Onion and Garlic
Agricultural pest management
Organic Weed Management
(Reviewed 1/07, updated 5/10)
Weed control in organic onions and garlic requires special attention to preventing weed problems before they start. Any method that reduces the amount of weed seed will reduce subsequent weeding costs during crop production. One of the best ways to prevent weed problems is to control existing weeds before they go to seed.
Onions and garlic present particular challenges to weed control because they do not effectively shade the ground, even when mature. As a result they are subject to weed germination and growth during their entire lifecycle. Emphasis must be placed on locating plantings in fields with low weed pressure or on techniques that reduce weed pressure before planting. It is particularly important to avoid fields with perennial weeds such as field bindweed and yellow or purple nutsedge. It is also important to locate fields away from areas infested with weeds with windblown seed such as groundsel, sowthistle, and horseweed. The goal of organic weed control techniques is to reduce weed pressure in order to produce the crop in an economical fashion.
The first step in developing a weed management program is to identify the weeds infesting the planting site. Become familiar with each weed's growth and reproductive habits in order to choose the most effective management options. See the weed photos linked to the weeds in the list of COMMON AND SCIENTIFIC NAMES OF WEEDS.
Soil solarization. Soil solarization can significantly reduce viable weed seed in the top layer of the soil. Soil solarization traps the sun's energy beneath a layer of clear plastic, increasing the temperature in the top foot of soil to levels lethal to many weed seeds as well as vegetative structures of perennial weeds. However, solarization does not control perennials as effectively as it controls annuals. Seedlings of bermudagrass, johnsongrass, and field bindweed are controlled, but not the plants. Yellow nutsedge is partially controlled while purple nutsedge is not significantly affected.
Effective soil solarization begins with preparing a smooth seed bed so that the plastic can be placed as close as possible to the soil surface. Disc to break up clods and then smooth the soil. Remove any material that will puncture or raise the plastic sheets such as rocks and weeds. Irrigate before or after applying the plastic because wet soil conducts heat better than dry soil. Cover the soil with plastic as soon as possible after irrigating. It is possible to irrigate after laying the plastic by installing the drip system before planting.
Use clear plastic that is 1.5 to 2 mils thick and impregnated with UV inhibitors to prevent premature breakdown of the material. Bury the sides of the plastic along the sides of the bed to create a seal on the soil; this also helps prevent the plastic from being blown away by wind. Machines that lay down the plastic are available to automate the process. The plastic must remain in place for a minimum of 4 weeks (length depends on the amount of solar radiation), and removed immediately before planting the crop. 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 onions and garlic.
In the desert and Central Valley, the plastic should be in place from June through August and can remain in place until planting begins. Solarization may not be as effective in cooler coastal areas as it does not heat the soil as deep. In these 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.
Preplant weed germination. Preplant weed germination involves the use of irrigation or rain to stimulate weed seed germination before planting onions or garlic. The emerged seedlings are then killed by shallow cultivation, flaming, an organic herbicide, or a combination of these treatments. This is done as close as possible to the date of planting to ensure that the weed spectrum does not change before planting the vegetable crop. Changes in the weed spectrum may occur because of changes in the season or weather.
The time of year, irrigation system, and the interval between irrigation and weed control all affect the efficacy of this technique. Waiting 14 days from irrigation to controlling weeds with shallow tillage can provide up to 50% control of weeds in the subsequent crop. If time permits, preplant weed germination can be repeated to further reduce weed populations. In addition, flaming or organic herbicide treatments can also be used to kill the flush of weeds anytime between the seeding of the crop and its emergence. However, flaming and organic herbicides are less effective on grass weeds. For all weed control methods, timing is important because small weed seedlings are more easily killed than larger weeds.
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 in order to avoid bringing up large numbers of viable weed seed back to the soil surface.
Cover crops. Cover crops are a key cultural practice in organic production, and they provide a variety of benefits to crop production. However, cover crops also have the potential to both increase or decrease weed pressure in vegetable production systems. Unfortunately, annual weeds frequently become established at the time of the cover crop, and depending upon the species of weed, they can grow in the cover crop and set seed unnoticed. Often weed plants 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 seed bank. Slow-growing winter cover crops can be particularly problematic for aggravating weed problems. For instance, many legumes and cereal/legume mixes allow substantial weed growth and seed set early in the growth cycle of the cover crop.
Fast-growing winter cover crops include cereals and mustards that provide complete ground cover in the first 30 days of the cover crop cycle. Competitive cover crops varieties include Merced rye (Secale cereale), white mustard (Sinapis alba) and Indian mustard (Brassica juncea). Adequate seeding rate is also an important factor in providing for rapid ground cover. 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 onions and garlic.
Cultivation. Cultivation is one of the most effective post-planting cultural practices. On double row 40-inch beds it is possible to cultivate 80% of the bed (assuming a 4-inch wide uncultivated strip is left for each seedline). However, planting configurations of onions typically utilize more than two seedlines per 40-inch bed, and the effectiveness of cultivation is greatly reduced. The first cultivations cut weeds out with coulters and knives, and later cultivations throw soil against the base of the plant to bury small weeds.
The goal of cultivation is to cut weed seedlings as close to the seed row as possible without disturbing the crop. New precision guidance systems for cultivation (i.e. EcoDan®) can help improve the accuracy of cultivation operations. More precise cultivation allows for reducing the width of the uncultivated band and thereby removing a higher percentage of the weeds. Uncontrolled weeds in the seedline are removed by hand or other mechanical means.
Herbicides. There are a few organically acceptable herbicides available for use in organic onion and garlic production. All of the organic herbicides are nonselective contact materials that contain various essential oils. Their best use is as a burn down treatment of the weeds before the emergence of the crop. The efficacy of all these materials is dependent upon the stage of growth of the weeds (i.e., less than 1-2 true leaves) and the rate of application. Higher ambient temperatures improve their efficacy.
Mulches. Dark-colored plastic mulches (i.e. black, brown and green) prevent light from reaching the soil surface, thereby preventing weed germination. Mulches are used to provide some weed control in organic crops such as peppers, tomatoes, and melons. They would be expensive to use in high-density crops such as onions and garlic, but could be justified where the crop receives a high premium.
Hand hoeing. Hand hoeing is generally necessary in organic onion and garlic production. It is difficult because of the close plant spacing and the multiple seedlines per bed. Successful employment of the above mentioned techniques can help make hand-weeding operations less time consuming and more effective.
UC IPM Pest Management Guidelines: Onion and Garlic
R. Smith, UC Cooperative Extension, Monterey County
Acknowledgment for contributions to Weeds:C. E. Bell, UC Cooperative Extension, San Diego County
D. W. Cudney, Botany and Plant Sciences, UC Riverside
G. J. Poole, UC Cooperative Extension, Los Angeles County