Weed Management For Organic Vegetable Production

Effective weed control in organic vegetable productions depends on an integration of sound cultural practices, careful cultivation, and hand labor. Preventing the production of weed seed in the field before planting will reduce subsequent weeding costs during crop production. Growers that have a zero-tolerance for weeds in all crops have lower weed pressure, and over time, the soil seedbank can be depleted.  Certain crops such as fast maturing, high-density plantings of spinach and lettuce can also deplete the soil weed seed bank because these crops are often harvested before many weed seeds can produce seed – provided the field is tilled immediately after harvest.

The first step in developing a weed management program is to survey the planting site and identify the weeds that are there. Become familiar with each weed's growth and reproductive habits to choose the most effective management options. For help in identifying common weeds, see the weed photo pages that are linked to the weed list in the section Common and Scientific Names of Weeds.

Vegetables produced in California include both warm-season crops like melons and tomato, as well as cool season crops like carrot, celery and lettuce. The warm-season crops are mostly grown in the central valley and low desert, while the cool-season crops are grown year-round at multiple locations depending on the season, e.g., in the low desert during winter and along the coast during summer. The wide range in vegetable production seasons and locations means that the weed spectrum varies quite widely. Therefore, organic vegetable weed control programs need to be flexible and robust.

It is important to locate plantings in fields with low weed pressure or to use weed management methods that reduce weed pressure before planting. It is also important to keep areas near production fields free from weeds with windblown seed. Some growers use transplants to give the vegetable crop a head start on the weeds. Transplants reduce the number of days to harvest and establish a canopy more quickly. They generally only need one weeding instead of two (as is done with direct-seeded crops) to get through the growth cycle. The goal of organic weed control techniques is to reduce weed pressure and give the crop an advantage over the weeds in order to produce the crop as economically as possible.

Crop Rotations and Field Sanitation

The previous crop can significantly affect weed pressure in a vegetable crop. A previous crop that has had effective weed control means that  fewer weed seed will enter the seedbank infecting later rotational crops. In addition, it is important to keep the areas surrounding the field free of weeds that have aerially dispersed seeds, such as groundsel and sowthistle.

Preplant Germination of Weeds

Preplant germination of weeds (pregermination) involves the use of irrigation or rain to stimulate weed seed germination before planting the crop. The emerged seedlings are then killed using various methods. The time of year, irrigation system, and the interval between irrigation and weed control all affect how well this technique works. Preplant germination can take place before or after beds are formed but must take place before a crop is planted.

Done properly, this method removes up to 50% of the weeds that would have otherwise emerged in the subsequent crop.

1. Preirrigate the field to germinate weeds at least two weeks before planting to assure that the weed spectrum does not change before planting.
• Changes in the weed spectrum may occur because of changes in the season or weather, such as from winter weeds to summer weeds. 
2. Allow weeds to emerge and for the field to dry enough to permit use of shallow tillage.
3. Cultivate as shallowly as possible to avoid bringing up dormant weed seed from deeper soil layers.
4. Shape the beds and apply water to stimulate another flush of weeds.
5. Kill the weeds using shallow cultivation, flaming, or applications of organic herbicides. Cultivate as shallowly as possible to avoid bringing up dormant weed seed from deeper soil layers.
6. Plant the crop immediately on these beds.

Flaming

On seeded crops, flaming can be used to kill a flush of weeds. This technique is particularly effective on crops that have slow seed germination like carrots. The timing of flaming is critical and is done just before the carrot seedlings emerge, but after the weeds have emerged.  It is most effective on small broadleaf weeds (i.e., less than 2 true leaves) but not grass weed species because the growing point is below the soil surface.

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 or sprout in the case of perennial weeds. The viability of buried weed seed declines over time; longer intervals between deep plowing and subsequent deep plowing (i.e., 3–5 years) are preferred to avoid bringing up large numbers of viable weed seed back to the soil surface.

Cover Crops

The use of cover crops is a key cultural practice in organic production. Cover crops provide a variety of benefits to crop production but can, potentially, both increase or decrease weed pressure in vegetable production systems. Unfortunately, annual weeds frequently become established at the time of the cover crop seeding, and depending upon the weed species, they can grow and set seed unnoticed in cover crops. 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, and allow for substantial contributions to the weed seed bank. Slow-growing winter cover crops (legumes and cereal/legume mixes) can be particularly problematic; many allow significant weed growth and seed set early in the growth cycle of the cover crop. Fast-growing winter cover crops, such as cereals and mustards, provide complete ground cover in the first 30 days of the cover crop cycle and are better able to compete with weeds. Competitive cereal and mustard cover crops varieties include Merced rye (Secale cereale), white mustard (Sinapis alba), and Indian mustard (Brassica juncea). An adequate seeding rate is also an important factor in providing for rapid ground cover. It is important to monitor the cover crops, particularly in the first 40 days following seeding, to make sure that they are not creating a weed problem for subsequent plantings.

Soil Solarization

Solarization can be used in areas with sufficient solar radiation like the central valley and low desert. Typically, the soil is irrigated and then covered with clear plastic for 4 to 6 weeks during the hot summer period. The effectiveness depends mainly on the heat that can be generated under the plastic during a short interval. Soil solarization can reduce amount of weed seed in the soil, as well as provide partial control of root knot nematode and soilborne fungal pathogens. For further information, contact your local farm advisor or see UC ANR Publication #21377, Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds.

Cultivation

Cultivation is one of the most effective postplant weed control practices that can be performed. Organic vegetable crops are typically cultivated at least twice. Traditional inter row cultivation can remove weeds in 80% of the field  while leaving  a 4-inch-wide uncultivated strip around each seed line.

New intelligent cultivators and laser weeders can cultivate in the seedline near the crop plants removing a high percent of the weeds.  These intelligent cultivators will not generally remove all of the weeds but rather remove a large percentage of the weeds thus increasing the efficiency of subsequent hand weeding operations. Alternatives to the intelligent cultivators are devices such as finger weeders and torsion weeders which can remove weeds very close to the crop without damage to the crop. However, the use of these weeders is limited in carrots due to the delicate nature of the crop until later in the crop cycle.

Hand Hoeing

Hand hoeing is essential in organic vegetable crops. Typically, the crop is weeded 30 to 40 days after seeding. At this time the plants are tender, and careful hand weeding is necessary in the seed row to safeguard the crop. Typically, the initial hand weeding and thinning operation is followed 2 weeks later by a final hand weeding before the crop canopy closes. At present, there are no successful techniques that can substitute for careful hand weeding of direct-seeded crops, but successful employment of the above-mentioned techniques can help make hand weeding operations less time consuming and more efficient.