Integrated Weed Management

Weeds compete with cole crops for nutrients, sunlight, and water. Weeds in and around the field sometimes harbor pest insects, nematodes, plant pathogens, or vertebrates that can invade or move to the crop after planting.

Weed control is especially important in precision-planted crops, where loss of seedlings to competition can substantially reduce the vigor and uniformity of the stand. Also, any weeds that mature to flowering can contribute to the soil weed seed bank and become weed problems in subsequent crops.

Cole crops are either direct-seeded or transplanted in the field. Brussels sprouts and cauliflower are primarily transplanted. In comparison with direct seeding, transplanted crops mature earlier in the field, have a more uniform stand, and need less weed management.

In direct-seeded crops, weed control is especially important during the first 30 days after planting. As the crop grows older, most cole crops compete well with weeds and shade them out.

There are also regional differences in weed problems:

• In cool, moist coastal areas, winter annual weed conditions are favored most of the year.
• In the southern desert, early fall plantings compete with weeds that germinate during warm conditions, and later fall plantings compete with winter annual weeds.
• In the San Joaquin Valley, grass weeds and early fall and winter weeds are most common.

It is important to rely on both cultural and chemical methods (integrated control) to keep weed numbers tolerable. Integrated management of pest insects, nematodes, and plant pathogens also contributes to successful weed management; damage and stunting due to other pests weakens the crop, making it less likely to outcompete weeds.

Irrigation Management

Throughout the season, manage irrigation carefully to improve the control of weeds and other pests:

• Keep furrows well maintained to prevent them from collecting water. Ponding water favors the growth of weeds with wind-dispersed seeds, water-loving weeds, and soilborne plant pathogens.
• Regularly monitor for uniformity and adjust sprinklers or drip systems to prevent uneven distribution of water.
  • Poor irrigation uniformity stresses patches of the crop and impairs the ability to outcompete weeds.
  • Regularly inspect and maintain emitters and lines for clogs and leaks.
• Skillfully schedule low-volume irrigation. Excess water can favor weed growth. Use CropManage to keep track of irrigation scheduling.

Herbicide Considerations

Herbicides may be applied before planting (preplant), after planting and before the crop emerges (postplant preemergence), or after planting and after crop emergence (postplant postemergence). Most herbicides used in cole crops control only weeds germinating from seeds. Herbicides can vary in effectiveness depending on field conditions and the targeted weed species. In some growing regions, fall-planted cole crops may receive a layby application (after the crop is well established) applied at the last opportunity for access to the field. The choice of timing depends on your schedule and the effective method for the particular herbicide and problem weeds in your field. For example, many preemergence herbicides are activated by sprinkler or drip irrigation, so the timing of the first irrigation may be the most important determinant of when to apply herbicide.

Herbicide choice depends largely on the weed species to be controlled, but is influenced by crop rotation, irrigation method, and soil type. Relatively few herbicides are available for use in cole crops, and the registered products are weak on key weeds such as shepherd's purse and common groundsel. Herbicides also vary in selectivity and potential for damaging the crop (phytotoxicity). When trying a new herbicide, or if you suspect that an herbicide may be damaging the crop, leave representative parts of a row untreated, to use in later comparison of weed efficacy or crop sensitivity. For more information on herbicide damage to nontarget plants, see UC ANR's Herbicide Symptoms diagnostic tool.

Many of the herbicides used in cole crops do not kill emerged weeds, and in these situations, it is best to cultivate soil to kill emerged weeds before making an herbicide application. An exception is oxyfluorfen, registered for postemergence application in broccoli and cauliflower. Other postemergence herbicides may control only grasses or require use of a hooded sprayer to shield the crop while controlling broadleaved weeds.

No single herbicide provides satisfactory control of all weeds; combinations of herbicides or sequential applications and cultivation and hand weeding will often be required. Check with your cooperative extension advisor or agricultural commissioner for the latest information on available herbicides and their compatibility and recommended rates.

Proper application is as important as the right choice of herbicide for controlling weeds without injuring the crop.

• Use application equipment suited to field conditions.
• Calibrate sprayers before each use.
• Test the pH and hardness of water used for the spray solution and adjust if needed.
• Know your soil type and adjust the application rate accordingly. Lower rates are usually recommended or required for sandy soils for the following reasons:
  • Sandy soils have fewer binding sites for herbicides than do clay soils. Herbicides are therefore more available to be taken up by plants' roots in these soils than they are in clay soils.
  • Herbicides are more effective at lower rates in sandy soils because herbicide uptake by weeds is higher.

It is important to time herbicide application relative to rain or irrigation for optimal efficacy and minimal potential for off-site movement and contamination. Rain or irrigation can increase the potential for runoff and leaching, and some herbicides lose effectiveness when leached or otherwise moved from the soil. However, application of preemergence herbicides must also generally be followed by appropriate irrigation or adequate rainfall to activate the herbicide. For herbicides requiring a post-application irrigation, take care to not overirrigate; about 1/2 inch of water during the first irrigation is often appropriate. If rainfall is insufficient to activate the herbicide, irrigation or mechanical incorporation will be necessary.

MONITORING

The type of weed management program depends mostly on the weed species present, where they are located, and how numerous they are. Survey each field for weeds:

• before harvesting the preceding crop,
• after planting but before the first cultivation, and
• between heading and harvest.

When surveying, identify, rate, map, and record the summer annuals, winter annuals, and perennial weeds.

To conduct a weed survey:

1. Walk through the field in a regular grid pattern. Also check the areas surrounding the field.
2. Rate the degree of infestation for each species on a weed survey form. Small weed seedlings may be difficult to identify, so scan the area for larger plants that are easier to identify. Use a numerical scale, or rate infestations as light, medium, or heavy.
3. Record what species of weeds are mature and producing seeds or other propagules; these weeds will be problems in succeeding crops.
4. Mark the main weedy locations on a map or use a GPS and computerized database such as GIS (geographic information system) to map weeds, if available. Take detailed notes of perennial weeds and other special weed problems to facilitate targeting of the separate practices needed for these weeds.

Maintain these records for each field to track long-term trends in weed species and infestation severity to help you assess the effectiveness of your management program and determine whether modifications are needed.

The recommended surveys, especially during weed monitoring in the previous crop, tell you where to apply specific weed control methods that are effective for the targeted weed species. For example, perennial weeds generally require different management than annual weeds.

WEED MANAGEMENT BEFORE PLANTING

Weed control is easier and less expensive in fields not infested with difficult-to-control weeds. Problem weeds include:

• common chickweed
• common groundsel
• cruciferous weeds such as mustards, London rocket, and shepherd's-purse
• prickly lettuce
• little mallow
• burning nettle
• hairy nightshade
• black nightshade
• nutsedges
• common purslane
• sowthistles

If problem weeds are numerous, the best strategy is to rotate to a crop that allows successful control. For example, yellow nutsedge in coastal areas may be best controlled by rotating to a crop that receives preplant fumigation, such as strawberry.

Fallow

Sanitation is critical for effective weed management, and fallow is an important time to clear out weeds. Certain weeds, such as purslane, shepherd's-purse, and burning nettle, can produce thousands of seed per individual weed in a single season. To reduce seed production, disc or mow fallow fields before weeds flower and produce seeds. To reduce weed seed dispersal into fields, regularly control weeds in areas such as fence lines, field edges, irrigation ditches, and roadsides.

Keep cultivation equipment and irrigation water free of weed seeds and vegetative propagules to avoid spreading weeds. Wash equipment with high-pressure water before leaving weedy areas or entering fields. Screen surface water sources of irrigation to avoid applying weed seeds in irrigation water.

Controlling weeds in fallow beds helps to reduce weed infestations in production fields. Emerged weeds on fallow beds can be controlled by shallow cultivation with a Lilliston® cultivator. Postemergence herbicides such as paraquat, glyphosate, pelargonic acid, and the combination of caprylic acid and capric acid can also be used to control emerged weeds on fallow beds. However, neither paraquat nor glyphosate completely control burning nettle, field bindweed, little mallow (cheeseweed), or nutsedge. Pelargonic acid and the combination of caprylic and capric acids will control little mallow.

Mechanically control Conyza (horseweed and fleabane) biotypes that are resistant to glyphosate, or use other herbicides with a different mode of action in fallow fields and nearby areas. Check herbicide efficacy two weeks after application to identify resistant weeds and control them before they produce seeds.

Deep Plowing

Plow deeply to bury weed seeds and perennial weed propagules below the depth at which they can germinate or regrow. The viability of buried seeds and propagules declines over time. Wait a longer interval before subsequent deep plowing (3–5 years) to reduce the number of buried weed seeds that will still be viable when brought back to the surface. Deep moldboard plowing 16 inches below the soil surface can temporarily reduce nutsedge infestations by 95 to 98%.

Plowing to bury seeds is not as effective against weeds that have hard-coated seeds, such as little mallow. These seeds can survive buried for years at greater depths and will germinate when they are brought to the surface by subsequent plowing.

Crop Rotation and Field Sanitation

Excellent weed control during the previous crop reduces the weed seed bank and makes the field less weedy over time. Keep areas around fields free of weeds that have aerially dispersed seeds such as groundsel and sowthistle. Clean equipment and tools before leaving weedy areas or entering fields. Screen surface water before irrigating with it to avoid applying weed seeds.

Cover Crops

Avoid slow-growing winter cover crops that allow substantial weed growth that sets seeds during early growth of the cover crop. Plant fast-growing cover crops, or if slow-growing cover crops must be used, use higher seeding rates to allow them to better compete with weeds. For example, Indian mustard (Brassica juncea), cereal rye (Secale cereale), and white mustard (Sinapis alba) can be used as winter cover crops and can provide complete ground cover within 30 days of planting, as long as an adequate seeding rate is used to favor rapid ground cover and uniform establishment. See Cover Cropping for Vegetable Production, ANR Publication 3517 (PDF), and the UC Davis Cover Crops Database for more information.

Drill cover crops in closely spaced rows (about 6 inches apart) to help ensure rapid canopy closure of the cover crop. This is generally preferable to broadcast seeding, as it does not require as much seed. If broadcast seeding must be used to plant the cover crop, use higher seeding rates to obtain adequate coverage.

Cover crops provide a variety of benefits, but also have the potential to increase weed pressure in subsequent crops. Annual weeds frequently establish in the cover crop; depending on the species, weeds can grow, set seed, and decompose while the cover crop is present. They can be difficult to detect and manage, and may go unnoticed, substantially contributing to the weed seed bank and causing the cover crop to act as a nurse crop for weeds. Monitor cover crops, particularly during the first 40 days after seeding; ensure they are not creating a weed problem for subsequent plantings.

Though relatively uncommon, cover crops can also be grown on beds at spacing typical for growing vegetables and cultivated to control weeds. Use cultivation equipment such as rotary hoes or tyne weeders that disturb the top few inches of soil to control weeds after the cover crop is established. At this time, the roots of weed seedlings will be smaller and more vulnerable to shallow tillage than the more deeply rooted cover crops.

Cultivation and Bed Preparation

Control wind-dispersed weed species in areas surrounding the field before they flower, if possible. Preplant plowing of fields, followed by irrigation and one or two discings before bed formation, will destroy many weeds. Because plowing brings buried seeds to the surface, it must be followed by rain or preirrigation to germinate a flush of weed seed, and then followed by cultivation, discing or spraying of the emerged weeds before they flower.

Shallow cultivation can significantly reduce problem weeds such as common groundsel and sowthistle in the Central Coast region, and may also reduce certain soilborne plant pathogens. Cultivation should bury seeds of these weeds a few inches below the soil surface to prevent them from germinating.

Level the land before planting to avoid water collection in low areas, which favors weed growth. Precisely align and space beds and carefully align cultivators so that weed cultivation of bed tops is effective. Use GPS-assisted, auto-guidance systems to properly prepare and cultivate the beds.

Pregermination of Weeds Before Shaping Beds

This method involves germinating and controlling weeds before forming seed beds. Irrigate or await rainfall to induce weed seed germination, then kill emerged seedlings with shallow cultivation, plowing, flaming, an herbicide, or a combination of these methods. If time permits, repeat this method to further reduce the weed seed bank.

Conduct the final pregermination as close as possible to bed shaping and planting to ensure that the germinating weed spectrum (seasonal variation in weed species) does not change before planting. The season and weather affect when the seeds of each weed species germinate.

Time of year, irrigation method, and the interval between irrigation and weed control affect the efficacy of pregermination. Shallow tillage 14 days after preirrigation reduces weed germination in the subsequent crop up to 50%.

Pregermination of Weeds After Shaping Beds (Stale Seedbed Method)

This method provides substantial weed control to beds that are shaped and ready to plant. Weeds are controlled just before seeding or transplanting after seed bed formation. Similar to pregermination before bed formation, weeds are germinated with irrigation or rainfall. When using this method, weeds should be controlled with minimal disturbance of soil.

1. Prepare the soil as if you are about to direct seed or transplant. Once beds are ready to plant, irrigate to germinate weed seeds in the top inch of soil. Soil should be sufficiently moist to encourage germination of weed seeds.
2. Wait as long as possible for weeds to germinate and emerge. Allow weeds to grow to the third leaf stage, or at least to the first true leaf.
3. Once soil is dry enough to allow equipment on the field, control emerged weeds with shallow cultivation, flaming, or an herbicide.

• Take care to not cultivate too deeply, otherwise additional weed seeds from deeper layers may be brought to the surface.
• If transplanting, control weeds just before transplanting.
• If direct seeding, control weeds before planting, just before crop emergence, or both if necessary, such as with direct-seeded broccoli.
• Apply a preemergence herbicide at the appropriate time relative to field preparation and the age and species of crop. Application timing can vary depending on whether the preemergence herbicide is incorporated by cultivation or irrigation.

4. Otherwise, minimize soil disturbance for as long as possible after planting.

Soil Solarization

Cover moist, bare soil with clear plastic for at least 4 to 6 weeks during a sunny, warm time of the year to control many weed species, soil-dwelling insects, nematodes, and pathogens in the top few inches of the soil. This method is not often used in cole crops because of the high cost, short crop cycle, and limited number of times immediately before planting that weather is suitable for solarization. For more information, see Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds, UC ANR Publication 21377.

Flaming

Use flaming any time before the crop emerges. This method is less effective on grasses than broadleaves because the growing point for grasses is below the ground.

Flaming can be used with the stale seedbed method to control small emerged weeds. It can also be used in organic operations to control weeds on winter fallow beds when the soil is too wet to cultivate but dry enough to drive through the field with a tractor.

The flame causes the water in the plant to expand, which ruptures the cell walls. Weeds must have less than two true leaves for greatest efficacy. Typically, flaming can be done through fields at 3 to 5 miles per hour, although this speed depends on the heat output of the unit being used. Propane-fueled flamers are the most common models used.

Windless conditions achieve the best results for this method, as winds can prevent the heat from reaching the target. Early morning or evening are the best times to observe the flame for adjustment.

After flaming, weeds that have been killed change from a glossy to a matte finish. This occurs very rapidly in most cases.

Herbicides

Preemergence herbicides are applied before weeds emerge. If using herbicides, create a custom herbicide weed susceptibility chart for your field and use it to make the best herbicide selection for your situation. See the Herbicide Treatment Table for information on specific herbicides that can be used at this time.

Preemergence herbicides can be applied before direct seeding or transplanting. These herbicides usually require rainfall, irrigation or mechanical incorporation to be effective.

Minimize soil disturbance during transplanting and for as long as possible after applying a preemergence herbicide. However, timely cultivation after weeds emerge may also be necessary. Transplants exposed to a properly applied preemergence herbicide may temporarily show leaf cupping or crinkling, but rapidly outgrow these symptoms if they are hardy and not severely stressed before transplanting.

Organically Acceptable Methods

The goals of organic weed management are to use organic-compliant practices and products to

• reduce weed infestations and weed seed production
• give the crop a competitive advantage over weeds
• produce the crop as economically as is feasible

Weed control in organic cole crops depends on the integration of good cultural practices, such as pregermination of weeds, sanitation, careful cultivation, and hand weeding. Cover cropping is a key cultural practice in many organic systems and should be done in a way that depletes the weed seed bank over time (see the Cover Crops section above). Additionally, the use of opaque plastic mulches on beds (with transplants in holes) can control weeds and reduce irrigation needs.

Practice sanitation in fields to prevent weed introduction by contaminated equipment, tools, transplants, and irrigation surface water. This will reduce the need for weed management during the current and subsequent crops.

It is very important to either plant cole crops in fields with few weeds or reduce weed pressure before planting. If possible, use transplants instead of direct seeding organic crops. This will establish the crop canopy more quickly and give it a head start on shading out weeds. Transplants also require fewer days to harvest and generally need only one weeding, whereas direct-seeded crops require two.

Cultivation can be used to control weeds any time before the crop is planted. Flaming can also kill weed foliage any time before transplanting or the emergence of direct-seeded crops, especially for crops that have slow germination. Flaming kills young broadleaf weeds with less than two true leaves, but grasses, perennials, and older broadleaves will generally regrow from belowground parts. Prior to planting (including during fallow periods), organic herbicides consisting of caprylic and capric acids can also be used to burn down flushes of weeds on fallow or stale beds. Because organic herbicides can be expensive, their cost should be taken into consideration when planning for their use.

WEED MANAGEMENT AFTER PLANTING

Control weeds during their critical seedling stage. Cole crops (with the exception of cabbage) compete well with weeds later in their crop cycle. However, some weeds (such as sowthistle) will grow taller than crop plants and continue to produce seed. Late-season weeding may be needed to reduce seed set and dispersal.

Cultivation

Cultivation typically controls weeds on 80% of a typical double-row, 40-inch bed. Good cultivation depends on precise alignment of cultivation knives around the uncultivated seed lines. When cole crops have two to three true leaves, sweeps or knives can be set as close as 2 inches on each side of the seed rows as long as they are set to cultivate shallowly; closer cultivation will cut crop feeder roots. Crowders can also be used to throw dirt to the base of the crop plant and bury small weed seedlings. On 40-inch beds with a single row of the crop (most often the case for cauliflower), there is more dirt to throw to the base of the crop plant, and burying weed plants is more effective. The thrown dirt both buries small weeds and prevents new weed seedlings from germinating.

Cut weed seedlings as close to the seed row as possible without disturbing the crop. Precision guidance systems such as Kult®, Steketee®, or Robocrop® use digital cameras and computer controls to shift the cultivator sled, which allows for more precise cultivation. The greater precision of these machines removes a higher percentage of the weeds and allows the grower to confidently reduce the uncultivated band width.

To remove weeds in the seed line, hand weed or use special implements such as finger or torsion weeders. Finger or torsion weeders are most effective at killing small weed seedlings in the white thread stage. If timing is correct they can efficiently remove a high percentage of weeds in the crop row, greatly reducing subsequent hand weeding time. These devices are more suitable for transplants and timing is critical to remove the weeds when they are still small enough for the fingers to dislodge them from the soil.

Automated weeders are now available that use either a split knife or a spinning blade to remove weeds from the seed line without damaging the crop. These machines were designed for use in transplanted crops; although they are sometimes also used in direct-seeded crops, they are most effective for transplants. Because the transplants are initially larger than germinating weeds, the machines use the size difference to know which plants to remove and which plants to keep. They do not remove all weeds from the seed line but make subsequent hand weeding operations quicker and more efficient.

Fields may be cultivated two or more times between planting and harvesting depending on the crop, location and season:

• Most cauliflower and Brussels sprouts are transplanted at low planting densities (for example, a single seed line on a 40-inch wide bed), so multiple mechanical cultivations can control weeds for the entire cropping season.
• Broccoli is usually planted to a stand and not thinned. Hand weed or cultivate at least once if economically feasible.
  • For direct-seeded broccoli, cultivate at about the two- to three-leaf stage and again two weeks later. About 80% of the bed can be cultivated on double-row, 40-inch beds, assuming a 4-inch-wide strip along each seed line is left uncultivated. Soil can be crowded to the base of the plant on the second cultivation, smothering small weeds.
• In coastal areas, fall-planted cole crops may require more cultivation than summer-planted crops.
  • Cool temperatures slow crop growth; it may take 45 to 60 days for cole crops to shade out weeds and eliminate the need for further weed control.

Hand Weeding

Use a long-handled blade to cut weeds at or slightly below the soil surface to avoid mechanical damage to the delicate crop stems and roots.

Carefully hand weed when needed during the first 30 to 40 days after planting. Depending on weed pressure, hand weed one or two more times in direct-seeded cole crops. Employ the methods above before crop emergence or transplanting to reduce the time needed for hand weeding and improve its effectiveness.

Flaming

Flaming can be used after planting any time before the crop emerges. Weeds must have less than two true leaves for maximum effectiveness. Windless conditions achieve the best results for this method, as winds can prevent the heat from reaching the target. Early morning and evening are the best times to observe the flame for adjustment. Flaming is also less effective on grass weeds than broadleaf weeds. Flame weeds at a pace of 3 to 5 miles per hour depending on the heat output of the flamer.

Surface Banding of Ammonium Nitrate Fertilizer

Surface banding with ammonium nitrate is an effective way to apply nitrogen and gives the crop a competitive advantage over weeds. The waxy cuticle that cole crops develop once they have at least three true leaves prevents damage to the crop unless the plants are very wet. Because many weeds lack this cuticle, the fertilizer will burn them, especially when applied on warm days. Use a shielded sprayer to prevent application to the growing point or emerging new leaves of the cole crop.

Table 1. Effect of Surface Banding of Ammonium Nitrate Fertilizer on Weed Species in Cole Crops.

Controlled	Partially controlled	Not controlled
chickweed*	purslane, common	annual grasses
groundsel, common		bindweed, field
little mallow (cheeseweed)		goosefoot, nettleleaf
mustards		lambsquarters, common
nettle, burning		nutsedge
nightshade, black		sowthistles
nightshade, hairy
pigweeds
pineappleweed
radish, wild
rocket, London
shepherd's-purse

*Must be in the cotyledon to 2-leaf stage.

Herbicides

To determine which herbicide is most likely to be effective against the particular weed species present, create a custom herbicide weed susceptibility chart for your field. See the Herbicide Treatment Table for information on specific herbicides to use at this time.

Organically Acceptable Methods

After planting, use hand weeding, flaming, and cultivation in an organically certified crop. Capric and caprylic acids may also be used after planting if applications are made with hooded spray equipment to prevent crop injury.

Careful hand weeding is typically critical for effective weed control in organic cole crops. However, transplanting instead of direct seeding reduces, and sometimes eliminates, the need to hand weed after planting.