Strawberries are highly susceptible to weed competition, especially at the initial stage after planting when the plants are small and frequent irrigation provides ideal conditions for weed germination. During stand establishment, weeds such as, little mallow, burclover, sweetclover, and filaree are problematic because their seeds survive the soil fumigation. Likewise, grass species and broadleaf weeds with windblown seeds, such as annual sowthistle, hairy fleabane, everlasting cudweed, and common groundsel may become problematic after planting. Perennial weeds such as field bindweed and yellow nutsedge are increasingly common in Southern California and Santa Barbara County strawberry production and may be a problem on the Central Coast, especially in fields where the crop is carried over into a second year of production. In areas where strawberries are carried over for 2 years, weed management during the second season consists of a combination of preemergence herbicides, mulches, and hand weeding.
In conventional strawberry fields, effective weed management requires a combination of cultural practices, preplant soil fumigation, and herbicide applications when necessary. Proper field and bed preparation is essential for a good weed control program. Fumigation with chloropicrin or a mixture of 1,3-dichloropropene/chloropicrin, followed by an application of oxyfluorfen, flumioxazin, metam sodium, or metam potassium, in conjunction with the use of opaque plastic mulches is a viable alternative to methyl bromide for most weed and pathogen control in California strawberries. However, these fumigants are not as effective as methyl bromide/chloropicrin at controlling nutsedges. The use of totally impermeable film (TIF) enhances weed control provided by 1,3-dichloropropene/chloropicrin (see DRIP FUMIGATION). For weeds that escape preplant controls, hand-weeding or selective herbicides (or both) are used. For weed control alone, the herbicides oxyfluorfen and flumioxazin are very effective on many annual weeds.
Several methods may be used as fumigation alternatives for weed management in strawberry production. In warmer, inland areas soil solarization can be effective at killing germinating weeds. However, the use of soil solarization is not effective in cool, coastal strawberry districts, where the best alternative method of weed control is the use of black, brown, or green mulch films. In some cases, organic mulches have been used instead of plastic mulch. Though the use of weed-free substrates eliminates initial weed control needs, infestation with wind-dispersed weeds can still occur. Soil treatment with steam is very effective in killing weeds if lethal temperatures are reached in soil where weed propagules (plant pieces that can give rise to new plants) are present, but it requires specialized equipment. Anaerobic soil disinfestation can reduce the numbers of many annual weeds but it has limited efficacy on perennial weeds. For more information on alternatives to fumigation, see the NON-FUMIGANT ALTERNATIVES section.
Field Selection and Monitoring
Survey the intended fields for the distribution and density of annual, biannual, and perennial weeds. Soil fumigation allows for the use of land that may have a weedy history, but less weedy sites are preferred. Certain weeds (e.g., hairy nightshade) host soilborne diseases (e.g., Verticillium wilt); by avoiding fields infested with these weeds, there could be a lower incidence of soilborne diseases. Weeds can be a source of food and habitat for mites and insect pests such as lygus bugs and greenhouse whitefly. Controlling weeds in the vegetative stage in and around strawberry fields helps to lower pest numbers and, additionally, prevent weed seed production and dispersal.
Before field preparation, scout the site for weeds and make notes of which weeds are present at the field site and in surrounding areas. Keep records on a monitoring form (example form . Control annual weeds before they produce seed. Control weeds in areas adjacent to strawberry fields (roadsides, ditches) before flowering to prevent their potential dispersal into the field. Pay particular attention to herbicide efficacy, since there are several modes of action to which weeds have developed resistance. Particularly common is resistance to glyphosate in horseweed and fleabane. During the early stages of plant establishment, check frequently for weeds (at least once every 3 weeks during the first 3 to 4 months after planting). Send weeding crews through and around fields, as needed, to remove perennials and purslane.
Weed Management Before Planting
Rotational crops are an important part of a weed control program. Rotations can be with vegetable crops, caneberries, or highly competitive cover crops (cereals, cereal plus legume mixtures, and mustards). Where the cropping cycle permits, sudangrass may be included in the rotation cycle as a summer annual green manure crop. Intensive cultivation for a vegetable crop rotation such as lettuce or a cole crop helps in controlling many problematic weeds. In tunneled caneberries, a common rotational crop, weed germination is confined to plant rows and tunnel post rows that have sufficient moisture. A densely planted small grain crop is highly competitive with weeds and prevents deposits and establishment of wind-dispersed weeds.
Alternative herbicides are also available in rotations. In small grains for example, translocated broadleaf herbicides can help to control infestations of field bindweed, and contact herbicides can control broadleaf annuals. In peppers or celery, S-metolachlor (Dual Magnum) can be used to control yellow nutsedge.
Before bed formation, sprinkler-irrigate to germinate weeds, thus reducing the weed seed reservoir in the soil. After weeds have germinated, remove the seedlings with cultivation or propane flaming. Because most California strawberries are planted in the fall, this practice can be accomplished mid-to-late summer in coastal climates where soil temperatures are usually cool enough for winter weeds to germinate year round. In the warm interior valleys, winter annuals may not germinate during this period.
Opaque mulches are usually dark-colored plastic films. These may be brown, black, or green, but they must restrict light from penetrating the film to be effective. Clear (transparent) plastic is sometimes used in summer in warmer areas to solarize the soil, but in winter it serves as a greenhouse and encourages both weed and strawberry plant growth. The use of clear plastic is a common practice on South Coast winter plantings because it promotes early yield (colored mulches delay fruit production). Growers who choose to use clear plastic in winter must use fumigants and herbicides to ensure that most weed seeds are killed. Plastic mulches with a clear strip covering bed-tops (to stimulate strawberry growth) and opaque strips on the sides of beds (to aid weed control) are also common.
When using opaque mulches, secure them to the soil before transplanting. Place strawberry plants in the soil after cutting a hole into the plastic at the desired spacing. Weed growth is greatly reduced with opaque mulches, but weeds will still grow in the holes where the strawberry is planted and need to be removed by hand. Use the smallest possible hole to minimize weed growth around the strawberry plants. Planting through slits in the mulch helps to minimize weed growth and seed deposition.
After the phase-out of methyl bromide, the most effective soil fumigation is a sequential application of chloropicrin or 1,3-dichloropropene/chloropicrin followed 5 to 7 days later by metam sodium or metam potassium. This combination of pesticides can provide effective control of weeds as well as soilborne pathogens, soil insects and nematodes.
Fumigation with 1,3-dichloropropene (1,3-D) plus chloropicrin mixture (Telone C35, Inline, Pic-Clor 60), chloropicrin, and metam sodium before bed preparation kills the seeds of most weeds and the reproductive structures of some perennials. Nearly all fumigant applications are either immediately covered with a plastic tarp or are injected through the drip irrigation system under a plastic tarp. Drip injection of fumigants such as 1,3-D plus chloropicrin mixture or chloropicrin often improves weed control compared to shank fumigation. However, it is important to thoroughly wet the bed during fumigant injection to ensure weed control on the edges of the bed. Where drip fumigation is used, only the bed is treated, and the row middles (furrows) are left unfumigated. Use soil-applied herbicides such as oxyfluorfen, napropamide, flumioxazin, or pendimethalin to control weeds in the row middles before planting.
Soil fumigants control weeds by killing both germinating seedlings and nongerminated seeds. Chloropicrin, 1,3-D plus chloropicrin mixture (Inline, Telone C35), and metam sodium kill weed seedlings and seeds by respiration inhibition. However, to kill weed seeds, fumigants must be able to penetrate the seed coat and kill the seed embryo. It is more effective to kill moistened seed, because the seed tissues swell with water and allow the fumigant to penetrate more thoroughly. Moist seeds also have higher respiration rates and are more susceptible to fumigants than dry seed with low respiration rates. Proper irrigation before fumigation is one of the keys to effective weed control with all fumigants. Soil temperature must be above 55°F for effective absorption of water by seeds. Preirrigation allows nondormant weed seeds to germinate, and germinating weed seedlings are readily killed by fumigation. Burclover, sweet clover, filaree, and little mallow are among the seeds that are difficult to kill with fumigation. These seeds have impermeable seed coats that limit moisture and chemical penetration, and they remain dormant in the soil.
For additional information on this process, see DRIP FUMIGATION.
Herbicides such as flumioxazin and oxyfluorfen are also quite effective on annual weeds and may be used in addition to fumigation, particularly on beds that will be covered with clear plastic mulch, and in furrows.
Oxyfluorfen (GoalTender) is registered in California as a fallow bed treatment that can be used before planting a strawberry field. It is useful for controlling weeds such as filaree and little mallow, which are not controlled well by the fumigation This treatment is compatible with drip-applied fumigants because it can be applied after the beds are formed but before the plastic mulch is installed. It must be applied 30 days before transplanting.
Oxyfluorfen has the potential for "lift-off" or co-distillation. Lift-off is not drift but instead is the movement of the herbicide with water vapor. Lift-off can move oxyfluorfen from the soil surface to susceptible strawberry foliage. Oxyfluorfen-treated soil can also be moved onto susceptible strawberry foliage as splash from sprinkler irrigation or rainfall. To ensure safety to the strawberry plants, only use oxyfluorfen if plastic mulch will be installed before strawberry transplanting.
Flumioxazin (Chateau) can be applied to bed tops in a manner similar to oxyfluorfen. Flumioxazin controls little mallow (cheeseweed), filaree, clover, and a wide range of other broadleaf weeds. Unlike oxyfluorfen, flumioxazin does not have a "lift-off" potential. It can also be applied to control weeds in furrows (with shielded sprayers) after transplanting but before strawberry flowers.
Pendimethalin (Prowl H2O) can be applied to bed tops in a manner similar to oxyfluorfen before transplanting. Pendimethalin can also be applied to the furrows after transplanting. Pendimethalin is useful for suppressing grass weeds like annual bluegrass.
Pelargonic acid (Scythe) is a postemergence herbicide that provides contact activity or burn down of a wide spectrum of weeds. It can be applied to control weeds in the furrows both before and after transplanting.
Caprylic and Capric acids (Suppress), registered for organic production, is an effective postemergence contact herbicide for most annual weeds, but it does not control perennial weeds or weeds that emerge after application.
Weed Management After Planting
During the early stages of plant establishment, mechanical (by hand through planting holes) removal of weeds from under the clear plastic mulch and from planting holes in all tarps may be necessary. Timely removal is essential to minimize weed competition.
Several herbicides are currently registered for use in newly planted strawberries. Napropamide (Devrinol) and DCPA (Dacthal) are preemergence herbicides that may be applied at transplanting or during the early growth stage of strawberry. Flumioxazin (Chateau) can be used to control weeds in furrows, especially if soil in furrows has not been fumigated. Apply it with shielded sprayers after transplanting, but before strawberry flowers. Unlike oxyfluorfen, flumioxazin does not have a 'lift-off' potential, but take caution to avoid flumioxazin drift to strawberry plants on bed tops. Suppress can be applied to furrows after planting as long as precautions are taken to assure no contact with the strawberry canopy.
Sethoxydim (Poast) and clethodim (SelectMax), postemergence herbicides registered for use in strawberries, are systemic grass herbicides that can be applied to control emerged grass weeds or cereal cover crops grown in furrow bottoms. Sethoxydim and clethodim are effective on many annual and perennial grassy weeds, but sethoxydim does not control annual bluegrass or annual ryegrass. Pelargonic acid (Scythe) is a contact herbicide that burns back a broad range of weeds and is useful in fumigated fields to provide weed control in the furrows.
Each herbicide has certain time restrictions for preharvest interval. When using any herbicide always read the product label for specific instructions.
If the soil is preplant fumigated, weeds that have a hard seed coat (little mallow, burclover, and filaree) may require additional control measures. Flumioxazin is effective on little mallow and filaree if applied before the weeds have emerged; napropamide can also be effective. If the application is delayed until the planting is established, emerged weeds must be removed before herbicide application.
For second-year strawberries, napropamide, pendimethalin, or DCPA can be applied following renovation. Overhead irrigation or rainfall is essential to incorporate the herbicides into the soil.
Tolerance to napropamide has been evaluated on several strawberry varieties. When strawberries are grown on sandy soils, maximum label rates of napropamide have caused strawberry runner inhibition and some reduction in the growth of the strawberry plant. To protect early strawberry development, try to limit or avoid early applications of napropamide.