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How to Manage Pests

UC Pest Management Guidelines

Cole Crops

Weed Management For Organic Broccoli Production

(Reviewed 6/07, updated 6/07)

In this Guideline: More about weeds in cole crops:

The goal of organic weed management techniques is to reduce weed pressure and/or give the crop an advantage over the weeds in order to produce the crop as economically as possible. Weed control in organic broccoli (and other cole crops) is dependent upon an integration of good 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.

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 in order 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.

Broccoli is produced in three distinct production districts (coastal valleys, San Joaquin Valley, and the desert) during the cooler times of the year (San Joaquin Valley and desert) and from spring to fall along the coast. Broccoli is typically grown on double row, 40-inch wide beds. It is very important to plant this crop in fields with low-weed pressure or to employ methods that reduce weed pressure before planting. Some growers utilize transplants to give the broccoli crop a head start on the weeds. Transplants establish a crop canopy more quickly and have fewer days to harvest. They generally only need one weeding instead of two.

Crop rotations and field sanitation.
The previous crop can significantly affect weed pressure in the broccoli crop. A previous crop that has had excellent weed control generates fewer weed seed to germinate in the broccoli crop. In addition, it is important to keep the areas surrounding the broccoli field free of weeds that have aerial dispersed seeds such as groundsel and sowthistle.

Pregermination of weeds before bed shaping. Pregermination involves the use of irrigation or rain to stimulate weed seed germination before planting broccoli. The emerged seedlings are then killed by shallow cultivation, flaming, an organic herbicide, or a combination of these treatments. Pregerminate as close as possible to the date of planting to assure that the weed spectrum does not change before planting the crop. (Changes in the weed spectrum may occur as a result 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 the preirrigation to control weeds with shallow tillage provides up to 50% weed control in the subsequent crop. If time permits, repeat the pregermination process to further reduce weed populations.

Pregermination of weeds after bed shaping. Once beds are shaped and ready to plant, water can be applied to stimulate a flush of weeds, thereby depleting the quantity of weed seed in the top inch of soil. The flush of weeds can be killed by shallow cultivation, flaming, or applications of organic herbicides. Care must be taken to not till too deeply or additional weed seed may be brought to the surface. The crop can then be planted immediately on these beds. This technique is called "stale" seedbed weed control and can provide substantial control.

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 and longer intervals between deep plowing and subsequent deep plowing (i.e. 3-5 years) is preferred in order 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, and depending upon the species of weed, 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, making substantial contribution to the seed bank. Slow-growing winter cover crops (legumes and cereal/legume mixes) can be particularly problematic in this manner and may allow substantial weed growth that sets seeds 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 your cover crops, particularly in the first 40 days following seeding, to make sure that they are not creating a weed problem for the subsequent broccoli plantings.

Soil sterilization. This technique is generally accomplished by heat generated by soil solarization. This technique is little used specifically for broccoli production given the high costs of these techniques and the relatively short crop cycle of broccoli.

Weed control before crop emerges. Flaming or organic herbicide treatments can be used to kill the flush of weeds anytime between seeding the crop and its emergence. This technique is particularly effective on crops that have slow seed germination. In addition, flaming and organic herbicides are effective on small (i.e. less than 2 true leaves) broadleaves but not effective on grass weeds.

Cultivation is one of the most effective postplant cultural practices that can be carried out. 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). Direct-seeded broccoli is frequently cultivated at about the two to three leaf stage and again 2 weeks later. The first cultivations remove early emerging weeds, and later cultivations cut out weeds that germinate later in the growth cycle.

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 and Robocrop) 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.

Removal of weeds from the seedline can be achieved by the use of specific weeding implements such as finger and torsion weeders. These devices are more suited to transplanted broccoli or later in the crop cycle of direct-seeded broccoli (i.e., at least 50 days following seeding) as their mode of action is more aggressive to the crop. They will not generally remove all of the weeds but rather remove an increased percentage of the weeds that will make subsequent hand-weeding operations more efficient.

Hand hoeing. Hand hoeing is generally necessary in organic broccoli. Broccoli is frequently planted or transplanted to a stand, and as such there is generally no need for a thinning operation. However, weeds are usually removed by hand in the first 30 to 40 days after seeding. Careful hand weeding is necessary at this early stage of the crop cycle because of the delicate stems of the crop. Depending on weed pressure, one or two subsequent hand weedings may be undertaken. 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 effective.



[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Cole Crops
UC ANR Publication 3442
R. F. Smith, UC Cooperative Extension, Monterey County
S.A. Fennimore, UC Cooperative Extension, Monterey County
O. Daugovish, UC Cooperative Extension, Ventura County
M. LeStrange, UC Cooperative Extension, Tulare County
Acknowledgments for contributions to Weeds:
C. E. Bell, UC Cooperative Extension, San Diego County
D. W. Cudney, Botany and Plant Sciences, UC Riverside
W. T. Lanini, Vegetable Crops/Weed Science, UC Davis

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