Integrated Weed Management

Carrot is a slow-growing crop that suffers severe yield loss from weed competition. Its thin, feathery leaves do not shade out competing plants and its long growing season creates an opportunity for successive flushes of weeds throughout the growing season. The first four weeks of crop growth are an especially critical period for weed control. At harvest, weeds also present problems when they become entangled in the equipment and in the crop.

An integrated weed management program is essential for carrot production because registered herbicides do not control many problem weeds in carrots, mechanical cultivation and hand hoeing are of limited value because of the high crop plant populations, and carrots are not competitive against weeds. A weed management program that combines good cultural practices with the use of herbicides will control many weed pests of carrots. However, the limited number of herbicides available and the diversity of weeds that grow in carrot fields make it difficult to maintain adequate control throughout the growing season. Generally, more than one herbicide is necessary to effectively control the different weed species. Usually, a preplant or preemergence herbicide is applied followed by one or more postemergence treatments.

The choice of herbicide depends upon the weed species that are expected to occur. Plantback restrictions need to be considered when selecting herbicides because soil residues of some products can limit the growth of sensitive rotational crops. Herbicide labels are the best source of plantback restrictions (i.e., rotational intervals).

Nonchemical options include solarization and flaming. Carrots have a long germination period and flaming must be performed after the weeds have emerged but before the carrots emerge. Carrots are planted with 4 to 8 seed lines on the top of a 40- to 42-inch bed. Since the bed top is only 22 to 24 inches wide, there is no room to cultivate the planted area. Only the shoulders and furrows are cultivated. Additionally, because of the high-density planting, hand weeding is expensive and can seriously damage the crop by removing carrots with the weeds.

Monitor fields and keep records of the weed species that occur in each field during the period of the year when the crop will be grown. Records of weeds occurring at planting time are especially important. Not only are these records valuable in planning which fields to use to grow carrots, they also help track the occurrence of hard-to-control weeds. Avoid fields with high populations of certain weeds such as sowthistle, shepherd's-purse, nightshades, and nutsedge.

Avoid planting carrots in fields last planted to cereals or in fields with known infestations of perennial weeds: available herbicides do not effectively control perennial weeds. To prevent the buildup of weed seed in the soil, cultivate weeds before they set seed in rotation crops. After harvest of the rotation crop, clean-cultivate the field or plant a green-manure crop to prevent weed infestations. In a typical farm mix of crops, carrots should be planted in the most weed-free fields.

Soil solarization can be used to control most weeds in carrots. It will also control or suppress some other important pests, such as nematodes and soilborne diseases. Soil solarization requires a summer 4- to 6-week fallow season for treatment; it fits in best with a fall-planted crop in the warm Central and Imperial valleys. Solarization is most effective in the top 6 inches of soil. For further information, see Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds (UC ANR Publication No. 21377).

Metam sodium is used in many carrot fields throughout California, principally for control of soilborne fungal diseases and nematodes. It will also kill emerged weeds, ungerminated weed seeds that have become softened by irrigation, and nutsedge shoots. Application is made about two weeks after a preirrigation before planting the crop. Typical application is through solid-set sprinklers or into a flood irrigation. The minimum time allowed between application and planting the crop is 14 days and can be up to 60 days depending upon environmental conditions, so planning ahead is important.

About two weeks before planting carrots, preirrigate the field to germinate weed seedlings and cultivate to destroy them. Carry out this operation as close to planting time as possible to assure that soil temperature and climatic conditions are similar to those that will occur during the crop germination period, thus maximizing the number of weeds controlled. Cultivate as shallowly as possible to avoid bringing up dormant weed seed from deeper soil layers.

Herbicides that are available to use preplant in carrots include paraquat (Gramoxone), trifluralin (Treflan),  glyphosate (Roundup), and pyraflufen ethyl (ET Herbicide/Defoliant). Glyphosate has been particularly helpful in controlling perennial weeds when used as a preplant treatment. Trifluralin will control many annual weeds when used for the entire carrot production season. These products can be used to control emerged weeds just before planting or before the crop emerges. Be sure the crop has not emerged, however, because emerged plants will be killed if contacted by these herbicides.

Weeds must be controlled in a carrot field throughout the growing season and require some postplant control measures. Close cultivation and hand hoeing must be done carefully to not cause injury to the crop. An herbicide is generally applied before or after the crop emerges, depending on the weed species to be controlled.

Preemergence

Trifluralin can also be used as a preemergence herbicide in carrots if applied by chemigation and done immediately after planting and before weed germination. This method of use will increase the activity on shallowly emerging weeds, such as common purslane, but will limit soil longevity and effectiveness on weeds germinating from deeper in the soil.

Linuron (Lorox) and prometryn (Caparol) are used for preemergence applications; be careful not to exceed label recommendations on rate. These herbicides will control annual broadleaf weeds better than trifluralin, particularly nightshade, mustards, and sowthistle.

S-metolachlor (Dual Magnum) can be used for preemergence application on yellow nutsedge, but care must be taken to select the correct rate. Lower amounts must be used on coarse-textured soils. This herbicide will provide partial control of yellow nutsedge and good control of several broadleaf and grass weeds, including nightshades.

Postemergence

Following crop emergence, clethodim (Select Max), fluazifop-P-butyl (Fusilade), Metribuzin (Tricor DF), and sethoxydim (Poast) can be applied for grass control, and linuron (Lorox) for annual broadleaf weeds and suppression of yellow nutsedge. Grass herbicides are effective in controlling small seedling annual grasses and some perennial grasses. Their effectiveness is reduced when grasses are under moisture stress. Later growth stages of annual grasses are more difficult to control. Of these herbicides, clethodim is the only one that will control annual bluegrass.

Linuron and prometryn can also be used as postemergence herbicides in carrots. Linuron is applied over the top of the crop when the carrots are 3 inches tall, whereas prometryn can be used until the end of the six-true-leaves-stage. Both control emerged weeds and also have soil residual activity against later emerging weeds. Linuron will suppress yellow nutsedge but has little to no effect on purple nutsedge. A repeat application of Linuron is allowed, but it is limited to a total of 1.5 lb a.i./acre per season. Some carryover can occur under certain conditions, creating a plantback problem. Consult the herbicide label before application.