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�..� %map, <br /> Also, weeds harbor insects and other agricultural pests. Rodents prefer to work where <br /> weed cover is present. Rodent control is more difficult where weeds prevent the grower <br /> from seeing signs of their activity. <br /> From the point of view of control, weeds may be divided into two groups; annuals and <br /> perennials. Annuals are more readily controlled than perennials, most of which are <br /> capable of re-growth from resistant roots or rhizomes following cultivation or chemical <br /> treatment. Bermuda-grass, johnsongrass, wild morning glory, and nutgrass are among <br /> the more serious perennial weeds occurring in avocado orchards. <br /> When the top growth of annuals is killed, re-growth from the roots does not occur; <br /> therefore, the basis of a program for the control of annuals is to destroy top growth <br /> before seeds are set. The same principle also applies to the control of perennials but <br /> with the additional, and more difficult, problem of destroying resistant underground plant <br /> parts. <br /> Weed control methods may be classified as mechanical, biological, and chemical. <br /> Mowing and cultivation are the mechanical methods most widely used in orchards. <br /> Biological methods find little application in avocado culture Chemical sprays, largely <br /> petroleum oils, are used extensively. <br /> Chemical methods for weed control may be divided into two main groups; (1) <br /> application of herbicides to plant foliage, and (2) application of chemicals to the soil to <br /> be taken up by the roots of the weeds. Use of foliage sprays is common in avocado <br /> culture. At the present time, not enough is known about the tolerance of avocados to <br /> soil sterilants to recommend use of soil-acting chemicals. However, this approach to <br /> weed control in avocado orchards offers the most promise for the future and is now <br /> being investigated. <br /> Petroleum oils are widely used for weed control in avocado orchards, therefore, <br /> problems related to their use will be discussed in some detail. Weed oils have the <br /> advantage that they kill all plant tissues wet by the oil, and have the disadvantage that <br /> they are not translocated into under ground parts of perennial species. Annuals are thus <br /> killed by one spraying. Perennials produce new top growth after each treatment and <br /> may be killed only by exhausting their underground food reserves by repeated <br /> sprayings for one or more growing seasons. <br /> Avocado growers should find it helpful to consider some of the basic principles of oil <br /> toxicity and the characteristics of oils that affect their efficiency as weed killers. An <br /> important characteristic of oil is its unique ability to wet foliage. Plants are covered by a <br /> waxy coating which, because of its oil-like nature, is repellent to water sprays and <br /> readily wet by oils. When applied to plants, oils form a spreading film which creeps <br /> along leaves and stems, penetrating and saturating the growing points of grasses and <br /> other plant parts sheltered by surrounding foliage. Oils penetrate the stomata and come <br /> into close contact with the living tissue. <br /> Wetting and penetrating action alone is not sufficient to explain the herbicidal properties <br /> of oils. Some oils, for example the petroleum fractions used as insecticides, have little <br /> toxicity to plants. Toxic materials must be present in the oil, or added to it, in order to <br /> make it an effective weed killer. Oils may be refined in such a way as to contain toxic <br />