and yellow nutsedge, as well as globe sedge and rice flatsedge.

In the lab, purple nutsedge was inoculated with Dactylaria higginsii conidial suspensions, which resulted in significant reductions in shoot numbers, shoot dry weight, and tuber dry weight. The fungus was also tested in the field against purple nutsedge. Three postemergence applications resulted in about a 90 percent mortality of purple nutsedge. Greenhouse studies of purple nutsedge in tomatoes showed that D. higginsii reduced nutsedge competition so that tomato yields were equal to those from the weed-free control. Laboratory and field trials are currently being conducted to determine the compatibility of D. higginsii with postemergence pesticide applications.

The loss of methyl bromide to control nutsedge poses particular problems for minor crops such as tomatoes, peppers, and strawberries, since there are few chemical herbicides registered for use. Even with methyl bromide, losses in fresh market tomatoes in Florida due to weed pressure have been estimated at more than $291 million. Without methyl bromide that figure could substantially increase.

But nutsedge is an equal opportunity pest, striking agricultural crops the world over. Though nutsedge has earned a reputation as the world's worst weed, it is by no means the only one.

Types of Weeds

Weeds fall into three types: annuals, biennials, and perennials. Annual weeds are plants that reproduce by seed and germinate each year. Some common annual weeds include foxtail, velvetleaf, and cocklebur.

Winter annuals germinate in the fall and complete their reproductive cycle in the spring or early summer. This type of weed is likely to be found in winter-sown grains, no-till crops, or pastures where the soil remains undisturbed during the winter months. Summer annuals germinate in the spring and seed out in late summer or fall. Summer annuals flourish when summer annual crops such as corn or soybeans are grown, competing directly with the crop for resources.

Biennial weeds live through two growing seasons, although they also reproduce via seed. The first year is devoted to vegetative growth, and the second year has both vegetative and reproductive growth. Because of the 2-year cycle of growth, these weeds are found in areas of low soil disturbance such as pastures, waterways, and fence rows.

Perennial weeds grow for more than two years and are quite versatile, able to thrive in both areas of reduced soil disturbance or fields of row crops.

Plant seeds are dispersed in a variety of ways. Leafy spurge seed pods build up pressure and explode, shooting seeds up to 15 feet away. Small mammals and birds eat some seeds, carrying them away from the parent plant. Some seeds, like those of the dandelion, can become airborne and float away. Other seeds can attach to the fur of animals, thus hitching a ride to new locations.

Weed Management

"Weeds have a huge impact in yield and increased labor costs in agriculture," says Rosskopf. But in order to control them, weeds must first be correctly identified. Field scouting allows for identification and recording of all weed species found. Severity of infestation also needs to be determined by noting the number of weeds per row and their height relative to crops.

There are various methods of weed control: mechanical, chemical, crop rotation, and biological control. Some other less-utilized methods include controlled burning and crop competition.

Mechanical control can involve several different methods. Burial, by which all growing plants are buried under the soil surface, is most effective on annual weeds. It is less effective on perennial weeds, which have underground root systems and are capable of regrowth. Another method of mechanical control is cultivation—cutting the root systems of weeds. However, care should be taken because deep cultivation can damage crop roots. Deep cultivation may also pull more weed seeds to the soil surface where they will germinate.

Crop rotation is yet another method. If the same crop is planted in the same field year after year, weeds will appear that have become tolerant of the cultural practices and herbicides used on the crop. By rotating to other crops, the types of cultural practices and herbicides will change, thus not allowing weeds to become adapted to any particular regimen.

Chemical control is flexible in that it can be administered preplanting, pre-emergence and postemergence. Application can be broadcast, band, directed, or spot treatment, depending on weed type, severity of infestation, and location.

Biological control has not been used as a weed control agent to a great degree, but that may be changing. While no side-by-side herbicide tests have been conducted yet, D. higginsii seems to provide excellent control of nutsedge by acting as a foliar plant pathogen. Rosskopf maintains, "since there aren't any herbicides that are close to methyl bromide in effectiveness against weeds, biological controls may fill the niche." A potential advantage to its use in existing production systems is that it can be applied like any other foliar herbicide.

"Sometimes, it is forgotten that weeds have such a big impact on production systems. But it is still an area we need to continue to address," says Rosskopf. Weeds have been around for millennia, but methods exist to control them—albeit not without a fight.

Last Updated: November 22, 2000