Integrated Weed Management

An integrated weed management program is essential for onion and garlic production because of the unique challenges posed by their planting densities and susceptibility to weed competition. These crops are slow-growing and shallow-rooted, planted at high densities, and susceptible to severe yield loss from weed competition. Their narrow, upright leaves do not compete well with weeds, and their long growing season allows for successive flushes of weeds. Weed control is often challenging in these crops because few herbicides are registered, mechanical cultivation is often limited in high-density plantings, and handweeding can be costly. A good integrated weed management plan can increase the ease and effectiveness of these management tactics.

Planting densities for onion and garlic pose unique challenges to weed management. These crops are sown at high plant densities and are not thinned to produce the greatest possible yield per acre. Onions are planted with 4 to 10 seed lines on beds that are 40 to 80 inches wide (furrow to furrow), or 36 inches wide in the intermountain area. Garlic is typically planted with 2 to 4 seed lines on 40-inch beds. These planting configurations limit cultivation to the furrow and narrow row middles on the bed top.

Emphasis must be placed on techniques that reduce weed pressure before planting, such as the use of the stale seedbed method, weed-free seed, or soil solarization. Any method that reduces the amount of weed seed in the soil will reduce weeding costs during crop production. Another good way to prevent weed problems is to control existing weeds before they go to seed.

Monitoring

Monitor the fields and keep records (PDF) of the weed species present in each field during the growing season. 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, and avoid fields with high amounts of difficult-to-control perennial weeds such as nutsedge, field bindweed, bermudagrass, johnsongrass, clovers, and Canada thistle. If weedy fields must be used, control weeds during fallow periods using stale seedbeds and herbicide application or shallow tillage. Fumigation with metam potassium or metam sodium can also be used to reduce the weed seed bank.

Nonchemical control options are primarily limited to the preplant period in onion and garlic production. For most weed control methods, timing is important because small weed seedlings are easier to kill than larger weeds.

Weed Management Before Planting

Nonchemical control options are primarily limited to the preplant period in onion and garlic production. For most weed control methods, timing is important because small weed seedlings are easier to kill than larger weeds.

Crop Rotation With Cover Crops

Cover crops are rarely used in onion and garlic production. However, they can provide a variety of benefits to the crop when used in crop rotations, especially if they are grown in the fall prior to planting Alliums.

Timing is the key to whether cover crops promote or inhibit weed growth. If cover crops become established quickly, they will suppress weeds. Adequate seeding rate of the cover crop is also an important factor in providing rapid ground cover and suppressing weeds. Vigorous cover crops that provide complete ground cover in the first 30 days of the cover crop cycle are very competitive with weeds and greatly limit weed growth. Competitive species include cereal rye (Secale cereale), white mustard (Sinapis alba), and Indian mustard (Brassica juncea). Avoid slow-growing winter cover crops, including legumes and many cereal-legume mixes, which allow substantial weed growth and set seed early in the growth cycle of the cover crop. Cover crop residues must have adequate time (at least 3 to 4 weeks) to break down in the soil before planting onion and garlic, which require shallow, precise seeding depth with good seed-to-soil contact.

Cover crop residues can increase pressure from certain diseases and insect pests. For example, cover crops can increase seedcorn maggot numbers. If a cover crop is used, incorporate the cover crop three to four weeks before planting onion or garlic to avoid tilling in green residues that attract this pest.

Cover crops also have the potential to increase weed pressure for the following reasons.

• Annual weeds frequently establish themselves at the time of the cover crop.
• Depending upon the species, weeds can grow in the cover crop and set seed unnoticed.
• Weeds often decompose before the end of the cover crop cycle, making their detection difficult. In such cases, the cover crops act as nurse crops to weeds, making substantial contribution to the weed seed bank.

It is important to monitor your cover crops, particularly in the first 40 days following seeding, to make sure that they are not creating a weed problem for subsequent plantings of onion and garlic.

Soil Solarization

Soil solarization traps the sun's heat beneath a layer of clear plastic and disinfests the soil with higher temperatures that kill weed seeds, vegetative structures, insects, and some disease-causing pathogens (such as the fungus that causes pink root).

Solarization effectively controls seedlings (but not mature plants) of bermudagrass, johnsongrass, and field bindweed, and is more effective on annual weeds than perennial weeds. It partially controls yellow nutsedge, but does not control purple nutsedge. Annual weeds with a harder seed coat such as little mallow and velvetleaf are often hard to kill with solarization.

Plan to solarize soil according to planting time: plastic must remain in place for a minimum of 4 weeks, and the best results occur when the crop is planted immediately after removing the plastic. For some crops, growers burn holes into the plastic mulch and transplant directly into them; however, this practice may be of limited value for high-density plantings of onion and garlic.

Because solarization requires a summer fallow, it works best with a fall-planted crop. In the desert and Central Valley, the plastic should be in place during June through August, and can remain in place until planting begins in the fall.

Solarization may not be practical in areas with short growing seasons, or in coastal areas, where it does not heat the soil as deep. In coastal areas, apply plastic in fall when there is less chance of fog (i.e. August and September). Cultivate solarized soil less than 3 inches deep to avoid bringing viable weed seeds to the surface, where they germinate.

For more details on how to effectively solarize soil, see Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds, UC ANR Publication 21377.

Flaming

Flaming can be used to kill or suppress the flush of weeds anytime between seeding the crop and its emergence. This technique is particularly effective on crops that have slow seed germination like onion, where nonprimed seed is used. Timing is critical: flaming must occur just prior to the emergence of the onion plants, when a good number of weeds are emerged. Flaming is more effective on small (i.e., less than two true leaves) broadleaves than on grass weed species.

Propane-fueled flamers are the most common type of flamer used for this method. Other equipment without an open flame includes hot water or steam applicators and infrared devices. Flamers can be handheld or mounted on a handcart or tractor. Mechanized flamers have multiple burners, while small devices typically have a single flame source.

Typically, flaming can be done at 3 to 5 miles per hour through fields, although this depends on the heat output of the unit being used. Briefly touch the basal stem area with the tip of the flame to disrupt the cells; do not flame weeds to the point where they char and burn. Best results occur under windless conditions, as winds can prevent the heat from reaching the target. Early morning or evening are the best times to observe the flame for adjustment.

Determine the correct working pace or travel speed by checking weeds after flaming a test area. Weeds are being killed if gently pressing their leaves between your thumb and index finger creates a water-soaked appearance, indicating that cell membranes have ruptured. Plants may wilt, change color, or appear unaffected soon after flaming. Even if no change in the weeds is evident immediately, proper flaming causes plants to yellow and die within several days.

Fire is a serious hazard when flaming weeds. Only an experienced operator with demonstrated skill and good judgment should be allowed to flame weeds. Wet conditions during the rainy season or after a thorough irrigation are often good times to flame. Work in the early morning or late evening when winds are lower and any open flame is more visible. Proper flaming should not create smoldering vegetation or air pollution other than fuel burning emissions.

Use good judgment to identify hazardous situations in which flaming should not be conducted due to the risk of starting a fire. Do not use flame weeders in dry areas or during the dry season. Be especially cautious around mulch and leaf litter. Keep fire suppression equipment (e.g., a fire extinguisher, shovel, water) handy in case of an accident.

Cultivation

To prevent the increase of weed seed in the soil, cultivate weeds before they set seed in cover or rotation crops. After incorporation of the cover crop (if a cover crop is used), clean cultivate the field before planting onions or garlic.

Deep Plowing

Deep plowing is a tillage technique that buries weed seed or propagules of perennial plants below the depth at which they can germinate. The viability of buried weed seed declines over time. A relatively long interval (3–5 years) is preferred between deep plowing and subsequent deep plowing, to avoid bringing up large numbers of viable weed seed back to the soil surface. In fields heavily infested with nutsedge, plow fields with a specialized moldboard plow that fully inverts the soil, to bury tubers 10 to 12 inches. This can reduce up to 95 to 98% of nutsedge in the field.

Stale Seedbeds

This method can provide substantial weed control. It involves controlling the final flush of weeds before planting, followed by minimal soil disturbance to reduce subsequent weed flushes. To do this, prepare a seedbed and preirrigate it to germinate weed seeds. Cultivate as shallow as possible to kill emerged seedlings and prevent bringing up weed seed from deeper soil layers. Other options for killing the flush of weeds include flaming and foliar herbicides. The crop can then be planted on these beds soon afterward. If an herbicide is used, be sure to follow label directions on plantback restrictions.

The time of year and the irrigation system used may also affect the efficacy of this technique. Irrigate and cultivate as close as possible to planting time to ensure that soil temperature and climatic conditions are similar to the crop germination period, and to maximize weed control. If the interval between irrigation-cultivation and planting is too long, the weed spectrum (seasonal variation in weed species) may change due to changes in season or weather. Using shallow tillage 14 days after irrigation can reduce up to 50% of weeds in the subsequent crop.

Herbicides

Herbicides, combined with good cultural practices, control most weed pests of onion and garlic. There are specific herbicides that are applied before planting or after planting.

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

Preplant treatments are used in fields with persistent perennial weed problems. Metam sodium will destroy most weeds present. Paraquat and glyphosate can be used before the crop is planted to control emerged weeds. Glyphosate has been particularly helpful in controlling perennial weeds the season before planting.

Organically Acceptable Methods

Use the above-mentioned nonherbicidal methods in an organically certified crop. The stale seedbed method and cultivation are especially important preplant weed management tactics in organic onions and garlic.

Weed Management After Planting

Cultivation

Cultivation is one of the most effective postplant cultural weed control practices. Because of the high-density plantings of onions and garlic where most of the bed top is occupied with the seedlings, cultivation is often limited to a narrow band on the bed and the shoulders of the bed and furrows. Not all planting configurations of onions allow for bedtop cultivation, but the middle of the bed can be cultivated in garlic production.

The goal of cultivation is to remove weed seedlings as close to the seed row as possible without disturbing the crop. Precision guidance systems for cultivation (e.g. EcoDan®) can help improve the accuracy of cultivation operations. More precise cultivation allows for reducing the width of the uncultivated band, thereby removing a higher percentage of the weeds. Uncontrolled weeds in the seed line can be removed by postemergence herbicides or by handweeding.

Handweeding

Handweeding, although effective, is costly. Handweeding is particularly difficult in onion due to close plant spacing and the placement of multiple seed lines in a single bed. Handweed carefully to avoid damaging or killing onion plants, especially those that are young. Preventive measures, precision cultivation, and herbicides can also make handweeding less time-consuming and more effective.

Flaming

Flaming can be used to kill the flush of weeds any time between seeding the crop and its emergence. This technique is particularly effective on crops that have slow seed germination. However, its use in onions is limited if the onions germinate too quickly. This is particularly true in fields where primed seed is used.

Herbicides

Foliar-applied herbicides may be used after crop emergence to control established weeds. Layby herbicides are applied to clean-cultivated soil, typically at the 4- to 5- leaf stage of crop development, to keep the crop weed-free until harvest. Most fields require one or more postplant preemergence applications and one layby application. Split applications are most common for controlling weeds throughout the season, and to prevent unacceptable crop injury from herbicides.

Herbicide application sequences in onion and garlic typically involve applying preemergence herbicides to control or slow the growth of weeds germinating shortly after planting, and applying postemergence herbicides during the 1- to 5-leaf stage to control any weeds that escape preemergence treatments. Layby herbicides can help prevent weed flushes after cultivation or weeds that tend to germinate later in the growing season.

Preemergence herbicides

Preemergence herbicides typically require activation through irrigation. In both onion and garlic, bensulide, ethofumesate, and pendimethalin can be used preemergence. However, preemergence use of pendimethalin is only registered for some areas and crops as directed by special labeling (see the Herbicide Treatment Table for more information regarding use at 75% radical emergence and the loop stage). In garlic, the above-mentioned materials can be used, as well as flumioxazin and oxyfluorfen. Pendimethalin controls many annual grasses and broadleaf weeds, and provides optimal weed control through sprinkler irrigation. In the southern San Joaquin Valley, a common practice is to combine application of flumioxazin or oxyfluorfen with pendimethalin to control weeds after planting garlic.

In both onion and garlic, occasionally postplant applications of glyphosate or paraquat are used to control germinated weeds before the crop emerges. The timing of this application is critical, because any emerged crop plants will be killed if contacted by these herbicides. If applied at more than 1% crop emergence, glyphosate may reduce the stand.

Postemergence herbicides

Several herbicides, including grass-selective herbicides, are available for postemergence use in both onion and garlic. Clethodim controls annual bluegrass, whereas other herbicides do not. See the Herbicide Treatment Table for more information.

Oxyfluorfen and bromoxynil can only be used on young onions (see the Herbicide Treatment Table for additional information). Oxyfluorfen is complimentary to bromoxynil in onion; together they control a wider spectrum of weeds than either do alone. They are usually used in sequence about 1 week apart depending on the crop growth rate and weeds present, and the order in which they are applied may vary according to experience. They can also be tank-mixed, or the sequence strategy can be combined with the tank mix method. If tank mixing these two chemicals, use a sufficient spray volume per acre and closely follow the label to avoid crop injury. Oxyfluorfen is commonly used when the crop has 1.5 fully developed true leaves, followed by an application of both oxyfluorfen and bromoxynil at the 2- and 3-leaf stages.

Ethofumesate can provide control of a variety of broadleaf and grass weeds. Dimethenamid controls yellow nutsedge if applied before nutsedge emerges.

Organically Acceptable Methods

After planting, use cultivation, flaming, and handweeding in an organically certified crop.

Layby

Cultivation

Cultivation will remove weeds from the furrow and sides of the bed. In the intermountain region, cultivate weeds between the 4- to 6- leaf stage. When drying out the field out for cultivation, avoid causing water stress to the crop. Take care to avoid injury to the root system on outside seed lines while cultivating.

Cultivation during the layby period can sometimes stimulate new flushes of weed species such as pigweed.

Herbicides

Pendimethalin is registered for use during the layby period in onion  and garlic. It may be applied over the top of the crop and activated with irrigation. It does not control emerged weeds, and is used after cultivation of furrows. Some carryover can occur under certain conditions, creating a plantback problem. Consult the herbicide label before application.

Organically Acceptable Methods

During the layby period, use cultivation and handweeding in an organically certified crop.

Preharvest

Late-season weeds can be a problem in onion and garlic fields in the Central Valley, so it may be necessary to impose preharvest weed management efforts. Problematic weeds during this period include nightshade, nutsedge, and field bindweed. Weed growth during this time can increase the relative humidity within the field, increasing the risk of bulb rot. Harvest efficiency can also be hindered as weeds become entangled in the topping and harvest equipment, particularly with field bindweed.

If tall annual weeds (like nightshade) occur in patches in the field, using hand knives to sever the shoot from the roots can be effective. Controlled weeds should be layed in the furrow bottoms or removed from the field so they do not interfere with topping and harvest equipment.

When the weed infestation is widespread across the field, it is more efficient to apply glyphosate as a preharvest aid application. This is effective at drying down weeds like nightshade, nutsedge, and field bindweed, and allows for improved harvest of the crop. Application of glyphosate as a preharvest aid is available for onion and garlic grown for processing under a Special Local Needs [Section 24(c)] registration (see the Herbicide Treatment Table for more information).