GALIFORNiA AeRICULTURAt EX OIL SPRAYS FOR WEEDING CARROTS AND RELATED CROPS A. S. Crafts carrots sprayel With stove oil carrots sprayed 1 With refined oil T ttlEIliClD f=C^ G f ©^ ^fcSTE ITNIV f R s^if yW^/^ I F o R*^^ WEED CONTROL BY OIL SPRAYING IN CARROTS AND RELATED CROPS . . . has largely replaced hand labor in many districts. While cost of spraying varies with the type of oil used, size of spray rig, and extent of operation, the expense will usually be much less than that for hand-weeding. The carrot grower has a choice of five methods of weed control 1. Cultivation 2. Hand- weeding 3. Preemergence spraying with Diesel or smudge-pot oils 4. Stove-oil spraying 5. Spraying with refined oils such as thinners, solvents, or the new, specially developed sprays Each method has a place and each should be used in its proper place. The grower must decide which method to use to meet the requirements of his particular situation. Some oils, although used as sprays, are not entirely satisfactory for carrot crops Heavy oils, such as Diesel fuel and smudge-pot oils, are toxic to carrots as well as weeds. These oils are suitable only for preemergence and general-contact spraying. Stove oil may be injurious to very young plants (in the two- leaf stage), and may cause an unpleasant flavor and odor on market carrots if they are sprayed when more mature. Certain cleaners' solvents and paint thinners are more vola- tile and hence more satisfactory; they are, however, more ex- pensive than stove oil. Distillates in the gasoline range are highly selective and leave little oily flavor, but they are too dangerous to use. Oil Sprays for Weeding Carrots and ReijAted ('rops 3 A satisfactory oil for killing weeds in a vegetable crop . . . should be moderately toxic, low in cost, and relatively free of objectionable flavor or odor. Improved oils (solvents and thinners) having these desirable qualities are now in production. Although more expensive, these oils are superior in many ways to stove and other oils: They are lighter, and therefore less toxic to crop plants, than Diesel oil. They are less volatile than gasoline, hence they are safer from the standpoint of inflammability. They are properly balanced and selective so that they kill weeds rapidly and completely without injury to carrots. They are low in highly flavored compounds, and sufficiently volatile so that they leave carrots within a few weeks after application. General pointers on the use of oil sprays Weeds should be sprayed when carrots are small ( 1 to 4 true leaves). Larger carrots absorb more oil and thus retain the flavor and odor longer. Small weed growth can be covered by 50 to 70 gallons per acre — 35 to 50 gallons are sufficient for spraying only the tops of beds. Larger weeds may require 75 to 100 gallons. Pressure of 100 pounds is sufficient for applying oil. Higher pressures cause loss of oil through drifting mist, and result in less efficient coverage of weeds. Seed crops of vegetables of the carrot family may be safely sprayed with selective oils — even two or more applications will not harm the crop. Avoid spraying seed stalks and flower- ing heads. Caution: Care is necessary in the use of any oil spray. A complete list of precautions is included at the end of this circular. Digitized by the Internet Archive in 2011 with funding from University of California, Davis Libraries http://www.archive.org/details/oilspraysforweed136craf OIL SPRAYS FOR WEEDING CARROTS AND RELATED CROPS A. S. CRAFTS^ Hand-weeding of vegetable crops is costly. Often the profits depend largely upon savings in the amount of hand labor required. In many districts, oil spraying is largely replacing hand labor in the weeding of carrots, celery, parsnips, parsley, and certain herbs of the carrot family. The cost of spraying will vary with the type of oil used, size of spray rig, and extent of the opera- tion, but it is usually much less than the expense of hand-weeding. METHODS OF WEED CONTROL IN CARROTS Several methods of weed control are available to the carrot grower. Each has a place and each should be used in its proper place. The following evalua- tion should aid the grower in selecting methods to meet specific needs. Cultivation or tillage. This method is useful where weed seeds are few and soil conditions favorable. It requires thorough seedbed preparation, correct depth of seeding, and accurate spacing of straight rows. Seeding of the crop plant may be delayed to allow germination of weed seeds. In this way, the first weed crop may be destroyed before seeding. As soon as carrots are 3 or more inches high, loose soil may be thrown gently over small weed seedlings to choke them out. Hand-weeding. This is justified when the weed-seed population is light but appreciable. It may be used to destroy occasional weeds that survive seedbed preparation or cultivation. Preemergence spraying. This method is useful on seedbeds of carrots, onions, and other crops that germinate slowly. Use Diesel or smudge-pot oil on the rows just before crop plants emerge. Stove-oil spraying. Where weather and soil conditions permit, carrots hav- ing one to four true leaves may be sprayed with stove oil. Spraying in the cotyledon stage (when only two narrow seed leaves are present) may result in injury, while if more than four true leaves are present, an oily flavor in market carrots usually results. When 60 gallons of stove oil per acre are used, the oily flavor should leave the carrots in 75 to 90 days. Cold weather and higher dosage prolong this time ; warm weather or lower dosage shortens it. Larg-e carrots absorb more oil than small ones, and the oily flavor lasts longer. ^ Professor of Botany and Botanist in tlie Experiment Station. [5] California Agricultural Extension Circular 136 Fig. 1.— Carrots sprayed with A, Diesel fuel; B, stove oil; C, D, light aromatic stock; E, unsprayed. Spraying with refined oil. Special carrot spray oils, that evaporate and leave the plant rapidly, are appearing on the market, and use of such oils will greatly reduce the risk of producing oily carrots. Since these oils are more selective, they will not damage younger carrots; because they are volatile, they may also be used on somewhat older ones without leaving an oily flavor In warm weather, spraying may be done within six weeks of harvest. After a preemergence spray with Diesel oil, one refined oil spray on the carrots may eliminate hand-weeding. PROPERTIES OF OILS In the table on page 9, a comparison is made of the properties of stove oil, Diesel oil, and one of the new oil sprays. Several factors are involved m this comparison, and in order to understand the effects of the different types of oil sprays, it is necessary to know something about the properties, or char- acteristics of oils, and their effects on plants. In this way, the grower will be better able to choose intelligently the correct spray for his particular need. Two main processes are involved in oil refining, distillation and separation. In the distillation process, the oil is heated in large stills, at increasingly higher temperatures. Some constituents (or fractions) of the oil are more volatile (evaporate more quickly) than others. These more volatile fractions Oil Sprays for Weeding Carrots and Related Crops evaporate first, at lower temperatures. The heavier, less volatile fractions re- quire higher temperatures. The oil fractions resulting from distillation are called distillates. A distillate contains a mixture of compounds which have not been separated by the distillation process. The main constituents are paraffinic or naphthenic oils; the distillate also contains unsaturated (olefinic and aromatic) compounds. If further separation is desired, the oil is treated with liquid sulfur dioxide, which dissolves the unsaturates so that they can be removed. (When a very highly refined oil is desired, such as a medicinal oil additional refining processes are employed. ) Toxicity. Oils vary widely in their toxic effect on plants, and tests show that the unsaturated compounds, or unsaturates, are responsible for most of the toxicity. They are also largely responsible for the objectionable flavor and odor m some oil sprays, such as stove oil. Light unsaturates (low boilin- point) cause a rapid burning of foliage, termed acute toxicity; heavy un" saturates injure the growing regions and cause a chlorosis (yellowing) of leaves The latter injury comes on much more slowly and is called chronic toxicity. Very light unsaturates, such as benzene and xylene, cause burning ot the leaves, but injury is not complete with dosages that just wet the plant because these oils evaporate before all tissues are killed. Such compounds occur in gasoline. Unsaturates of medium weight are very toxic to grasses and most weeds but they do not kill plants of the carrot family, except at high concentrations' 8 California Agricultural Extension Circular 136 They usually occur in unrefined petroleum distillates, such as stove oil, at concentrations between 20 and 30 per cent — amounts that kill weeds but not carrots. Heavy unsaturates, such as those occurring in Diesel and other heavy fuels, kill plants slowly by chronic toxicity ; carrots as well as weeds are affected. Stove oil. The gravity, or density, of an oil has to do with its weight. In a gravity test (as devised by the American Petroleum Institute), an oil with a low-degree reading would be heavier than an oil having a higher reading. For example, in the table on page 9, Shell Weedkiller No. 10, wdth a gravity of 43.3°, is a lighter oil than stove oil, with a gravity of about 37°. For use as oil sprays on carrot crops, an oil should not fall very much below 38° A.P.I. The commonly used stove oil is cheap, plentiful, and of intermediate tox- icity. Although it causes slight damage to young carrots, its flavor will leave the carrot if spraying is done early enough. Experiments prove, however, that a more highly refined oil may have higher selectivity and leave less ob- jectionable flavor. Although slightly injurious to carrots, stove oil has proved popular as a carrot spray because of its cheapness and availability. Its gravity (around 37° A.P.I.) is near the limit for oils having chronic toxicity, and it has a slight stunting effect upon the crop. It is the only unrefined distillate pro- duced in quantit}^ which falls within the gravity range tolerated by carrots and has the correct concentration of unsaturates. Some stove-oil samples hav- ing gravities in the neighborhood of 34° have caused serious damage to carrot crops. Such oils are actually misnamed — they come within the gravity range of Diesel oil. Light distillates. The temperature at which an oil ignites is the flash point. Oils with a low flash point (highly volatile), such as gasoline, are dangerous for use as sprays. Experiments have proved, however, that such oils are highly selective and leave less oily flavor than stove oil. They are volatile, so that such flavor as is left disappears more rapidly and the carrots are free of oil a month after being sprayed. The unsaturates of gasoline include toluene, xylene, and others ; when used directly, these compounds kill young carrots; when diluted 3 to 1 with kero- sene, they bring about selective killing of weeds as does stove oil. These same compounds occur, at the proper concentration to cause selective weed killing, in a number of refined oil products that are sold as paint thinners and cleaners' solvents. In the East, where stove oil is not readily available, some of these more highly refined fractions have been tested as carrot sprays. Among them are kerosene, Sovasol No. 5, Sovasol No. 75, Stoddard Solvent, Varsol No. 2, and the paint thinners Mineral Spirits and Sun Spirits. All these products are derived from gasoline stock. These oils were used alone and in various com- binations. Kerosene proved too low in toxicity to be widely useful. Sovasol No. 5, Stoddard Solvent, Varsol No. 2, Mineral Spirits, and Sun Spirits are suitably selective for weeding carrots. Being light in gravity, they evaporate more rapidly than does stove oil ; they leave much less oily flavor if properly Oil Sprays for Weeding Carrots and Related Crops 9 applied. Though somewhat more expensive than stove oil, these products are still relatively cheap if hand-weeding is the only alternative. Stoddard Solvent and various grades of light cleaning oils are available in California in limited amounts. If any of them are tried as carrot sprays, it must be kept in mind that they are not produced for herbicidal purposes and, whereas their properties may be fairly constant with respect to their require- ments as solvents, they may vary widely in toxicity. All such materials should he tested on small plots hefore being used in quantity as herbicides. New oil weed killers. Oil refiners have been testing oil fractions as weed killers for carrot crops. They are presenting several products, similar to those mentioned above, that are superior in many ways to stove oil. Being intermedi- ate in boiling range, they do not contain the heavy fractions that make Diesel oil unfit for carrot spraying ; they also lack the light volatile end that makes gasoline unsafe from the standpoint of inflammability. They are properly balanced so that they kill the weeds rapidly and completely without injury to the carrots. And they are low in highly flavored compounds and sufficiently volatile to leave the carrots within a few weeks after application. The intro- Table 1 COMPARISON OF SPECIFICATIONS OF DIESEL OIL,* STOVE OIL,t AND SHELL WEEDKILLER NO. 10 Diesel oil stove oil Shell WeedkiUer No. 10 Gravity A.P.I 32.6° 37.2° 43.3° Viscosity (SayboltUniv. at 100° F) 39.6 sec. 32.2 sec. less than 30 sec. Flash pointi 176° F 146° F 106° F Distillation temperatures : Initial boiling point 453° F 510° F 626° F 680+° F 387° F 443° F 522° F 573° F 314° F 10 per cent point 326° F 50 per cent point 90 per cent point 370° F Final boiling point 395° F Unsulfonated residue § 79.6 per cent (approx.) 86.6 per cent (approx.) Per cent by weight aromatics 22 Sulfur content 0.47 per cent 0.29 per cent * Average of 23 random samples. t Average of 21 random samples. t In comparison with the oils shown, gasoline has a flash point of 0° F. This emphasizes the great hazard in any possible use of gasoline as a herbicide. § Since the unsulfonated residue of the stove oil and Diesel oil represents the saturated compounds, the unsaturate concentrations would be approximately 13.40 per cent (stove oil) and approximately 20.40 per cent (Diesel oil), values similar to the 22 per cent aromatics of Shell WeedkiUer No. 10. 10 California Agricultural Extension Circular 136 duction of such special spray oils is a distinct improvement. In addition to providing a herbicide which is relatively foolproof with respect to crop in jury, they should greatly reduce the production of oily carrots — a product that is rapidly gaining an unenviable reputation in vegetable markets through- out the country. The grower is therefore faced with the choice between a cheap oil that stunts his carrots slightly and leaves an objectionable flavor for 75 to 90 days, and a more expensive one that causes no injury and produces little flavor or odor. Specifications of oils. In the oil trade, all oil fractions are described by sets of specifications either required by law or used by the manufacturer as a standard of quality. Every commercial product must meet these specifications. Such specifications describe the chemical and physical properties of the frac- tion within rather broad limits, and are intended to indicate a product's ability to do the job for which it was manufactured. Thus specifications which show a fraction to be suitable for cleaning fluid would not necessarily mean that such a product would be safe or effective for use as a weed kiUer. Table 1 indicates the differences in specifications among stove oil, Diesel oil, and one of the refined oil sprays. Shell Weedkiller No. 10. The grower should keep in mind, in using these specifications, that they have been designed to provide a means of describing oil fractions in terms of their use as fuels, solvents, thinners, lubricants, and so forth. There are no specifica- tions for weed-hilling oils. In fact, the development of such specifications will be very difficult because toxicity depends upon quality as well as the amount of unsaturates, and the chemical constitution of these compounds is not well known. With our present knowledge of oil toxicity, the only reliable means for evaluating a selective oil is a biological test performed in the field rather than specifications. Hence, in purchasing herbicidal oils, the grower will have to rely upon the integrity of the manufacturer. In this connection it should be emphasized that small differences in specifications may result in large differ- ences in toxicity. Manufacturers who offer special weed oils should assume responsibility for their weed-killing properties, since the manufacturer alone knows the nature of his product. SPRAYING RECOMMENDATIONS The time to spray. As a rule, weeds are more easily killed when they are small, hence spraying should be done soon after the weeds appear. With respect to season, carrots in the Salinas Valley have been sprayed during every month of the year, with good results. Spraying when the plants are wet has no detrimental effect because the oil wets the foliage and the water is blown off. Rain soon after the oil application likewise does not interfere with the results. Wind prevents proper coverage and causes loss. Commercial operators often spray at night when the air is still, if winds are prevalent during the day. Toxicity of the oil both to the weeds and the crop is increased by heat. Spraying while temperatures are above 90° F may be dangerous and should Oil Sprays for Weeding Carrots and Related Crops 11 be avoided; such temperatures, however, are seldom encountered when carrots are in the small seedling stages. Young carrots sprayed with stove oil have been killed by frost, whereas unsprayed carrots are hardy. Light oils, being less persistent, should cause less trouble of this kind. Dosage for market carrots. Most young weeds can be killed by spraying with 50 to 70 gallons of herbicidal oil per acre, applied to the entire field area; for the tops of beds alone, 35 to 50 gallons are sufficient, the furrows being left for cultivation. If the weeds are young and succulent, this dosage may be reduced somewhat. In no case should more than 75 gallons per acre be applied on bed plantings, or 100 gallons on fiat plantings. At the higher rates, if stove oil is used, it may be necessary to leave the carrots in the ground past the normal harvest date to allow the oil to disappear so that the carrots become palatable. If lighter, more highly refined fractions are used, higher dosages and later spraying may be possible. AVith proper management of seedbed preparation, planting dates, and tillage operations, oil sprays applied accord- ing to the recommendations given above should handle most weeds. Occa- sionally, inclement weather delays weeding ; when this occurs, one of the improved oil sprays is necessary if a palatable carrot is to be produced. Spray pressure. Viscosity relates to the flowing quality of an oil. The viscosity of an oil to be used as a spray determines somewhat the amount of pressure needed and the size of the spray orifices, since the heavier oils will not break up into droplets readily nor flow as rapidh^ as will the lighter ones. Viscosity is also a factor in determining how much of the oil penetrates the plant surfaces. A heavy oil will remain on the plant longer than will a lighter, more volatile one, and thus may penetrate in larger amounts and have a more toxic effect. Although pressures of from 250 to 400 pounds per square inch have been widely used in carrot spraying, 100 pounds is sufficient and desirable since it results in less loss from drifting mist. Excessive pressure will cause even greater loss of a refined carrot oil because of its low viscosity. Experiments using the range of from 100 to 400 pounds have proved the lower pressure to be sufficient. To obtain the same volume delivery at the lower pressure, larger nozzle orifices are necesary. These also are an advantage be- cause they do not clog so rapidly. Spray equipment. The arrangement of nozzles, the adjustment of the boom, and the orifice sizes required to deliver the proper dosages at different pressures and ground speeds are discussed in the mimeographed circular, "Equipment for Spraying Weeds," obtainable from the Botany Division, Col- lege of Agriculture, University of California, Davis. A new circular on spray equipment will be published in the present series, at a future date. Spraying seed crops. In spraying seed crops of carrots, parsnips, celery, and other members of the carrot family, the recommendations given above should be followed except the one concerning the size of plants. Spraying may be done at any stage of growth up to the time plants are mature. More 12 California Agricultural Extension Circular 136 than one spray may be applied when necessary. It is not advisable to actu- ally spray the seedstalks but if it is necessary to spray after bolting starts, two nozzles may be used for each row, directed so that the spray fans meet just above the soil level, thus avoiding the seedstalks. Many hundred acres of carrot seed crops have been weeded with oil with no detectable harm to the seed. Oil sprays should not be applied during excessively hot weather. caution 1. Use only stove oil or a special fraction having a gravity rating in A.P.I, units of 37° or above, for killing weeds in carrots. 2. Never use Diesel or similar heavy fuel oils to spray carrots. Diesel may be used only for preemergence spraying. 3. Apply just enough oil to wet the plants ; more runs off and is lost. 4. Apply stove oil only to young carrots having 1 to 4 true leaves. A more refined oil, however, may be used up to within 6 weeks of harvest. 5. Move the sprayer through the field at constant speed. Adequate screens should be used in the suction line of the pump so that nozzles do not clog. 6. Shut off the spray before stopping in the field to clean nozzles, 7. If excess oil is accidentally applied in one spot, hoe out those plants, be- cause a few heavily contaminated carrots may cause rejection of a whole ship- ment. 8. Do not harvest carrots until the oil flavor and odor have disappeared. This may be determined by preparing, cooking, and tasting random samples from the rows. 9. The atomized oil from spray nozzles is highly inflammable. Keep open flames away from spray equipment. Cooperative Extension work in Agriculture and Home Economics, College of Agriculture, University of California, and United States Department of Agriculture cooperating. Distributed in furtherance of the Acts of Congress of May 8, and June 30, 1914. B. H. Crocheron, Director, California Agricultural Extension Service. 8im-5,'47(A2942)