How to Manage Pests

UC Pest Management Guidelines

Onions and garlic are slow growing, shallow-rooted crops that can suffer severe yield loss from weed competition. Their narrow, upright leaves do not compete well with weeds. In addition, their long growing season allows for successive flushes of weeds.

Onions are sown at high plant densities and not thinned in order to have the greatest possible yield per acre. They are planted with 4 to 6 seedlines on the top of beds that are 40 to 42 inches wide from furrow to furrow. Because the bed top is only 22 to 24 inches wide, there is no room to cultivate the planted area, and only the shoulders and furrows are cultivated. Hand-weeding is difficult because of the high plant densities and frequently does as much damage as good. Garlic is not planted as densely as onion with only 1 to 2 seedlines per bed; but like onion, it is not thinned.

An integrated weed management program is essential for onion and garlic production because these crops are not competitive against weeds, the selection of herbicides is limited, and mechanical cultivation and hand-hoeing are of minimal value because mechanical cultivation is too difficult to perform on beds with multiple seedlines and hand-hoeing is too costly.

MONITORING

Monitor the 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. Pay special attention to weeds likely to be present at planting time. Not only are these records valuable in choosing the most suitable fields for growing onions and garlic, they also help track the occurrence of hard-to-control weeds. Plant onion and garlic in the most weed-free fields available, avoiding fields with high populations of difficult-to-control weeds such as nutsedge, field bindweed, bermudagrass, and clovers.

Nonchemical control. Nonchemical control options are limited primarily to the preplant period in onion and garlic production.

Preplant. To prevent the buildup of weed seed in the soil, cultivate weeds before they set seed in rotation crops. After harvest, clean cultivate the field or plant a green manure crop to limit weed infestations. Fields heavily invested with nutsedge can be plowed with moldboard plows to bury tubers 10 to 12 inches. This can reduce a nutsedge population by 95 to 98%.

Just before planting onion or garlic, preirrigate the field to germinate weed seedlings and cultivate to destroy them. Preirrigating and cultivating as close to planting time as possible ensures that soil temperature and climatic conditions are similar to the crop germination period, thus maximizing the number of weeds controlled. Cultivate as shallowly as possible so that weed seed is not brought up from deeper soil layers.

Soil solarization is a nonchemical, soil pasteurization process that will control most weeds in onion and garlic. It will also control some important diseases of these crops, such as pink root. To solarize, place clear plastic on top of the moist, clean seed bed for 4 to 6 weeks during the hottest part of the year. Because soil solarization requires a summer fallow season for treatment, it fits in best with a fall-planted crop.

Herbicides, combined with good cultural practices, control many weed pests of onion and garlic. Herbicides may be applied before planting or after planting. Foliar-applied herbicides may be utilized after the crop emerges. Layby herbicides are applied to clean cultivated soil at a certain stage of crop development to keep the crop weed-free until harvest. Most fields require a postplant preemergence application and one layby treatment. Preplant treatments are used in fields with persistent perennial weed problems.

Herbicide selection depends upon the weed species that are expected to occur. Plantback restrictions need to be considered when selecting herbicides; soil residues of these products can limit the growth of sensitive rotational crops. Herbicide labels are the best source of information on plantback restrictions.

BEFORE PLANTING (Onion and Garlic). Preplant treatments of metam sodium will destroy most weeds present. Paraquat and glyphosate (Roundup) can be used before the crop is planted to control emerged weeds. Glyphosate has been particularly helpful to control perennial weeds the season before planting.

AFTER PLANTING (Onion). DCPA is used in most onion crops for control of annual grasses and broadleaf weeds. It is applied after planting but before weeds emerge and often sprinkler-irrigated for incorporation. DCPA does not control all weed species but is extremely important in reducing weed competition during the critical early stage of crop establishment.

Bensulide (Prefar) can be used safely as a preemergent herbicide in onions at 4 lb a.i./acre but may cause crop injury at 6 lb a.i., the maximum label rate. Bensulide is most effective on annual grasses and on a few broadleaf species such as knotweed, pigweed, and purslane.

Occasionally an application of a glyphosate or paraquat is necessary to control germinated weeds before the crop emerges. The timing of this treatment is critical because any emerged crop plants will be killed if contacted by these herbicides.

After the crop emerges bromoxynil (Buctril), oxyfluorfen (Goal), pendimethalin (Prowl), sethoxydim (Poast), fluazifop-P-butyl (Fusilade), and clethodim (Prism) can be used; bromoxynil and oxyfluorfen, however, can only be used when the onion crop is young (2- to 4-leaf stage).

Bromoxynil is very effective for control of small broadleaf weeds. Onion injury can occur if label directions are not followed closely; do not treat before the crop has two true leaves and only if the crop leaf cuticle has a waxy appearance. Cloudy days, wind and sandblasting, frost, other pesticide applications, or insect feeding injury can reduce the thickness of the cuticle. If applied to plants with thin cuticles, bromoxynil can cause severe crop injury.

Oxyfluorfen is registered as a postemergent herbicide in onion. It is used at the two- to four-leaf stage of the crop, like bromoxynil. It also controls small broadleaf weeds and some grasses. Oxyfluorfen is complimentary to bromoxynil, between them they control a wider spectrum of weeds than either alone. They are usually used in sequence, about 1 week apart, but this depends on the crop growth rate and the weeds; the order in which they are applied varies according to experience.

Sethoxydim and fluazifop-P-butyl are effective in controlling small, annual grass seedlings 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. Neither herbicide will control annual bluegrass. Clethodim is registered for use on onion (dry bulb onion) and garlic. It effectively controls annual bluegrass along with most other grass species.

In onions, nitrogenous fertilizers such as ammonium nitrate, ammonium thiosulfate, or mixtures of urea and sulfuric acid (N-Phuric or Enquick) applied when the plants have one true leaf will stimulate the growth of onion seedlings and increase their competitiveness with broadleaf weeds, which are burned back by the fertilizer.

AFTER PLANTING (Garlic). DCPA is used in garlic crops for control of annual grasses and broadleaf weeds. It is applied after planting but before weeds emerge. In garlic, furrow irrigation is often used to germinate the seed (cloves); a thorough postplant wetting of the garlic beds will activate DCPA and provide good weed control. DCPA does not control all weed species but is extremely important in reducing weed competition during the critical early stage of crop establishment.

Pendimethalin (Prowl) controls many annual grasses and broadleaf weeds, but sprinkler irrigation is essential for optimum weed control.

Occasionally a foliar application of glyphosate or paraquat is necessary to control germinated weeds before the crop emerges. The timing of this treatment is critical because any emerged crop plants will be killed if contacted by these herbicides.

After the crop emerges bromoxynil (Buctril), sethoxydim (Poast), fluazifop-P-butyl (Fusilade), and clethodim (Prism) can be used.

Bromoxynil is very effective for control of small broadleaf weeds. Garlic is more tolerant to bromoxynil than onion and this material can be used on garlic any time after the crop emerges but before it reaches 12 inches in height.

Sethoxydim and fluazifop-P-butyl are effective in controlling small, annual grass seedlings 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. Neither herbicide will control annual bluegrass, which clethodim controls along with most other grass species.

LAYBY (Onion and Garlic). DCPA is registered for use during the layby period for both onion and garlic, whereas pendimethalin is registered for use on garlic only. Both herbicides control late-emerging grasses and some annual broadleaf weeds, thus providing cleaner fields at harvest. They are applied over the top of the crop and activated with irrigation. They do not control emerged weeds and are used after cultivation of furrows. Some carryover can occur under certain conditions, creating a plantback problem. Consult the herbicide label before application.

PUBLICATION

UC IPM Pest Management Guidelines: Onion and Garlic
UC ANR Publication 3453
Weeds
R. Smith, UC Cooperative Extension, Monterey Co.
S. A. Fennimore, Vegetable Crops/Weed Science, UC Davis/Salinas
S. Orloff, UC Cooperative Extension, Siskiyou Co.
G. J. Poole, UC Cooperative Extension, Los Angeles Co.
Acknowledgment for contributions to the weed section:
C. E. Bell, UC Cooperative Extension, San Diego Co.
D. W. Cudney, Botany and Plant Sciences, UC Riverside

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