Agricultural pest management
Integrated Weed Management
(Reviewed 6/08, updated 6/12)
Strawberries are highly susceptible to weed competition especially right after planting when the plants are small and frequent irrigation provides ideal conditions for weed germination. Most weeds that invade strawberries are annuals. During stand establishment, little mallow, burclover, sweet clover, and filaree are common weeds because their seeds survive fumigation. After planting, grasses and broadleaf weeds with windblown seeds, including sowthistle and common groundsel, may become problems. In certain sites, perennial weeds such as field bindweed and bermudagrass or yellow nutsedge may require control, 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 winter 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 additional herbicide applications when necessary. Proper field and bed preparation is essential for a good weed control program. For weed and pathogen control, fumigation with a mixture of 1,3-dichloropropene/chloropicrin or chloropicrin alone, followed by an application of metam sodium or metam potassium, in conjunction with the use of opaque plastic mulches is a viable method of control in California strawberries in response to the phase-out of methyl bromide. However, alternative fumigants that are currently available are not as effective as methyl bromide/chloropicrin for controlling yellow nutsedge. The use of totally impermeable film (TIF) such as VaporSafe enhances weed control provided by 1,3-dichloropropene/chloropicrin (see DRIP FUMIGATION). For weeds that escape preplant controls, hand-weeding and/or selective herbicides are used. For weed control alone, the herbicides oxyfluorfen and flumioxazin are very effective on many annual weeds. As with the fumigation sequence, they do not manage nutsedge.
As an alternative to fumigation, some growers use soil solarization in warmer, inland areas. 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 ones.
Select sites with good drainage in areas with good quality water. Survey intended fields for 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 (Verticillium wilt); by avoiding land infested with these weeds, one would expect a lower incidence of soilborne diseases. Weeds can also provide 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 can prevent pest build and, additionally, prevent weed seed production and dispersal.
Before field preparation, survey the site for weeds and make notes of weed problems at the field site and in surrounding areas. Keep records on a monitoring form (example form— ). Control annual weeds before they produce seed. 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.
Soil fumigation has been widely used to control weeds in California strawberries. If fumigation is not used, cultivation, mulches, or soil solarization can also provide weed control. These strategies can also be used in conjunction with soil fumigation to further enhance weed, pathogen, and nematode control.
Crop rotation. Rotational crops can be an important part of a weed control program. Rotations can be cash crops such as lettuce or cole crops, or highly competitive noncash cover crops (cereals, cereal/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 of a vegetable crop rotation such as lettuce or a cole crop helps control many problem weeds. A densely planted cereal rye cover crop or small grain crop is highly competitive with weeds and provides better weed control than a legume cover crop. In addition, alternative herbicides are 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 that are not controlled by soil fumigation. In peppers or celery, yellow nutsedge may be controlled with S-metalachlor (Dual Magnum).
Cultivation. Following bed formation, sprinkler irrigate to germinate weeds, thus reducing the weed seed reservoir in the soil. After weeds have germinated, remove the seedlings with minimal soil disturbance, for instance by paraquat application or with propane flaming. Cultivation equipment may be used for furrows, but the sides and tops of beds must be hand-weeded. Avoid cultivating too deeply to reduce the risk of bringing up weed seeds from lower depths. 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. Opaque mulches are usually some type of dark-colored plastic. These may be brown, black, or green, but they must restrict light from penetrating the film to be effective. Blue plastic is not effective. Clear 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 growth and strawberry plant growth. The use of clear plastic is standard practice on South Coast winter plantings because it promotes early yield (colored mulches delay fruit production). Growers that choose to use clear plastic in winter must use effective fumigants to ensure that most weed seeds are killed.
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 hole where the strawberry plant is 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.
Soil solarization. In summer, clear plastic applied to preshaped beds several weeks before planting will solarize the soil and reduce the number of weed seeds and soilborne disease organisms. On the central coast, this practice requires at least 12 to 15 weeks in order to obtain pest management benefits; consequently, solarization is usually not practical in this region. Solarization is much more effective in areas of the state where temperatures are consistently (30-45 days) hot enough in summer to produce soil temperatures of at least 122°F. Solarization can be even more effective if the residue of a cruciferous crop (especially broccoli or mustards) is incorporated into the soil just before the plastic is installed, or following an application of metam sodium (40 gal/acre). For more details on how to effectively solarize soil, see Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds. For optimum results, check the plastic for good adhesion to the soil and for any holes that might have developed and need repair.
Fumigation. With 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 materials can provide effective control of weeds as well as soilborne pathogens, soil insects and nematodes.
Fumigation with methyl bromide, 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 plastic mulch or are injected through the drip irrigation system under plastic mulch. Drip injection of fumigants such as 1,3-D plus chloropicrin mixture or chloropicrin often improves the 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 are left unfumigated. Soil-applied herbicides such as napropamide, flumioxazin, or pendimethalin can be used to control weeds in the row middles.
Soil fumigants control weeds by killing both germinating seedlings and ungerminated seeds. Methyl bromide, 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. Among the seeds that are difficult to kill with fumigation are burclover, sweet clover, filaree, and little mallow seed. 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. Herbicides such as flumioxazin and oxyfluorfen are also quite effective on annual weeds and may be used with similar success instead of fumigation.
Oxyfluorfen (Goal) 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 fumigants. This treatment is compatible with drip-applied fumigants because it can be applied after the beds are formed but before the tarps are installed. It must be applied 30 days before transplanting.
Oxyfluorfen has the potential for "lift off" or codistillation. 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 control or burn down of a wide spectrum of weeds. It can be applied to control weeds in the row middles both before and after transplanting.
During the early stages of plant establishment, mechanical (by hand) 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 competition.
If strawberry leaves are pruned in winter, an opaque plastic mulch can be used to control weeds. This provides good control for much of the harvest period. Organic mulches could also be applied at this period. To effectively control weeds, at least 2 inches of mulch are necessary and must be maintained to keep weeds from growing through the mulch. However, organic mulches may increase problems with gray mold, snails, slugs, earwigs, and possibly other insects.
Herbicides. 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 stage of strawberry development. Flumioxazin (Chateau) can be used to control weeds in furrows, especially if they were not 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.
Sethoxydim (Poast) and clethodim (Prism), postemergence herbicides registered for use in strawberries, are systemic grass herbicides that can be applied to control grass weeds after they have emerged. Pelargonic acid (Scythe) is a contact herbicide that controls a broad range of weeds and is useful in fumigated fields to provide weed control in the row middles.
Each herbicide has certain time restrictions that apply to its preharvest interval. When using any herbicide always read the 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. Napropamide is effective on little mallow and filaree if applied before the weeds have emerged. If the application is delayed until the planting is established, emerged weeds must be removed before application.
For second-year strawberries, napropamide, pendimethalin, and DCPA can be applied following renovation. Overhead irrigation or rainfall is essential to incorporate the herbicides into the soil.
Strawberry 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 development of the strawberry plant. Limit initial use to obtain the benefit associated with this herbicide, while minimizing the risks.
Sethoxydim and clethodim are effective on many annual and perennial grassy weeds, but sethoxydim does not control annual bluegrass or annual ryegrass.
UC IPM Pest Management Guidelines: Strawberry
S. A. Fennimore, Weed Science/Plant Sciences, UC Davis/Salinas
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