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PREVENTIVE WEED CONTROL 67 articles capable of spreading plant diseases and insects to farms, forage areas, gardens, and forests. The Federal Plant Pest Act of 1957 regulates the entry and movement of plant pests as such and provides for emergency action as needed to prevent their spread. Several aspects of these acts relating to weeds are worthy of mention. In general, foreign plants or plant parts imported into the United States must be free of sand or soil. Inspections at ports of entry also give attention to any article suspected of carrying a plant pest. Such inspections probably prevent some weed seeds from being imported. However, although the Fed- eral Seed Act regulates crop seeds and the extent to which they may be con- taminated, restrictions on imports of weed seeds are generally not provided by the quarantine laws. Few tangible benefits in reducing the spread of weed species can therefore be expected from these laws. Federal quarantine over an area in which a weed pest was spreading rapidly, posing a great threat to agriculture, was first carried out in 1957 against witchweed. Both federal and state quarantines of areas infested by witchweed assisted in the containment of this parasitic plant (see Chapter 3). . The federal quarantine, however, classified witchweed as a parasitic plant that caused a dangerous disease to major crops of the grass family. Thus, for purposes of the quarantine, witchweed was considered a disease rather than a weed. Quarantine could not be imposed for witchweed or any other plant as a weed under existing authority. State Quarantine Laws Most states have some form of quarantine laws, but they are primarily con- cerned with insects and diseases and not with weeds. Some state noxious- weed-control laws include a quarantine provision that can be carried out if all other methods of weed control fail. SEED CERTIFICATION Seed certification is a system employed in the United States to keep pedi- gree records of seed multiplications of crop and turf varieties and make genetically pure seeds and propagating materials available to consumers. Seed certification is the responsibility of individual state crop-improvement associations, which set standards for clean seeds. In 1919, seed-certification agencies in both Canada and the United States joined together to form the International Crop Improvement Association. This organization has developed uniform rules and procedures for certification and serves as a clearinghouse for information on seed improvement. It has helped standardize inspection and approval procedures for seed certification
68 WEED CONTROL and has provided constructive leadership in educating farmers on the value of clean, pure seeds. In the United States, seed-certification agencies in 44 states were members of the International Crop Improvement Association in 1966. The association recognizes four classes of seeds: certified, registered, foundation, and breeder. Seeds in any of these classes in interstate commerce must meet the labeling provisions of the Federal Seed Act. Standards for maximum tolerance for weed seeds in individual crops vary with the seed that is being certified. An effort is made to hold the level of impurities to a prac- tical limit that is consistent with the state of agricultural technology. Drill-box surveys of the quality of field-crop seeds planted by farmers have been made in more than 20 states and Canadian provinces since 1951. In general, the results show that the quality of small-grain seeds sown by farmers has not kept pace with the strides that have been made in developing better seed-cleaning machinery and knowledge of seed processing. Only about 5 percent of seeds were certified, and these were found to contain 1.2 weed seeds per pound, in comparison with 160 weed seeds per pound for noncerti- fied seeds purchased from dealers or taken from farmersâ bins. Although seed- certification programs have direct control over only a small percentage of the small grain that is planted, their beneficial effect on the quality of seeds planted by farmers will grow through continuing seed-certification and educa- tional programs.
CHAPTER D Weed Control by Physical Methods Physical methods of weed control have been employed for centuries in all crops throughout the world. Many agricultural practices are dictated by the need for tillage operations. The wide spacing of row crops is governed by the necessity of maneuvering between crop rows to till the soil and eliminate weeds. On a worldwide basis, this is most evident in countries that are still dependent on animal power or human power for tillage purposes. Under such conditions, row spacings are not rigid and vary considerably, depending on the width of the power source. In areas where tractors are widely used, row spacings are defined more rigidly to adapt cultural practices to the tillage implements. Although recently developed herbicides are supplementing traditional methods of weed control, physical methods are still dominant in weed con- trol. Even the most modern farming operations still use these methods for controlling weeds. It is important, therefore, to understand physical methods of weed control so that they may be used most effectively. WEED CHARACTERISTICS CONSIDERED IN PHYSICAL CONTROL GROWTH HABIT AND LIFE-SPAN Although any plant species may be a weed, it must have special character- istics and competitive advantages to become a serious pest. Certain of these characteristics must be considered in employing physical-control methods. In 69
70 WEED CONTROL general, the more nearly a weed species resembles a crop plant in its ecologi- â cal requirements, the more difficult it is to control without injuring the crop plant. If the weedy plant is an annual or biennial, the production and germina- tion of seeds greatly influence that plantâs ability to become a problem, re- gardless of whether it is to be controlled mechanically or chemically. Pro- duction of large numbers of seeds that remain viable for a long period of time and the ability of these seeds to produce vigorous seedlings increase the difficulty in controlling a plant. Many species produce several thousand seeds per plant, and many of these seeds remain viable for years. Viability of a seed can be prolonged by dormancy, which is controlled by various physiological mechanisms and such environmental factors as temperature, moisture, oxygen, and light. Mechanisms that disseminate weeds over long distances assist them in invading new areas. However, special dissemination mechanisms are not re- quired for a weed infestation to develop throughout a field after a few plants become established and produce seeds. Once a weed is established in a given location, mechanical control of the species is affected primarily by other factors. The period of germination is another significant factor in the physical control of weeds developing from seeds. Some species germinate during a relatively short period in the year, while others germinate almost any time during the year. To be effective, the timing and duration of a control method should be correlated with the weedâs germination period. In the control of perennial weeds, there are two primary objectives: the prevention of seeding and the eradication of existing plants. Seed production is the major factor in the spread of weeds from one geographi- cal area to another. Within a growerâs field or a restricted locality, how- ever, the development and spread of vegetative portions of the plant may be equally important. As with seeds, dormancy of persistent vegetative portions of perennial plants requires consideration in mechanical control. For example, tuber chains of nutsedge display terminal dominance, causing mid-chain tubers to remain dormant. Breaking tuber chains by tillage induces germination of nutsedge tubers and thereby influences the degree of control attained by subsequent practices. If no further control efforts are made, the infestation will spread because of the increase in population of individual plants. If new shoots from the tubers are controlled shortly after they emerge, the number of tubers will eventually be reduced. Growth habits of perennial plants influence the type of cultural methods employed for their control. Shallow-rooted perennials such as bermudagrass and quackgrass usually have rhizomes only in the surface foot of soil, making
WEED CONTROL BY PHYSICAL METHODS 71 it possible to drag the rhizomes to the surface where they can be dried out by sun and air. Deep-rooted perennial plants send up shoots from as deep as 4 feet or more below the soil surface. Control by tillage should aim at exhausting food reserves through repeated removal of shoots. However, cultivation of perennial weeds often results in deeper rooting. For example, Canada thistle growing undisturbed roots pri- marily in the surface foot of soil, whereas under cultivation it may root as deep as 6 feet. Many studies have been carried out on depletion of food re- serves in perennial weeds, and results show considerable variation. This may be due to environmental differences that influence regeneration of carbohy- drate reserves. It is generally agreed that by delaying the removal of plant foliage for several days after emergence, greater depletion is obtained than by removing plant tops as they emerge. The tops continue drawing food reserves from the roots for several days after emergence, and, depending on the species, may not begin to translocate food to the roots until 10 to 14 days after emergence. Equally important are the reduced number of cultiva- tions needed for control. High soil fertility and adequate moisture appear to be associated with greater success in controlling perennial weeds by till- age, possibly because a more rapid regrowth results in depletion of food reserves. A number of factors that are associated with the growth habits of âbiennial and annual weeds make them well adapted to compete with crop plants and must be considered in physical control. Competitive ability and rapidity of replacing depleted food reserves are associated with the photo- synthate production of a plant. Factors such as area and spatial arrange- ment of leaves affect photosynthetic productivity. Depth and distribution of roots vary with species and soil types and conditions. Another charac- teristic of a plant species influencing the success of mechanical control is the root- or crown-ability of a plant. Great differences exist in the ability of plants to become established from cuttings. Under moist soil conditions, these differences affect transplanting of weeds by tillage. HABITAT AND ENVIRONMENT Many soil factors influence the selection of mechanical-control methods. Usually, it is desirable to grow crops as monocultures in otherwise bare soil. While weeds become more readily established in such habitats, there is an opportunity to use a variety of control measures. Certain crops are favored by unusual moisture conditions that influence both the weed species that can compete and the weed-control practices that can be em- ployed. In the bog environment of cranberries or the flooded fields in
72 WEED CONTROL which rice is grown, for example, cultivation cannot be used to control weeds. Examples of limitations imposed on control techniques by soil conditions are numerous. The presence of stones may prevent tillage or mowing. A high organic-matter content, such as in peat soil, may pose a fire hazard if flaming is chosen as a control measure. The water-table level or moisture- retention capacity of the soil may prevent successful control by allowing survival of weeds uprooted during tillage. Topography also influences weed-control practices. Rough terrain in rangeland areas may be essentially impassable for equipment. Even in crop- lands, steep fields may limit the types of equipment that can be used. In any case, the possibility of erosion must be considered if large areas or steep sites are tilled. Even very flat topography may be subject to wind erosion if weeds are removed by tillage. CLASSIFICATION AND PRINCIPLES OF PHYSICAL CONTROL METHODS TILLAGE CONTROL METHODS Weed control by tillage physically alters the weed relationship with the soil. Tillage may remove weeds from the soil, resulting in their death, or merely weaken plants through root pruning or other injury, reducing their competi- tive ability. Tillage may also bury weeds. In addition to its weed-control function, tillage may be performed to alter physical conditions of the soil or turn under crop residues. Manual Means Hand pulling is probably the oldest method of controlling weeds. It has survived all other innovations, although its large-scale use is dwindling. Hand pulling can be very effective, particularly against annual and biennial plants, provided the root system is extracted. With perennial weeds, hand pulling is effective on seedlings. Once perennials are established, hand pulling is almost fruitless. This method may be a practical way of removing a few scattered weeds from a field. Hand pulling or hoeing are often the preferred methods for home, vegetable, or flower gardens containing a va- riety of desirable plants in a small area. Herbicides usually are not practical for such areas. Spudding is a physical method of removing individual plants from the soil with a specialized implement. This practice is more successful than hand pulling, if weeds have a taproot. A spud usually has a metal blade with a
WEED CONTROL BY PHYSICAL METHODS 73 forked, sharpened end that may be pushed into the soil to sever roots. The spudding operation is accompanied by hand pulling of the plant after roots are severed. Hand pulling without spudding often results in the removal of top growth only, allowing regeneration of top growth from the taproot. The practice of spudding is usually restricted to small operations, mainly home Iawns and gardens, since the physical effort required is excessive. Hoeing is designed to uproot or cut weeds below the crown or at the base of the plant. Hoeing is still carried out in many crops, although it has been used less frequently in large-scale operations since the introduction of selective herbicides. It can be effective in controlling annual weeds and many biennials but is only partially successful in controlling established perennials unless hoeing is continuous. Hoeing is a faster operation than hand pulling or spudding and hence is more widely used in economic agri- culture. Most weeds can be controlled by hoeing, but intensity of hoeing varies according to weed species. Annual weeds are the most susceptible to the hoeâbroad-leaved weeds in particular. With annual grasses, the cut should be made below the soil surface to prevent regeneration of the plant from the crown. Biennials vary in susceptibility to the hoe, depending on the regenerative ability of their root system. Those having a rosette-type growth with little or no taproot are easy prey to the hoe, but many of those with a well-developed taproot regenerate from adventitious buds if the root is not extracted. Seedlings of perennial species are controlled as easily with a hoe as most annual weeds, but established perennials require long periods of continuous hoeing for eradication. Repeated hoeing of top growth at proper intervals depletes food reserves in the roots of perennial plants. Machine Tillage Tillage with implements powered by animals or machines is a common prac- tice throughout the world. Animal-drawn tillage equipment is rapidly de- creasing in the more developed agricultural regions and has been replaced by motorized tillage. In many respects, motorized tillage is not as efficient as hand pulling, spudding, or hoeing. It cannot completely remove weeds from within crop rows, and competition within rows is the most deleterious form of competi- tion. This disadvantage, however, is more than offset by the shorter time required to perform the control operation. Functions of Tillage Tillage reduces or eliminates weed competition for moisture, nutrients, light, and carbon dioxide and thereby improves crop growth. It also serves the
74 WEED CONTROL function of turning under crop residues or altering the physical conditions of the soil, which may be necessary to break up clay pans and other soil barriers or to create an improved seedbed environment for crop plants. Early studies comparing tillage that alters the physical characteristics of the soil with tillage conducted only to control weeds are primarily of academic interest, since selective weed-control methods other than tillage were not then available. Development of herbicides has made it possible to study the effects of tillage as an agricultural practice independent of weed control. In general, it has been found that tillage is of little benefit, other than for weed control, under most soil conditions. In heavy soils, tillage offers bene- fits beyond weed control. It is usually thought that it closes cracks in dry soil and promotes soil conditions that favor absorption of rain. Because of advances in methods of weed control, the role of row culti- vation is increasingly subject to question. In general, the major function of row cultivation is weed control. On soils of good tilth, there usually is no advantage in cultivation if weeds can be controlled in some other manner. On soils in poor physical condition, there may be an advantage in cultivation during certain seasons. However, direct benefits from cultivation, other than weed control, are difficult to demonstrate for many soils. Recent studies of chemical land preparation indicate that seeds of many crop plants germinate and the plants become established under a wide variety of soil conditions. Where weeds have been killed by herbicides and crop seed planted directly through dead sod, results have often compared favorably with more conventional methods of seedbed preparation. The type of top growth and rooting characteristics of previous vegetation, of course, have considerable influence on the physical condition of soil. More definitive research is needed comparing effects from various types of vegetation with the effects of tillage on soil condition. Although soil physical condition often dictates the number of tillage operations required after plowing, there has been a trend toward reducing the amount of tillage. Some authorities recommend disking a seedbed several times to kill several crops of weeds prior to planting, but this ap- proach is questionable in many cases. With earlier equipment that was ineffective against large weeds, it may have been necessary to disk weeds each time they were a few inches high. With modern equipment and a trend toward earlier planting, it seems logical to disk only enough to prepare a satisfactory seedbed. This may mean disking once just before planting, especially on fall-plowed land. Data from an Illinois study of a light-colored silt loam with 2.6 percent Organic matter suggested that three diskings prior to planting tended to cause crusting, making row cultivation advantageous. However, when soil was disked only once prior to planting, the three-year average yields did not indicate any benefit from row cultivation.
WEED CONTROL BY PHYSICAL METHODS 75 Although some farmers have reduced the amount of seedbed preparation, most have been slow to adopt this newer method. In many cases, researchers have obtained data indicating that reduced cultivation is beneficial. Fre- quently, special equipment is required for changes in production practices. Additional herbicides and equipment also need to be developed before farmers can be expected to adopt such new methods. Factors Influencing Frequency and Effectiveness The effectiveness of tillage for weed control is influenced by a number of factors. A primary factor is the kind of weed to be controlled. Annual weeds, biennial weeds without a well-developed taproot, and perennial weed seedlings are readily controlled by tillage. In general, the younger the plants, the easier they are to control. As plants become established, they often de- velop extensive root systems, build up food reserves in the roots, and cover the ground more extensively, making tillage operations difficult and less ef- fective. The effectiveness of tillage also depends on thoroughness. The greater the disturbance of the weed-soil association, the greater the effective- ness of tillage. Control of mature perennial weeds by tillage is a more complex opera- tion. Usually it involves depletion of food reserves through continued de- struction of top growth. Perennial weeds with deep, extensive root systems or rhizomes are more difficult to control than perennial weeds with shallow, restricted root or rhizome systems. Rhizomatous or stoloniferous perennial weeds may be spread by tillage, and a continuous program is required to destroy new growth after each cultivation. The number and frequency of cultivations required to control perennial weeds vary with a particular species and are partially dependent on the soil and climatic conditions encountered. For example, in experiments con- ducted at Hays, Kansas, from 1935 to 1940, cultivation performed 12 days after each emergence eliminated field bindweed in about 16 operations, as compared with 33 cultivations immediately after emergence. The actual number of days between cultivations will vary from one species to another under different conditions, but the objective is to kill the top growth pre- cisely at the time when food reserves cease to be depleted from the roots and begin to be replenished from the new shoots. WEED GROWTH HABITS INFLUENCING CONTROL PRACTICES The choice of whether tillage should be employed is often determined by growth habits of specific weeds. Frequently, growth habits may favor con- trol methods other than tillage. Weeds with an erect growth habit are easier
76 WEED CONTROL to pull by hand than prostrate or rosette-type weeds. Weeds with a confined, shallow root system are readily controlled by hand pulling. Weeds that are short or prostrate, such as prostrate knotweed, are difficult to control by mowing. Mowing is beneficial in preventing seed formation in plants that send up a seed stalk and is somewhat effective against weeds of weak regen- erative powers. Erect perennials can be eliminated by repeated mowing or cutting until the root reserves are exhausted. Prostrate perennials such as field bindweed, however, are not subject to cutting, and mechanical tillage of the soil is necessary for eliminating the tops. Woody plants are particu- larly difficult to eliminate by cultivation, and often much larger equipment (bulldozers or large tractors with chains) is required for their removal. Tillage Machinery A wide range of tillage implements are available for weed control. The basic tillage implement is the plow (Figure 1). The plow has varied in shape from a curved piece of wood to precisely designed moldboard and disk implements. Specialized tillage tools, such as the rotary hoe and harrow, are designed for control of weeds in emerging or established crops. The rotary hoe is a popular implement for control of weeds soon after FIGURE 1 Although many minimum tillage methods have been advocated, most land i still plowed for crop production. (Photo courtesy of Ellery Knake, University of [llinois.)
WEED CONTROL BY PHYSICAL METHODS 77 FIGURE 2 _ Rotary hoeing not only helps control weeds, but often aids crop emergence where the soil is crusted. (Photo courtesy of Ellery Knake, University of Illinois.) planting. In some areas, it is used on about three fourths of the row-crop acreage. It is generally employed once as a rapid, economical method of controlling weed seedlings before row cultivation begins. The rotary hoe is most effective against small weed seedlings before or soon after emergence. It is sometimes used to break a soil crust and aid crop emergence (Figure 2). Best results are obtained if the soil surface is slightly crusted. The rotary hoe should have sufficient weight and be operated fast enough to obtain adequate movement of soil to dislodge the weeds. Usually, the size of weeds rather than the size of the crop determines timing. It is often advantageous to hoe during the warmer part of the day when the crop is less turgid. Care should be taken to avoid crop injury when the crop is in the crook or spike stage. The harrow is similar in action to the rotary hoe, but the fingers or spikes remain stationary and do not revolve. The resistance of the spikes on harrows depends on the type. The spike-toothed harrow has rigid spikes, while the spring-tooth harrow has flexible tines. A variety of specialty harrows are used for particular situations. Where weeds are to be destroyed with a minimum of soil upheaval, blade or sweep implements are used. Their cutting edges are pulled a few inches below the soil surface and sever plants from their roots. Cultivators have been used for many years. Their main purpose is to control weeds between crop rows, although they also smother weed seed-
78 WEED CONTROL lings adjacent to crop plants by moving loose soil into the crop row. Weed control by tillage is economical and readily understood by the farmer. The main disadvantages of tillage are possible injuries to crop roots, limited effectiveness against weeds within crop rows (Figure 3), the necessity for proper timing, the burial and subsequent storage of weed seeds, detri- mental effects on soil physical properties, the inability to cultivate on relatively wet soil, and increased soil erosion. Mowing and Cutting The first cutting implements were sharp rocks, and their use dates back thou- sands of years. These eventually gave way to more-complex machines of sharp metal. Manual cutting implements are still widely used throughout the world. They include the machete, scythe, hoe, and a wide variety of other implements. Manual implements are employed in areas inaccessible to power-driven equipment or where power equipment does not exist. For example, the scythe is still widely used for manual control of ditch bank vegetation. Often it supplements power-driven mechanical devices such as the sickle and rotary mowers. Axes and other heavy timber-cutting manual equipment may be used to remove brushy weeds from forested areas. However, these imple- ments also are being replaced by mechanized equipment. FIGURE 3 = Cultivation between rows is often ineffective in controlling weeds within the row. (Photo courtesy of Ellery Knake, University of Illinois.)
WEED CONTROL BY PHYSICAL METHODS 79 Machine-operated cutters are used extensively. Weeds to be cut may be herbaceous as in hayfields or woody as in rangelands. Machine mowing is a common method along roadsides where shoulders should not be disturbed, along ditch banks, and on lawns. The two primary types of mowers are the sickle mower with oscillating blades and the rotary mower. Mowing controls weeds in two ways. If properly timed, mowing prevents plants from producing seeds. Repeated mowings also aid in control of some perennial plants by depleting the underground food supply. Mowing is ineffective against prostrate or short weeds growing close to the ground. Prostrate or low-growing weeds, therefore, are often invaders of frequently cut areas. Cutting the tops of some plants destroys apical domi- nance, and dormant buds may start growing, resulting in more stems on the plant than were present prior to cutting. Most weeds should be cut in the bud stage or earlier, since they may produce viable seed even if the heads are cut during flowering. Most peren- nial weeds should be cut in the early bloom stage, since this is usually the period in which the weed has the lowest amount of food reserves in its roots. Unfortunately, mowing is a practice often performed when most convenient rather than when most effective. Mowing may prove to be a desirable weed-control method in areas where bare ground is subject to erosion and plant root systems are needed to hold the soil in place. Mowing and cutting are also effective in crops that have a slow initial period of growth followed by an accelerated growth rate. In such cases, fast-growing weeds can be severely retarded by mowing, thus al- lowing the crop to develop. Flooding, Dredging, Draining, and Chaining Flooding is used for weed control in such crops as rice and taro, which are able to grow under flooded conditions. For example, taro planting slips are placed in 3 to 4 inches of water, and the paddy is kept flooded through- out the life of the crop except during periodic fertilizer applications. How- ever, flooding is not an unmixed blessing when used for weed control in this manner. While it provides control of many weeds, aquatic weeds often be- come problems. Flooding is sometimes used for the control of perennial weeds. The popularity of this method increased somewhat after the observation that many perennial weeds were controlled in areas flooded for the production of such crops as rice. Successful weed control from flooding depends on complete submergence of weeds for a period ranging from one to two months. It is important that no regrowth appear above the water. Many failures have been reported where plants were allowed to emerge above the
80 WEED CONTROL water level and thus survive. The susceptibility of perennial weeds to flood- ing varies with different species. Russian knapweed and hoary cress have been controlled quite successfully with this method. Where perennial weeds are susceptible to control by flooding and the terrain is suitable, this method may be superior to the use of herbicides applied to the soil. Flooding leaves no herbicide residues in the soil, and the area can be returned to production immediately. Disadvantages of flooding are the high expense of building and maintaining dikes, lack of available water in some areas, and wide differences in susceptibility of dif- ferent weeds to flooding. Dredging, drainage, and chaining are methods used for the control of aquatic weeds growing in swamps, drainage ditches, and irrigation canals. Dredging is an expensive method of controlling weeds. Several irrigation districts have demonstrated that dredging can be reduced to a minimum or eliminated through complete suppression of aquatic weeds with chemicals. Much silt deposition is caused by slow rates of water flow, resulting from aquatic weed growth. As a weed-control method, dredging is usually em- ployed as a last resort to physically remove weed growth from areas with excessive silt or from the margins of impounded water. The method is seldom used before weed growth becomes a serious problem. The ultimate success of a dredging operation depends on the plant organs removed. If rhizomes, tubers, and other propagating organs are not removed, weed con- trol is only temporary. Often, these organs are located in mud at consid- erable depths, and thus it becomes necessary to remove considerable quan- tities of soil or mud along with the weeds. In some instances, removal of bottom mud breaks the seal of canal bottoms and causes seepage problems. Drainage of swamps, canals, and other wet sites alters the environment sufficiently to prevent the growth of most hydrophytes. Drainage is an excellent practice for controlling cattail, bulrush, reed canarygrass, and other troublesome weeds. These weeds are more susceptible to eradication or control if their habitat is dried up. Drying of canals and laterals is an inexpensive method of controlling aquatic weeds. However, the gradient of the channels must be such that water drains out quickly, leaving no low, ponded areas. Pondweeds must be dried thoroughly if drainage is to be effective. Although drainage is an effective method against hydrophytes, mesophytes often invade an area after draining unless it is reflooded. In the western United States, Canada thistle is one of the species that frequently in- vade newly drained areas. The practice of dragging heavy chains along the bottom of canals, laterals, and drains to tear out or pull submersed vegetation from the substrata has been widely used in the past (Figure 4). Two tractors, one on either side of the canal, move in the same direction, pulling the chain
WEED CONTROL BY PHYSICAL METHODS 81 FIGURE 4 _â_ Chaining is a method that was widely used in the past for removal of aquatic weeds from irrigation canals. (Photo courtesy of U.S. Bureau of Reclamation.) between them. Several types of chains are used, the most common being a ship-anchor chain. Numerous innovations and changes have been made in chains to increase their effectiveness. Weeds broken loose by chains or other equipment are carried downstream to a collection point where they are flushed into waste areas. If such collection areas are not available, re- moval is accomplished by hand equipment or dragline. Tremendous quantities of debris and silt are stirred up by chains and collect in head gates, siphon tubes, and sprinkler systems. Accordingly, only small quantities of the water can be used for irrigation during a chaining op- eration. This disadvantage, together with the fact that chaining is expensive and generally does only a poor job of controlling aquatic weeds, has led most irrigation districts to employ aquatic herbicides. Although chaining is still employed to some extent, its usage is decreasing rapidly. Use of Heat Heat is often an economical and practical means of controlling weeds. It greatly reduces the amount of hand labor required for weed control, particu- larly when used in combination with other practices. Heat is used widely to dispose of accumulated vegetation that is unsightly or poses a health hazard. Burning cannot be equaled in some situations of weed control. For ex- ample, existing vegetative cover having low economic value can be eliminated from rangelands, permitting establishment of more desirable grasses. Con- trolled burning has been used for many years as a cultural practice in forest management. Controlled burning eliminates fuel before a large accumulation occurs, thus reducing the hazard of forest fires. Site-preparation to establish
82 WEED CONTROL new plantings and release desirable species is often accomplished by con- trolled burning. Three to four annual burns are among the least expensive ways to remove hardwood undercover in a pine forest. The pine trees will withstand heat that kills undesirable hardwood species. Precautions are re- quired to prevent fire from escaping. In Hawaii, large areas of land are covered by dense, impenetrable stands of trees and brush. Because of steep topography, such lands are difficult to clear by conventional machine methods such as bulldozing, brush-raking, or chaining. Burning thus becomes a valuable method of eliminating unwanted ferns, grasses, trees, and brush on potentially valuable jungle lands. In using flame, man has tried to improve upon nature. Machines have been designed to burn petroleum products as fuel, creating temperatures of about 2,000°F that kill only young plants. Flame may be used as a nonse- lective method of killing plants or to selectively kill weeds without injuring a crop. The hooded burner, often used for selective flaming, may be adapted for nonselective use. Flame does not kill by actual combustion. Burning of plant tissue oc- curs only after repeated flamings and should be kept at a minimum to avoid crop injury. The death of a plant by flame appears to be associated with in- jury to the cells of the leaves and stem. With the cells destroyed, the seed- ling has no means of recovery or regeneration. The critical temperature above which cambium cells may be injured appears to be about 130°F. The initial flaming on cotton is normally made when stems are about 3/16 of an inch in diameter at ground level. At this stage, the cambium layer is protected by an insulating layer of bark. The succulent weed seedlings have no such protection, and the heat need travel only a short distance to reach vital cells. The larger leaf-surface area of the weeds absorbs more heat than the round surface of the cotton stem. Alfalfa is flamed to control dodder and other weeds; flame is used to control weeds in some grasses and obtain cleaner grass-seed crops; potato vines are flamed when tubers are mature to facilitate mechanical harvesting; and drainage and irrigation ditches are kept free from weeds by nonselective flaming. Selective flaming is performed in a wide variety of crops such as cotton (Figure 5), corn, soybeans, and grain sorghum and in oil crops such as castor beans and sesame. Flame is used to a limited extent in fruits such as blueberries, raspberries, and grapes. The experimental use of flame showed promise in such vegetable crops as onions, carrots, asparagus, and potatoes. Selective killing of weeds and crop plants with flame is accomplished through several factors. Different kinds of plants are able to withstand dif- ferent amounts of heat, depending on their stem structure, age, size, and shape. Weed kill is accomplished by moving an intense flame along the base
WEED CONTROL BY PHYSICAL METHODS 83 FIGURE 5 Two-row flamer employed for conventional flaming of the drill in cotton. (Photo courtesy of Agricultural Research Service.) of a weed-infested crop. The intensity and duration of exposure are impor- tant factors in obtaining effective results with flame. Desired exposures are obtained by varying tractor speed. The size and intensity of the flame are controlled by an orifice tip and fuel pressure. Selectivity through careful timing can be illustrated with corn. Research shows that corn should not be flamed from the time that it is 2 to 12 inches high. Before the corn is 2 inches tall, the growing point is underground. Al- though flaming kills both the exposed corn leaves and weeds, the corn re- covers with no permanent injury to vegetative growth or yield. After the corn is 12 inches tall, young weeds can be killed by flame without injury to the com. The objectives of using heat for weed control should be examined care- fully. Very often too much is expected from burning or flaming. Persons who are not fully acquainted with limitations of these methods are then dis- appointed by the results. For example, field-burning is more effective in disposing of accumulated vegetation than in killing weeds. It is often used after weeds are mature and viable seeds are present. In such cases, fire seldom reduces seed viability enough to achieve any practical benefit.
84 WEED CONTROL Several flaming applications may be necessary for satisfactory weed con- trol. The initial purchase price of a flamer is often much more expensive than a sprayer. In addition, the heat generated by the burner may be ex- cessive for the operator. If weeds are as resistant as the crop to the flame, the method is not feasible. The flamer is generally more difficult to adjust than a sprayer. In most crops, weed control cannot be obtained from flame alone. It is most effective when used as a supplement to other weed-control practices such as pre-emergence and postemergence herbicides. Despite the limitations of flaming, it also has a number of important ad- vantages in weed control. With the greater use of herbicides and increased awareness and concern for residues in soil and crops, the freedom of flame from any residue hazard has special merit. With a well-adjusted flamer, results are usually not greatly influenced by environmental conditions. There are no drift hazards to adjacent crops from flaming. Flaming can often reduce other plant pests such as insects and diseases, facilitate mechanical harvesting by eliminating trash, serve as a control measure when the crop is too wet for mechanical methods, and permit almost immediate inspection of weed con- trol. Much of the current research on flame weed control is concerned with the most efficient utilization of heat per unit of fuel and in the transfer of the heat to the appropriate site in the plant. Research is also being conducted on the economics of flaming and in the development of ways to use flaming in conjunction with other methods of weed control. Smothering Weeds with Nonliving Materials Smothering achieves weed control by excluding light from the photosynthetic portions of a plant. Partial smothering by a thin layer of mulching material can weaken a plant by reducing its photosynthetic capacity and creating an unfavorable microclimate, thereby aiding in control. Particles of organic substances that are moderately resistant to decomposition are commonly used for mulching. These include straw, sawdust, bark dust, and other plant parts of various sizes. Moderately thin layers of these types of mulch control weeds that germinate near the soil surface and certain perennial weeds such as dan- delion and plantain. However, they are virtually useless for control of many perennial weeds with vigorous shoots, such as blackberry, or a strong rhizome system, such as field bindweed. Mulching materials applied as a continuous sheet, such as paper or plastic, are more effective for weed control than materials used in particle form. The use of paper mulches for weed control in pineapple originated many years ago in Hawaii.
WEED CONTROL BY PHYSICAL METHODS 85 Present interest in sheet materials for mulching is centered around the use of polyethylene film. To be effective, the film must provide a low enough light transmission to eliminate photosyntheses. Seedbeds are covered with strips of black polyethylene 12 to 36 inches wide, occupying 40 to 60 per- cent of the soil surface. In addition to forming a mechanical barrier to weed growth, the mulch also serves as a seal for fumigants and as a moisture barrier. It also increases soil temperatures, which enhances plant growth. In ornamental plantings, a combination of mulching materials is sometimes used. A layer of polyethylene film provides an effective barrier to weed emergence. This is covered with a layer of peat moss, bark dust, or crushed rock, which holds the film in place and protects it. The use of polyethylene mulch does not necessarily insure complete control of all weeds in the seed- bed. Punctures and tears often occur in mulches, resulting in exposure of soil. Nutsedge and other perennial weeds often grow through plastic mulches. Mulching materials have certain limitations as weed-control agents. Most of these materials are expensive to obtain or apply, and, therefore, are most commonly used on high-value crops with relatively small total acreages. The use of most types of mulches is also largely limited to emerged or established crops, which further reduces their potential in the total crop picture.
CHAPTER 6 The Biological Control of Weeds Vast areas of the earth are covered by weeds that either do not yield to chem- icals or against which the use of chemicals is too expensive or impractical. However, the use of insects has been strikingly successful in controlling some of the worldâs worst weeds, such as pricklypear and St. Johnswort. The role of natural enemiesâparasites, predators, and pathogensâin reducing the popu- lation of a plant or animal species is known as biological control. When suit- able and effective agents are available, this method is cheap and permanent, involving no repetitious treatments or corrective measures year after year. ECOLOGICAL BASIS AND PRINCIPLES OF BIOLOGICAL WEED CONTROL The objective in biological control is never eradication; it is reduction of a weedâs density to noneconomic levels. This may be accomplished by direct or indirect action of the organisms employed. Biological control of weeds has been viewed conservatively for two rea- sons: the possibility that risks are too great when compared with the chances of obtaining an adequate degree of control; and conflict in the acceptance of a given plant as a weed, particularly since introduced agents may readily move from areas where the plant is a weed to areas where it is considered of value. Biological control is based on the fact that there are enemy organisms capable of controlling certain weeds. The method has usually been applied against alien weeds; indeed, the abundance of alien species is frequently due to their having escaped enemies in their native environments. The introduction 86
THE BIOLOGICAL CONTROL OF WEEDS 87 of such enemies has often resulted in great reductions in the abundance of alien pests. However, the method is not applicable only to alien weeds. Enemies of related species of plants may be effectively used against either alien or native plants that are not their normal hosts. A natural enemy of a type not having a reciprocal-density relation with its host plant would be either ineffective or dangerous to introduce because of risk to other plants. For this reason, unspecialized feeders are not used in biological control. Thus, we are concerned here with a relation where the enemy is food-limited and the weed host is enemy-limited in a typical host-parasite interaction. By the very nature of the controlling mechanism, the absolute impact of an enemyâs action is automatically lessened as the density of the host declines. Other factors that cause mortality or reduced reproduction may then exert a greater impact, relative to that of the enemy, than they did at the crest of the enemyâs controlling action. Yet, fundamentally, the enemyâs action even at this low density is still vital and is as important as ever. Its action alone, among those forces operating in the existing regime, is capable of preventing return of the weed to its former abundance. It has been shown experimentally that full regulation may be achieved, even though a regulating action contributes only a small amount of stress, provided that density-independent, nonregulating forces cause a relatively heavy and consistent stress and that food shortage may be the factor regu- lating a given phytophagous organism, even though only a small portion of that food supply is actually destroyed. The decision to introduce a given insect for control of a weed cannot be entirely objective. The urgency of the problem will influence the amount of acceptable risk. We have, thus, an acceptable calculated risk balanced against preponderance of interest. It is unreasonable to expect that an in- troduced insect will not engage in abortive feeding upon some crop plant under stress. In fact, the capacity to breed upon a given plant is commonly a better criterion in selection than an insectâs feeding habits. Biological control can never be the solution to every weed problem, since there are not effective agents for every problem. If a weed is toxic to livestock, and eradication is required, this method is inappropriate. Also, many weeds are erratic in their occurrence in time and space, and the need for control may be urgent. While these factors lessen the likelihood of ob- taining success with biological control, they do not preclude the possibility, for a good enemy may reduce the subsequent need for other remedial mea- sures by markedly reducing the reservoir of seeds. The greatest disadvantage is that biological control is by its very nature a selective form of weed con- trol. It cannot be used to control a complex of many weeds. It has striking utility where a single, aggressive weed is devastatingly troublesome.
