Asparagus is a perennial crop that is produced in several regions of California. Major counties of production include San Joaquin and Monterey. Each of these areas has specific weed problems and differences in climate and soil types that affect weed management decisions.
Weed management in asparagus can be divided into two periods: stand establishment, which lasts about 2 years, and mature plantings. Weed control during stand establishment is complicated in asparagus by the fact that three methods of establishment are used: direct seeding, transplanting seedlings, or transplanting dormant crowns. During stand establishment, tailor weed management techniques to the establishment method. If weeds are left uncontrolled during this period, a weak asparagus stand can develop and limit the potential of the field for the rest of its stand life. Producing a uniform, vigorous asparagus stand and maintaining it in this condition can mitigate most weed control problems. Asparagus is a very good competitor with most annual weeds when it is a mature fern, especially if the stand is uniform and vigorous.
Once established, an asparagus field has a stand life of 5 to 20 years. At first, in newly established fields, annual weeds are the main problem, but as time passes, perennial weeds often become the major concern. These perennial weeds include yellow nutsedge, field bindweed, swamp smartweed, johnsongrass, and bermudagrass. If perennial weeds become established, they can be troublesome throughout the growing period by reducing crown vigor and density and ultimately asparagus yield and quality. Perennial weeds are difficult to control culturally or chemically without injury to the asparagus; therefore, prevention becomes a major tool in combating these weed pests.
To help prevent infestation of the field by perennial weeds, be sure that seeds, tubers, stolons, rhizomes, and rootstocks of perennial weeds are not moved into asparagus fields with planting materials or on cultivation equipment. If spot infestations of perennial weeds are noted in the field, mark the area with flags and mechanically remove the infestation. Following removal, monitor the area for at least 2 to 7 years to make sure that reinfestation from propagules or seed does not occur.
Established asparagus fields are harvested from early spring through early summer. Spears, which develop from the underground crown, are cut on a 1- to 3-day cycle, depending on temperature. During the harvest period, spears provide virtually no shade to reduce weed competition. Annual weeds can be a problem in established stands at this time because the beds are open and exposed to light. It is most important to have beds weed-free to facilitate harvest and increase soil temperature.
Preemergence herbicides can be applied either pre- or post-cutting to control many of the annual weeds that cause problems in the crop. On the other hand, monitoring for perennial weeds must be carried out throughout the year and treatment made as soon as they are detected. Initially, perennial weeds tend to develop at the head and tail of fields; spot-treat infestations immediately with a foliar herbicide to prevent their spread into the field. If they do spread and weeds have to be removed mechanically, harvest will be interrupted for about 10 days while the field is disced, which reduces profits.
After the last harvest of the season, asparagus spears are allowed to grow into the fern stage, during which the asparagus plant replenishes the carbohydrate supply in the crown for the next season. The period between harvesting spears and allowing the spears to grow into ferns is a good time for controlling both annual and perennial weeds. Also, light tillage can be utilized, and some herbicides can be applied in a timely manner. Once the spears have grown into ferns, cultivation and hand removal of weeds during the fern stage is difficult because equipment movement is restricted due to the dense fern growth. However, dense fern growth restricts light, thereby minimizing much of the late emerging annual weed growth.
When a field is taken out of asparagus, rotate it to annual crops for several years to reduce levels of perennial weeds, soilborne disease inoculum, and salt buildup in the soil. If perennial weed populations are very high, it may be necessary to fallow the field for a year, using a combination of tillage and herbicides to return the field to a condition suitable for other crops.
Weed management is most effective when herbicides are used in conjunction with cultural practices. Cultural practices such as proper field selection, fallow treatment, cultivation, and hand removal help to improve herbicide performance by leaving fewer weeds; therefore, a combination of both cultural and chemical control methods can give the best overall result.
Herbicides are often used in sequence or in combination to broaden the weed control spectrum; a single herbicide will seldom control all weeds present. To avoid a buildup of resistant weeds, use preemergence herbicides in combination or alternated with another preemergence herbicide.
Monitor fields that are intended for planting to asparagus several seasons before planting. Keep an inventory of the weeds encountered and their relative abundance in the field. Such a record can be helpful in planning which cultural techniques will be most helpful and which herbicides are most likely to be successful. Perennial weeds can be a major problem in asparagus plantings. Select alternative fields for asparagus if yellow nutsedge, johnsongrass, bermudagrass, swamp smartweed, or field bindweed are present.
Weed Management Before Planting
Choose fields that are known to be free of perennial weeds. Fallow the field for as long as possible before planting, irrigating often to germinate weeds and cultivating shallowly to destroy them. Do not cultivate too deeply as a new supply of weed seed may be brought up from deeper soil layers.
Before planting, metam sodium or other soil fumigants are often used to control soilborne diseases and nematodes, but these materials can also be used to control annual weeds and reduce perennial weed propagules. Take care to assure that the soil is well cultivated and moist for at least 1 week before an application of metam sodium.
Paraquat (Gramoxone Inteon) and glyphosate (Roundup) can be used as preplant or preemergence treatments to control emerged weeds before planting or before asparagus emergence. Make sure the emerged asparagus is not contacted by these herbicides or the plants will be killed.
Weed Management After Planting
During the first 2 years after planting, the asparagus plants become established. Weed control is critical to the long-term well-being of the crop during this period, and it relies on both cultural and chemical controls.
For all three methods of stand establishment, planting beds are used and fields can be lightly cultivated several times during the season to throw soil onto the bed tops, thus keeping weed competition to a minimum.
Direct field seeding
In direct seeding, asparagus is seeded into raised beds that may be cultivated during stand establishment to control weeds in the furrow and bed shoulders. The seedling asparagus is slow to emerge, requiring from 14 to 21 days. Once emerged it continues to grow slowly and is not competitive with most weed seedlings. This emphasizes the need to plant into fields that have a low soil weed seedbank or that have been fallow irrigated and cultivated to reduce the weed seedbank. If it is necessary to remove weed seedlings from the rows of seedling asparagus, this can be done with herbicides or by hand removal. Weed management at this stage is important in order to establish a uniform, competitive asparagus stand. Control weeds for the first 3 to 4 months in seedling asparagus until a heavy fern cover is established.
Linuron (Lorox) controls a broad spectrum of broadleaf and grass weeds and has both soil and foliar activity. Apply linuron as a directed spray to minimize contact with asparagus foliage when asparagus seedlings have from 6 to 18 inches of growth.
Sethoxydim (Poast) is used for controlling most annual grass species, except annual bluegrass. It is also effective in the control of some perennial grass species; however, more than one application is necessary. Its effectiveness requires that grasses not be under moisture stress. Later growth stages of annual grasses are more difficult to control.
Fluazifop-p-butyl (Fusilade) is a selective systemic grass herbicide that must be applied before grasses are 3 to 4 inches tall for best control. Asparagus seedlings are relatively tolerant of fluazifop, which can be applied as a broadcast application.
Planting of 10- to 12-week-old transplants hastens the establishment process compared to direct seeding. Transplants are planted into trenches and usually sprinkler irrigated. This procedure allows weeds to germinate within the planted area. The young asparagus seedling is a poor competitor; thus early weed management is essential for plant survival and growth. As with direct-seeded asparagus, it is necessary to control weeds until a heavy uniform fern cover is established. The same materials used for direct-seeded asparagus can be used for transplanted seedlings.
Crown planting is done when the asparagus crowns are dormant. The crowns are set into trenches and covered with 2 to 4 inches of soil, followed by rainfall or furrow or sprinkler irrigation to settle the soil around the crowns. Weed emergence soon follows, often before asparagus emergence. Rapidly growing weeds must be removed from within the planted beds. As the asparagus grows, the furrow and sides of the beds can be cultivated, which throws some soil onto the bed tops. This soil fills in around the plants and provides some weed control as small weed seedlings may be buried. As with the previous two methods of stand establishment, timely cultivations and hand removal of weeds or herbicide treatments are needed during the first season until a uniform fern cover is produced.
Paraquat (Gramoxone Inteon) is a contact herbicide that is effective against both grasses and broadleaf weeds and must be applied before asparagus spears emerge. It is most effective when applied as a broadcast application to weeds that are in the two- to four-leaf stage.
Linuron (Lorox) can be used on crown-planted seedling fields to control a broad spectrum of broadleaf and grass weeds and has both soil and foliar activity. Use linuron as a directed spray to minimize contact with asparagus foliage when asparagus seedlings have from 6 to 18 inches of growth.
Diuron (Karmex and others) may be used in the San Joaquin Delta only on high organic matter or clay content soils. It is a broad-spectrum preemergence herbicide that is useful in controlling emerging annual weeds; however, it is not very effective in the control of common groundsel, sowthistle, volunteer cereals, and wild oats.
Fluazifop may be applied for grass control after the asparagus spears have emerged. It is most effective when applied before the grasses are 6 inches tall.
The term "established asparagus" refers to plantings that are 2 or more years old. Once crop plants are established, focus weed management efforts first on limiting establishment and spread of perennial weeds, which can reduce the vigor and quality of the asparagus stand, and second on controlling annual weeds to avoid competition during the cutting season.
Weed control in established asparagus is only possible during a relatively short window of opportunity that lasts about 4 months. This period begins with preharvest cultivations, when beds are tilled and shaped before the harvest season. It also includes the harvest period, when shallow cultivations can be used to control weeds; limit cultivation to the furrows, however, because cultivation on the bed tops will interrupt harvest for a period of up to 10 days. The postharvest cultivation, which is possible until the fern limits mechanical activity, is the last chance during the growing season to control weeds by cultivation; after this time the asparagus fern becomes too tall to permit cultivation. Perennial weeds can be difficult to control because of the relatively limited opportunity to cultivate.
Preemergence (When spears are not present)
Glyphosate (Roundup) and paraquat (Gramoxone Inteon) may be used on established beds before spears emerge to control newly emerged annual weeds. Asparagus emerged at the time of application will be injured by these herbicides and spears will be unmarketable.
Metribuzin (Metribuzin 75) has preemergence and postemergence activity on newly emerged annual weeds. If the field is to be cultivated or rototilled, apply after bed preparation. Irrigation or rainfall is necessary to activate this herbicide.
Diuron (Karmex and others) is useful for the control of many emerging annual weeds, but does not control common groundsel, sowthistle, volunteer cereals, and wild oats. Apply it as a band or broadcast application to weed-free beds and incorporate it mechanically or with irrigation if rainfall does not occur. Do not use it on soils with less than 2% organic matter; use lower rates on coarse-textured soils.
Flumioxazin (Chateau) is useful for the control of a wide spectrum of broadleaf weeds. Apply it no less than 14 days before spears emerge and before weeds emerge, or burn the weeds back with a tank-mix material. Requires 0.25 inch of rainfall or irrigation to activate.
Halosulfuron (Sandea) can be applied before the cutting season to control broadleaf weeds. Do not use an adjuvant with sprays applied before the harvest period.
Napropamide (Devrinol) is useful in the control of winter annual weeds, such as common groundsel, which are difficult to control with other asparagus herbicides. It has no postemergence activity and should be used after bed preparation, before weeds emerge. Napropamide requires shallow mechanical incorporation (one to two inches deep), and if rainfall does not occur, it must be irrigated.
Trifluralin (Treflan and others) and pendimethalin (Prowl H20) are active in the control of many grasses and broadleaf weeds, with the exception of those in the sunflower, mustard, little mallow (cheeseweed), and legume families. Use trifluralin and pendimethalin before spears emerge. Trifluralin can be used after the cutting season but before ferns develop. They have no postemergence activity on weeds and must be applied prior to weed germination. Trifluralin can suppress the growth of bermudagrass if applied at this time. Trifluralin will also suppress field bindweed at high label rates. Trifluralin must be mechanically incorporated immediately after application two times in opposite directions with discs or rolling cultivators, or one time with a power-driven incorporator.
Linuron (Lorox) can be used before harvest. Linuron has a broad spectrum of annual weed control activity. It also has both foliar and soil activity. Its residual soil activity is shorter than other residual asparagus herbicides. This makes it a better choice to use in the last season of an asparagus planting to avoid long-lasting soil residues that could affect succeeding crops. See herbicide labels for plantback restrictions.
Postemergence (After spears emerge)
Dicamba (Banvel) is useful for the control of annual broadleaved weeds and troublesome perennials, such as field bindweed and swamp smartweed. It is applied immediately after spear cutting or as directed sprays to avoid spear and fern contact. Spears that are twisted or malformed as a result of treatment should be cut and discarded. Be sure to comply with all state and county regulations as to proximity to susceptible crops and other restrictions regarding their use.
Linuron (Lorox) can be applied immediately after cutting, but do not harvest within one day after application.
Halosulfuron (Sandea) can be applied during the cutting season to control broadleaf weeds. Do not use an adjuvant with sprays applied during the harvest period.
The grass herbicides sethoxydim (Poast) and fluazifop-p-butyl (Fusilade) can be used on emerged spears. Both have a one-day preharvest interval and work best on actively growing grasses.
Postharvest (Before or at the onset of the fern growth)
Halosulfuron (Sandea) can be applied following the cutting season to control broadleaf weeds and yellow nutsedge. A nonionic surfactant or crop oil concentrate may be used with postharvest applications only.
Diuron (Karmex and others) can be used during the postharvest period, but care needs to be taken to not exceed the seasonal limitations.
Linuron (Lorox) can be used during the fern stage by using sprays directed to the base of the fern.
Metribuzin (Metribuzin 75) can be used following the final harvest but before spears emerge that will form the fern.
Trifluralin (Treflan and others) can be mechanically incorporated following harvest and before fern growth to suppress grasses and broadleaf weeds.
The grass herbicides sethoxydim (Poast) and fluazifop-p-butyl (Fusilade) can be used on emerged spears and both have a one-day preharvest interval. They all work best on actively growing grasses.
Glyphosate (Roundup) may also be used as a postharvest treatment when all remaining spears have been removed (clean cut). It is useful in controlling emerged annual and perennial weeds; use higher rates of application to control perennials. Direct contact of the spray with asparagus fern can cause serious injury. Glyphosate is useful for spot-treating perennial weeds around the edges of asparagus fields to prevent these weed infestations from spreading into the field on incorporation and cultivation equipment.