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 4 weeks of crop growth is 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 not feasible 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 pre-plant or pre-emergence herbicide is applied, followed by one or more post-emergence treatments.

The choice of herbicide depends upon the weed species that are expected to occur. Plant-back 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 plant-back restrictions.

Non-chemical 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 6 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 not practiced, and can seriously damage the crop by removing carrots with the weeds.

Monitoring
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, nightshades, nutsedge, pigweed, and purslane.

Management Before Planting
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. Soil solarization requires a summer fallow season for treatment; it fits in best with a fall-planted crop.

Metam sodium is used in carrot fields principally for control of soil-borne fungal diseases and nematodes. It will also kill emerged weeds, ungerminated weed seed that has become softened by an irrigation, and nutsedge shoots. Application is made about 2 weeks after a pre-irrigation, which should be made before planting the crop. The typical application method 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.

To control nutsedges, EPTC may be used during the fallow season, if allowed. Apply it in summer before a fall carrot crop to reduce nutsedge tuber populations. Be sure to use it according to label directions; the herbicide must be applied 90 days before carrot sowing, and the field must be irrigated 30 days before planting.

About 2 weeks before planting carrots, pre-irrigate 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 pre-plant in carrots include paraquat (Gramoxone) and glyphosate (Roundup). 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. Glyphosate has been particularly helpful in controlling perennial weeds when used as a pre-plant treatment.

Trifluralin is a commonly-used pre-plant incorporated herbicide in carrots. It will control many annual weeds when used for the entire carrot production season.

Management After Planting
Weeds must be controlled in a carrot field throughout the growing season, and require some post-plant control measures. Close cultivation and hand-hoeing are not practiced, because the injury to the crop is generally greater than the benefit received. An herbicide is generally applied before or after the crop emerges, depending on the weed species to be controlled.

Pre-emergence. Trifluralin can also be used as a pre-emergence herbicide in carrots. Sprinkler irrigation is required for incorporation and activation of the material, if it is applied after planting. This method 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) can be used for pre-emergence application. Crop safety is marginal; be careful not to exceed label recommendations on rate. This herbicide will control annual broadleaf weeds better than trifluralin, particularly nightshade, mustards, and sowthistle.

Post-emergence. Following crop emergence, fluazifop-P-butyl (Fusilade) can be applied for grass control, and linuron (Lorox) for annual broadleaf weeds and yellow nutsedge. Fluazifop-P-butyl is effective in controlling small seedling annual grasses and some perennial grasses. Effectiveness is reduced when grasses are under moisture stress. Later growth stages of annual grasses are more difficult to control. Follow label instructions regarding the use of adjuvants with fluazifop-P-butyl.

Linuron can also be used as a post-emergence herbicide in carrots. It is applied over the top of the crop when the carrots are 3 inches tall. It controls emerged weeds, and also has soil residual activity against later emerging weeds. Linuron will control (suppress) yellow nutsedge, but has little to no effect on purple nutsedge. A repeat application is allowed, but a total of 3 lb a.i./A is the limit per season. Some carry-over can occur under certain conditions, creating a plant-back problem. Consult the herbicide label before application.