Weeds reduce yields by competing for space, light, water, and nutrients, weakening crop stand, and by reducing harvest efficiency. Some weeds can also increase pest problems by serving as hosts for insects, diseases, or nematodes. Weeds are most competitive if they emerge prior to or at planting until about 6 to 8 weeks after crop emergence (fewer weeks when using transplants). After 6 to 8 weeks, tomatoes become more competitive and they are usually less affected by late-germinating weeds. However, even late germinating weeds can produce seed and, in some instances, interfere with harvest.
Effective weed management in tomatoes involves crop rotation practices, cultivation, proper field preparation, sanitation, irrigation management, and proper selection of herbicides. When combined with good cultural practices, available herbicides can control many of the weed species that are found in tomato fields. Herbicide choice depends on the weed species that are present, the cultural practices followed by the grower, and the crops planted following tomato.
In tomato production, many growers establish beds in fall to facilitate early spring planting; some beds may be treated with an herbicide at this time. Herbicides may also be applied several weeks before planting, at the time of planting, after planting but before crop and weed emergence, after crop and weed emergence, or after thinning or transplanting. Herbicides can be classified according to their use as preemergence (controls weeds after the seeds germinate but before they emerge from the soil and usually provides residual control) and postemergence (controls emerged weeds but gives little or no residual control). Preemergence herbicides are absorbed by roots, emerging shoots (hypocotyl), or both. Postemergence herbicides are absorbed by leaves and stems of weeds. Some herbicides have both preemergence and postemergence activity.
Herbicides work best if soil moisture is adequate for plant growth. However, do not apply these materials when the soil is too wet because soil compaction can occur during application and the herbicide may not be uniformly mixed in the soil where mechanical incorporation is required. Preemergence herbicides will kill germinating seeds but not dry seeds. Postemergence herbicides work best on plants that are not moisture stressed. Nonstressed plants more effectively translocate the herbicide from where it is absorbed (mostly leaves) to the site of action.
Identification and knowledge of target weeds is essential for weed management in tomatoes because it influences management decisions, like herbicide selection. Conduct weed surveys on each field at least twice a year: the first after crop planting but before weeding, and the second just before harvest. Record observations on a monitoring form . Note the location of weeds producing seed as carefully as possible. Weed clusters are often stable through time and may require additional spot treatment to achieve effective control. Records from previous crops will indicate which weeds escaped control and will likely infest the tomato crop. Also, examine fence rows and ditch banks because these are other sources for weed invasion. Give special attention to surveying perennials and marking their locations on a map of the field for follow-up control action.
Weed Management Postharvest and Before Planting
Crop rotation can effectively reduce difficult weed problems by altering the environmental conditions that favor a particular weed species or by permitting the use of alternative methods to control these weeds. Corn is considered a good rotational crop for tomatoes because some corn herbicides have the ability to control nightshade, yellow nutsedge, and field bindweed, and corn is not a host for dodder. Alfalfa hay is also a good choice for a rotational crop because its frequent cutting cycle reduces many weeds, and herbicides available to use on this crop eliminate most other weeds. Other crops considered as useful rotational crops with tomatoes include wheat, cotton, rice, dry beans, onions, carrots, and safflower. Rotational crops that are not recommended include other solanaceous crops such as potatoes, peppers, and eggplant because they are genetically similar, and all but potatoes have similar cultural practices. Similar herbicides are used in these crops, which often result in common uncontrolled weeds. For the same reasons, monocropping of tomatoes is also not recommended.
Many major weed problems can be reduced by avoiding fields that are severely infested with weeds such as nightshades, little mallow, field bindweed, nutsedge, and parasitic dodder. Irrigation water can also be a source of weeds; keep canal banks free of weeds or install a weed screen on the inlets from canals. Avoid moving weed seed into fields on equipment. When equipment has been used in a weedy field, clean it before entering other fields.
Inverting the top soil profile (at least 12 inches deep) with a specialized moldboard plow (like the Kverneland plow) can effectively reduce nightshade and nutsedge populations by deeply burying seeds and tubers. In situations where this is not possible, such as with drip irrigation or conservation tillage systems, preirrigation or rainfall can germinate nightshade species before planting and the weed seedlings can be uprooted by a light cultivation or treatment with certain postemergence herbicides to reduce the population.
Soil solarization can provide control of many soilborne diseases, nematodes, and weed pests. Solarization is primarily effective only in June, July, or August and needs to be applied for 4 to 6 weeks. 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.
Metam sodium, metam potassium, and to a lesser degree 1,3-D or 1,3-D plus chloropicrin can provide control of many weeds and other soilborne pests. Adequate time is needed between fumigant application and crop transplanting to prevent phytotoxicity.
Herbicide applications made to fall or winter beds allow some fields to be planted early that could not otherwise be cultivated because of wet soil conditions. Adequate rainfall is necessary, however, to break down some fall-applied herbicides so that tomatoes planted in spring will not be injured. Ensure fallow bed herbicide applications to control annual weeds are made to clean cultivated beds. For fields that had weed problems during the season, especially perennial weeds such as field bindweed and little mallow, control weeds postharvest. Irrigate and then apply contact herbicides.
Some herbicides are best applied just before planting and incorporated into the soil. The entire bed top may be treated or band treatments applied over the seedline. Band treatments proportionally reduce the herbicide cost, usually require sprinkler irrigation for incorporation/activation, and usually reduce the risk of herbicide carryover into the next crop. An additional method of weed control, generally cultivation, will be required between crop rows. A postemergence weed treatment also may be necessary before planting to control weeds that have already emerged. Herbicide mixtures may be used to enhance weed control; select herbicide mixtures based on the benefits expected from increased weed control, the soil residual activity of the herbicides, and the degree of tomato selectivity.
Weed Management at Planting
Manipulating planting dates might be used to take advantage of weed germination under different temperatures. For example, early plantings under cooler soil temperatures usually escape barnyardgrass competition during the tomato seedling stage. However, such decisions must balance weed control benefits against other, possibly negative, effects on tomato production.
With direct-seeded fields, rapid, even stand establishment allows the crop to better compete with weeds. Full tomato stands with good shading of the soil surface reduce the ability of late germinating weeds to compete. Stand establishment is optimized when soil temperatures are 70° to 80°F; under these conditions tomatoes generally emerge within 10 days. Under cool soil temperatures (50° to 60°F), emergence is often uneven and may take longer than 3 weeks.
A soil cap (2- to 4-inch mound of soil) over the seedline at planting can reduce the first flush of weeds competing with the crop seedlings. Discs, or other implements, form the soil cap just after planting. The cap is removed just after tomato seedlings germinate but before rapid elongation of the hypocotyl (emerging shoot). Rains could delay cap removal, leading to potential tomato stand loss. High temperatures during cap removal can cause scalding of the emerging tomato seedlings and result in stand loss as well. However, under good conditions, weed seeds that germinate in the soil cap are destroyed when the cap is removed and fast-growing weeds that germinate in the original bed are often scraped off by the cap removal operation.
Transplanting into fields with high potential weed populations provides the crop an initial growth advantage over the weeds. Compared to direct seeding, more herbicides are available for transplants, especially preemergence herbicides, and in many cases, initial hand weeding can be eliminated or greatly reduced. Subsequent cultivations can further reduce weed populations along the sides of the planted row. Within the crop row, soil can be directed toward the base of the tomatoes, covering small, emerged weeds and creating a dry mulch. As hybrid tomato varieties have become more expensive, transplanting has become a more widely used method, thus reducing herbicide inputs, hoeing, and thinning costs.
Weed Management After Planting
Preventing weeds from going to seed helps reduce weed populations in subsequent crops; this also applies to areas adjacent to the cropped fields. Irrigation water can also be a source of weeds; keep canal banks free of weeds or install a weed screen on the inlets from canals. Avoid moving weed seeds into fields on equipment. When equipment has been used in a weedy field, clean it before entering other fields.
Subsurface drip irrigation can also aid weed control by keeping bed tops dry. If tomatoes are transplanted, supplemental irrigation may not be needed initially. If tomatoes are direct seeded, a second irrigation system (furrow or sprinkler) may be needed to germinate the tomato seed, which also favors germination of weed seeds. Improve weed control by ensuring the crop is well established and free of emerged weeds before subsurface drip irrigation is used. Perennial weeds, such as field bindweed, are likely to remain a problem with this system.
With furrow irrigation systems, maintaining alternate row irrigation helps keep the bed tops from becoming overly wet while maintaining adequate soil moisture for the crop. By keeping the bed tops drier, less weeds are likely to germinate in the soil surface.
Cultivation and Hand Weeding
After crop emergence, mechanically cultivate close to the seedline to reduce the amount of hand-weeding needed. To avoid excessive competition with the tomatoes and to make removal easier, cultivate when weeds are small. Removing weeds in the seedling stage permits a shallow sweep cultivation, which helps to avoid bringing more weed seeds near the surface where they might germinate. When tomato seedlings are about 5 inches tall, cultivation tools can be arranged to create a dry layer of soil (dry mulch) on the seedline to help prevent weed seeds from germinating and to smother small, emerged weeds. A single seedline per bed facilitates cultivation and is less costly to hand-weed than two seed lines.
Cultivation is effective at controlling many weeds in tomatoes; one exception is the parasitic weed dodder. Once dodder attaches to the tomato plant, it does not require connection with the soil and cannot be selectively controlled with cultivation. To reduce dodder problems, control broadleaf weeds that act as alternate hosts for dodder, allowing it to spread onto tomatoes. Especially eliminate tomato plants that have dodder attached at thinning and again 2 weeks later. As tomato plants grow, continue to look for dodder and if found, manage it to reduce seed production and further spread.
Nightshades can be controlled by hand-weeding, but it is not easy to distinguish nightshade from the tomato plant and often many are left in the tomato row. Other weeds that are not easily controlled with herbicides such as dodder, broomrape, and velvetleaf need to be rogued out. If they already have fruit or seeds, carry them out of the field if possible. Use hoeing, flaming or spot applications of foliar-applied herbicides to eliminate these weeds from fencerows and other areas around the field.
In some situations, flaming can be used before tomato plants emerge or as a directed application after planting. Flaming the bed just before tomato emergence eliminates emerged weeds so the crop comes up without competition. An advantage to flamers over cultivation is that the soil is not disturbed so new weed seeds are not brought to the surface. Disadvantages of flaming include the high cost of fuel (usually propane) and the inability to control large weeds, especially grasses, without injuring the tomatoes.
After planting, postemergence herbicides can be applied to emerged weeds, either before or after the tomatoes have emerged. Layby treatments are usually applied as directed or shielded sprays on each side of the seedline and immediately incorporated with a power or ground-driven tiller, usually when the tomato crop is at the 6- to 8-true-leaf stage of growth.
Variable rate layby applications (applying full rate in the furrow and reduced rates next to the crop) can reduce herbicide cost with no loss in weed control or tomato yield. Variable rate treatments can be applied by changing the nozzle sizes on a standard layby setup, using one size in the furrows (e.g., 8002) and a smaller size (8001) near the crop.