An effective weed management program takes into account the type of weeds present, crop rotation, cultivation, available herbicides, and the competitive ability of the potato crop. Competition from early season weeds will reduce yields if they are not controlled within 4 to 6 weeks after potatoes emerge. Weeds that emerge after vines have covered the rows usually will not compete with the potato crop; however, they may reduce yields by interfering with harvest and can produce seed that will cause infestation of subsequent crops. Weeds frequently become more serious if crop growth is delayed by adverse conditions early in the season. Weed problems can be reduced by establishing a vigorous stand of potatoes.
Weeds and potato management vary in the two major potato-growing regions of California: Kern County and the Tule Lake region. In the Kern County region, soils are sandy or sandy loam with organic matter levels below 1%. Plantings are made from late November until early March for the spring and summer crop and in July for the much smaller acreage winter crop. In the Tule Lake region, soils are largely silty clay loams with organic matter levels of 3 to 15%; plantings are made in May and are harvested in fall. Consequently, not only do cultural practices vary in the two regions but weed species and herbicide usage as well.
To plan a weed management program, you must know what kinds of weeds are present, which ones are most abundant, and whether their abundance is changing. Regularly survey each field to determine which weed species are present and the effectiveness of previous weed control measures. During the early part of the potato season, especially before row closure, make weekly surveys. Pay special attention to perennials and, where they occur, dodder infestations. Also be sure to record annual weeds that have produced seed. Where potatoes are in the crop rotation, note weeds in the previous crops as well.
To survey the weeds, randomly walk through the field and rate the degree of infestation for each weed species. Sketch a map of the field and mark where perennials occur. Maps are very useful for rechecking the same areas each season to determine if special controls are needed during fallow or rotational crops. This information will become increasingly valuable over the years in predicting weed problems. Record weed survey information (example form—PDF).
Some potato varieties are competitive with weeds and can be grown without herbicides if conditions, such as soil moisture, are conducive to weed emergence before the potatoes. Most weeds can then be removed by a cultivation that is timed just before potato emergence. A few weeds, however, such as nutsedges and nightshades, emerge with and after the potatoes and are best controlled with crop rotation or herbicides.
The competitive ability of the potato plant varies considerably among the different varieties. If you want to grow potatoes without herbicides, choose one of the more competitive varieties. White Rose, Ute russets, Chieftain, and Red LaSoda are very aggressive and cover rows quickly. The new Russet Norkotah tends to be a bit more upright and varies in cover, while the Tejon White is somewhat less aggressive. Sometimes disease, lack of nitrogen, air pollution, or climatic changes can alter how these varieties grow. Varieties that are harvested green for chipping have less problems with weeds, including nutsedges, invading after early season cultivation because the vine canopy remains until harvest.
Crop rotation and cultivation can both be used effectively to manage weeds. Crop rotation is useful in controlling difficult weed problems because it allows for a greater variety of weed control methods. Cultivation can also greatly reduce weed populations if already emerged.
Herbicides are a principal component in most potato weed control programs. Herbicide choice depends not only on present weed species, but on soil type, cultural practices, potato cultivar herbicide tolerance, and herbicide availability. In many fields two or three herbicide applications may be needed during the potato growing season. Use herbicides that will control the weed species in your field with the appropriate application method, rate, and timing, and if possible, kill weeds before they grow beyond the seedling stage.
When carefully used, herbicides labeled for use in potatoes grown in California control most annuals that presently infest potato fields, as well as some perennial weeds such as nutsedge, johnsongrass and bermudagrass. There are herbicides which are soil-active only (will not control emerged weeds), foliar-active only (will not control weeds which have not emerged) and herbicides that have both soil and foliar activity. Depending upon the label and the weeds, herbicides can be applied preplant; after planting, preemergence to weeds or potatoes or both; or postemergence to potatoes and weeds.
When soil-applied, preplant or layby herbicide applications are used on high organic matter soils, elevated rates may be required for acceptable weed control. Carefully follow all herbicide label recommendations for use rates on various soil types.
In some areas, vine-kill or desiccants are applied in preparation for harvest. These are usually contact, postemergence (foliar) herbicides. In case of field-stored potatoes, roll dead vines to break the stems of tall weeds and compress soil to prevent soil cracking. Use postemergence herbicides if additional weed control is necessary.
Weed Management Before Planting
Cereals, including small grains, corn, and milo (sorghum), are valuable in rotation; herbicides that cannot be used in potatoes can be used for controlling problem weeds in these crops and after their harvest. Perennial weeds can also be controlled after a grain harvest by discing and watering to encourage weed growth in fall and then applying a postemergence, translocated herbicide to the growing weeds. Dry or irrigated fallowing after May/June harvests in Kern County permits control of many weeds. Crop rotations also can help to control disease and nematodes.
Alfalfa is a good rotation crop on marginal soils that will not support potato production in short rotations. Alfalfa shades out many annual weeds and repeated mowing of alfalfa rids the field of emerged weeds before they produce seed and helps reduce infestations of some troublesome weeds. However, alfalfa is susceptible to some root-knot nematode species which may diminish the crop’s usefulness in a potato rotation. Other crops closely related to potatoes, such as tomato, pepper, and eggplant are not good rotation crops because the available herbicides are similar to those used in potato. Other than an increase in the weed species not controlled by these herbicides, this fact may not allow for effective herbicide mode of action rotation to prevent or delay herbicide resistance development.
Potato fields should always be tilled after previous crops are harvested to reduce weed seed or propagule production. Further tillage should be done if weeds grow and approach their reproduction stages before potatoes are planted.
A preplant herbicide is used during field preparation or fertilizer application before planting potatoes. Preplant, soil-active herbicides usually are incorporated into the soil mechanically or with sprinkler irrigation before weeds emerge in order to kill germinating weed seeds or just as weed seedlings are emerging. Nonselective foliar-active herbicides such as paraquat and glyphosate also can be applied before planting to control emerged weeds.
For nutsedge control in Kern County, the soil-active herbicide EPTC should be applied by March 1. It may be applied preplant or postplant, but two or three applications a season are usually necessary. Metribuzin can be used in Siskiyou and Modoc County. Trifluralin and EPTC or Pendimethalin and EPTC are often used as preemergent weed control tank-mixes.
Weed Management After Planting
Depending on the type of irrigation system used, potato fields can be cultivated at several periods before the canopy closes over the rows to manage most weeds.
In Kern County, timing the plantings to take advantage of the considerable competitive nature of potatoes is a most beneficial method. On spring potatoes planted in winter, rows can be cultivated during the winter, just before potato emergence until the plants are 4- to 8-inch tall, if necessary. This removes winter weeds and often the first flush of spring weeds. On February plantings, a cultivation during the period just before emergence to the 4- to 8-inch stage, is usually adequate for season long control of most annual weeds because the potato plants grow so rapidly and outcompete most weeds, with the exception of nutsedges. In summer plantings, the intense heat favors rapid weed growth. Cultivate when weeds are small. Multiple cultivations are sometimes needed to control multiple weed flushes. Delay irrigation after cultivation as long as possible to prevent re-rooting and new weed germination.
In areas where potatoes are irrigated with solid set sprinklers, most growers are reluctant to cultivate after potatoes have significant shoot and root development due to the risk of root pruning and physical damage to shoots. Untimely hot spell can severely reduce yields or quality if solid set sprinklers are broken apart to permit cultivation and not put back together quickly to resume irrigation. Potatoes should be kept on the wet side of moist; they transpire a lot of water in hot weather from their rather limited root system. If used, cultivations should be very shallow and done about 3 to 5 days following irrigation. Another concern with tillage is the problem of soil compaction.
In the Tule Lake region, well-timed cultivations can greatly reduce the need for chemical control. Postplant hilling cultivations can eliminate weeds already emerged. However, in order to be effective, cultivation should occur when these weeds are less than 1 inch tall, enough soil is thrown over the weeds to kill them, and weeds which lay on top of the soil are allowed to dry out and die. Later emerging weed seedlings can be controlled by a second cultivation or by postemergence, foliar-active herbicides. A second cultivation before the row closes over may even eliminate the need for chemical weed control. If postemergence chemical weed control is still required, early cultivation will reduce early weed pressures as well as the size of the weeds needing to be controlled. To maximize weed control effectiveness and to minimize compaction, cultivate when soils are relatively dry. Fields just cultivated should not be irrigated for at least 24 hours to allow for complete desiccation of the uprooted weeds.
Depending on the weeds present, preemergence herbicides applied after potato planting also may be needed in addition to the preplant application. After planting, a combination of two or three preemergence herbicides is applied during or just after hilling. Cultivation just prior to preemergence herbicide application can kill weeds already-emerged. If additional control is necessary after potatoes emerge, certain herbicides may be applied through sprinkler irrigation (chemigation). Postemergence, foliar-active herbicides are sprayed onto emerged weeds, most effectively while they are small and actively growing.
Preemergence herbicides are normally applied during the first three weeks after planting and are incorporated into the soil using sprinkler irrigation or mechanically during cultivation and hilling. Application after the last cultivation is usually preferred to prevent disturbing herbicide-treated soil in the row. Soil-active, preemergence herbicides safe to emerged potatoes and applied two to three weeks after planting extend residual control. In this scenario, postemergence, foliar-active herbicides should be included in the tank-mix if emerged weeds are present.
Nonselective postemergence herbicides such as paraquat, glyphosate, or carfentrazone can be applied after planting but before crop emergence or crop injury will occur. Rimsulfuron and metribuzin have both soil and foliar activity and so they can be applied alone or in combination before or after weed or potato emergence. Rimsulfuron is also often split-applied before and after potato emergence or twice postemergence. Read variety fact sheets before using metribuzin as potato varieties vary in their tolerance to this herbicide. Selective postemergence grass herbicides such as clethodim or sethoxydim can be applied to control grass weeds.
Killing potato vines before natural senescence can aid in tuber maturity and skin-set and also kill some weed species still green in the field, but the benefits from vine killing are largely dependent on variety. Flail mowing, rolling, and chemical desiccation are the three traditional methods for vine killing. Mowing and desiccation help prevent weeds from interfering with mechanical harvest. They also may aid in preventing weed maturity and seed production.
In Tule Lake and other potato regions with high organic matter content soils, s-metolachlor (Dual Magnum II) can be applied to the soil surface before potato emergence for control of nightshade, yellow nutsedge, and other weeds. It is preferable to incorporate s-metolachlor by sprinkler irrigation, but it also can be incorporated by shallow tillage after planting. Dimethenamid-p and pendimethalin can also be applied preemergence on high organic matter soils present in the Tule Lake area. In such case, use the high rate of dimethenamid-p; application must be followed by rainfall, irrigation or shallow mechanical incorporation. Where soils have more than 0.5% organic matter, a postemergence metribuzin application or metribuzin and rimsulfuron combination may be used to control emerged broadleaf weeds. For best results, apply herbicides when weeds are small, less than 1 inch tall. Metribuzin is not recommended on some sensitive potato varieties, which includes early-season white-skinned and most red-skinned varieties.
If using EPTC in Kern County and an application has already been made preplant, another (sequential) application can be made after planting with rolling cultivators and incorporation into the beds, or by chemigation through solid set sprinklers after the beds are cultivated and shaped. Since this herbicide is volatile, follow the label carefully for incorporation timing. Results are sometimes erratic, especially when rain does not occur, and irrigations are not made soon enough after application to activate it before weeds emerge.
Pendimethalin or trifluralin usually are not needed if EPTC is used sequentially. Pendimethalin is often used postplant with EPTC to suppress nightshade. Pendimethalin applied at low label rates then sprinkler-incorporated before potatoes emerge effectively controls several wed species including pigweeds, lambsquarters, nightshades, and mustards with reasonable crop safety. On high organic matter soils, control may only be partial. A tank mix of pendimethalin and dimethenamid-p can also be beneficial and often provides better control than either product alone. Note that dimethenamid-p must be applied as preemergence to the potatoes. When pendimethalin is applied after potato emergence, the risk of crop injury increases; do not use after potatoes reach 6 inches in height. In varieties that have less top growth, adding a low rate of pendimethalin to the postplant application of EPTC enhances late season grass and pigweed control. On winter plantings, emerged nettle and winter mustards can be controlled with paraquat before potatoes emerge.
Weed Management at Harvest
In areas where potatoes are grown for early harvest, weed numbers in subsequent years may be reduced with weed control at this time since they are destroyed before their seeds mature. Alternately, after the vines and weeds have been shredded, a 2- to 6-inch soil mound can be created over the beds to help maintain soil moisture around the potatoes during the skin-set period. At the same time, weed germination is prevented because the surface soil is dry.
At harvest, rolled beds can be treated with paraquat or diquat for potato vine kill and to kill nutsedge and other weed foliage to stop further growth immediately. Diquat can be very effective. For vigorous potato varieties that often have late-season growth, two applications, spaced one week apart, may be necessary. Follow the label for correct rates. Burning with propane is sometimes used for vine kill, more to prevent early blight tuber infections and facilitate rapid mechanical harvest than for weed control. It can desiccate weeds with one pass over, but a second pass is required to ignite the weed vegetation. Do not use glyphosate!
Normally desiccants are not needed in Kern County, especially on chipping varieties; the vines desiccate when irrigation is discontinued. Diquat and glufosinate can be used for vine kill for stored and seed potatoes. Stem end browning caused by the use of desiccants is a concern of growers but rarely occurs in California.