Integrated weed management involves the use of multiple strategies to manage weeds in a manner that is economically and environmentally sound. It is important to integrate a variety of weed control techniques during the first weeks of growth because dry beans are not competitive with early emerging weeds at this stage. Such strategies can include cultural, mechanical, and chemical methods. Regardless of the strategy employed, any method that reduces the amount of weed seeds in the field will diminish weed numbers over time. One of the best ways to prevent weed problems is to control existing weeds before they produce new seed.
It is necessary to carefully plan an integrated weed management program. The type of program developed depends on bean variety, species of weeds, crop rotation, soil type, and irrigation system used in growing the crop.
If growing beans organically, it is especially important to plant into fields free of most weeds, since control options are generally limited to cultivation and hand weeding.
Monitoring And Field Planning
- Monitor the field during the following three time periods:
- Identify summer annuals, winter annuals, and perennial weeds.
- Record observations on a monitoring form (PDF). Anticipating certain weeds can aid greatly in designing the optimum weed control program, so determine the dominant weed species present and maintain records of both the weed species and severity of infestations for each field planted to beans. For beans planted in late spring, summer weeds and grasses are usually the common competitors.
Avoid fields that are severely infested with perennial weeds such as bermudagrass, johnsongrass, nutsedge (nutgrass), and field bindweed. Weed control can also be difficult in fields heavily infested with certain annual weeds, such as nightshade, groundcherry, and annual morningglory. Beans planted in the fall or winter season (such as garbanzos) usually have mustard species (including shepherd's-purse or London rocket) and volunteer cereals that need to be managed.
Weed Management Before Planting
Cultural techniques aimed at suppressing weed growth are necessary parts of any weed control program. These practices are often combined with herbicide application.
Select varieties according to the weed conditions in the field. Vine-type bean varieties are very competitive to weeds and prevent weed germination if allowed to cover and shade the ground before irrigation is applied. Vine-type varieties like lima bean are suited to 30- to 42-inch centers or two rows on 60-inch beds. Plant bush varieties in narrow rows (30-inch centers or closer) or in three rows on a 60-inch bed.
Irrigation and Cultivation
Preirrigation is the ideal method for controlling weeds at germination. To reduce the weed seed bank
- preirrigate the field to germinate weed seeds into sprouts.
- remove weed seedlings via a shallow tillage or postemergence herbicides before planting.
- plant beans immediately into the moisture present under the thin layer of dry soil created through shallow cultivation. There is no need for immediate irrigation. Bean seedlings can then establish before weeds germinate.
This system reduces the incidence of seedling diseases and promotes the development of vigorous seedlings that are more competitive against weeds. If one week or more passes between bed preparation and planting, weed seedlings may begin to emerge, increasing competition and negatively impacting stand development.
If irrigation is preferred for germination, sprinklers are a safer method than furrow irrigation as they avoid problems from excess water saturation while providing moisture for bean seed germination. Furrow irrigation can cause soil crusting and lead to seed rot. In addition, furrow irrigation on heavy clay soils is prone to excessive saturation. Be aware that sprinkler and furrow irrigation practices both allow weed seeds to germinate at the same time or before the bean seeds, resulting in resource competition between the weed and the bean seedlings.
Herbicides registered for use in beans differ in the weeds they control and the conditions under which they are optimally used. Rates may vary depending on the texture, moisture, and organic matter content of the soil. In addition, different classes of beans vary in their tolerance to different herbicides, so herbicides must be chosen based on the class of beans to be grown. Refer to the herbicide label to make sure the herbicide is registered for a specific type of bean.
Herbicides can be applied either pre- or postemergence, according to the growth stage of the weeds.
Preemergence herbicides are applied and incorporated into the soil before weeds have emerged, whereas postemergence herbicides control weeds that have already emerged. Some herbicides have both pre- and postemergence activity. Herbicide combinations frequently are the most effective way to control multiple weed species.
Most preplant herbicides are applied to the soil surface and mechanically mixed into the soil before the crop is planted. These herbicides are called preplant incorporated herbicides and require soil moisture for best performance.
Herbicide application methods:
- Apply to existing beds and incorporate with a power tiller or rolling cultivator.
- Broadcast over field, followed by discing and forming raised beds.
It is important to thoroughly incorporate preplant herbicides 2 to 4 inches deep. When incorporating with a disc or cultivator, work the soil to double the depth of incorporation desired (i.e., 4–8 inches). Pull disc and harrows across the field twice at right angles. A power incorporator will incorporate to the depth for which it is set. A ground-driven tiller (rolling cultivator) must be set properly to the correct depth for thorough soil mixing and pulled at the proper speed. Two passes are required with a rolling cultivator for proper depth and soil incorporation.
Weed Management After Planting
Cultivation and handweeding
After bean plants have emerged and reached the third to fourth trifoliate leaf stage, cultivation with sweeps, knives, or similar equipment can significantly reduce weed growth that may have survived earlier cultural or herbicide control methods. However, cultivation can result in a loss of soil moisture. Cultivate shallowly with care to avoid damaging bean seedlings. Injuries predispose seedling to soil diseases and retard crop growth. Cultivation should be avoided as plants reach the flowering stage. Commonly, fields are cultivated once or twice before row closure. Cultivation may be combined with fertilizer side-dressing. In some cases, particularly where herbicides are not used, hand-weeding may be required to remove large weeds escaping control.
Once bean plants and weeds have emerged, only a couple of postplant, postemergence herbicides are available for grass and broadleaf management. Herbicide performance is maximized if weeds have not experienced moisture stress.
Weed Management Before Harvest
Late season weed infestations often hinder harvest operations and increase harvesting and cleaning costs. Weed trash can also lower bean quality, and the presence of certain toxic weeds (e.g., nightshade berries) can render the beans unsuitable for canning. In addition, weeds produce a lot of seed that remains in the soil and can infest future crops.
Groundcherry, black nightshade, and hairy nightshade berries severely reduce bean quality during harvesting because they do not dry in windrows. During threshing, berry juices stain the beans. The sticky juices on the beans collect dirt and debris during harvest and warehouse handling that cannot be cleaned off. The sticky seeds of these weeds may also slow down or clog the thresher. Moisture from berries can lower the quality of beans in storage. Berries left in the field will result in weed infestations the following year. If present, remove these weeds from the field before harvesting.
Grass weeds such as barnyardgrass can make harvest operations more time consuming because their fibrous root systems interfere with cutting and threshing. Additionally, fields overrun by grasses may attract large numbers of armyworms.
Carfentrazone or flumioxazin may be applied to beans at maturity as a preharvest application to burn down broadleaf weeds such as annual morningglory, nightshades, pigweeds, and lambsquarters.. Make applications when the crop is fully grown and beans have begun to dry down because these applications will also burn down bean foliage. Complete coverage is essential for best results. Sodium chlorate which is used to facilitate beans dry down before harvest also burns down existing weeds.