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Among the most important considerations in connection with the extermination of weeds are prevention of the maturing of seed and the sowing of good seed only. In the long run the more expensive clover seed is better than the cheaper grade. Manure should be thoroughly decomposed to prevent the scattering of weed seeds. I found that seeds of various weeds placed in manure that underwent decomposition were destroyed.
Cultivation.—No other method is so good for the extermination of weeds as cultivation by keeping the fields clean. With ordinary cultivation the annual weeds are readily destroyed. Where the field is very weedy it may be necessary with perennial weeds to summer fallow, and perhaps we may as well give the methods for destroying a few of the perennial weeds. Take quack grass as an illustration, which is the most injurious weed in the state of Iowa, especially in the northern part of the state.
The difficulty in exterminating weeds is due, in many cases, to the prolonged vitality which many weed seeds possess. Indian mallow, shoofly and other members of the mallow family retain their vitality for years. The seeds of many Leguminosae display the same characteristic. Cases may be cited where fields which had been in meadow for years upon being plowed soon abounded in the greater ragweed, in the grain crop. Such weed seeds may have been scattered by freshets, but the coat; of the involucre being very hard the seeds were thus able to delay germinating. In the case of the cocklebur, one “seed” germinates one year and the other the next. When the seeds' coats are hard the seeds may retain their vitality for long periods. Manure should not be applied on fields until it has been thoroughly composted, and thus the weed seeds contained in it thoroughly destroyed. This is not the usual practice. When fresh manure is spread on the field, the straw should be reasonably clean. Clean seed should be sown. In the general treatment of weeds, in order to exterminate them, it is first always important to prevent the formation of seed and secondly to prevent the formation of roots. This subject will be discussed under the following heads: (1) Rotation of crops. (2) Treatment of annual weeds. (3) Treatment for biennials. (4) Treatment for perennials. (5) Treatment for special weeds: a, in meadow and pasture; b, in grain fields; ba, among small grains; bb, in corn fields; c, in gardens; d, on roadsides, in yards; f, lawns; g, summer fallow. (6) Treatment with herbicides. (7) Fungi destructive to weeds. Rotation of Crops.-It is a well-known fact that a soil long cultivated with the same crop is generally not as remunerative as where rotation of crops prevails. A piece of land grown continually in wheat, oats, or flax will leave the field full of foul weed seeds. The continuous cropping of flax and oats in the northwestern states has left the fields so full of mustard that in some places there is almost as much mustard as oats or flax. The continuous cropping with corn, combined with poor cultivation, has left many fields in southern Iowa and northern Missouri full of cocklebur. The continuous cropping with oats has often resulted in large quantities of wild oats growing with the tame. The continuous use of the pasture without renewal has caused blue vervain to become a bad weed. Professor Spillman gives the following as a definite suitable rotation: When sod is plowed, the land is planted in corn, then wheat is sown and grass follows. A freshly broken sod, brought into good condition will be much easier to take care of than a piece of wheat or oats ground. It will contain a far smaller number of weeds, and consequently fewer weed seeds. Corn in such ground is usually quite clean by the time it is “laid by,” and there is less chance for weed seeds to mature than in a “small grain” field. This method of cropping insures a smaller number of weed seeds when this corn field is placed in small grain the following year. After the small grain is removed it is always advisable to plow as soon as possible and then harrow in order to expose the roots of weeds already in the soil. If the small-grain crop is followed by clover a crop fairly free from weeds should be produced. It would be better if the clover meadow could be converted into blue grass pasture. When there is a tendency for weeds to appear, the blue grass should be encouraged to occupy the vacant spaces. In the East, where alfalfa is grown it should be treated in the same way as clover, but Mr. J. E. Wing of Ohio keeps all of his land, so far as possible, in alfalfa. In the West, especially in the Rocky Mountain country, it is far better to retain an alfalfa meadow for a long term of years. There are some notable illustrations in western Wisconsin, eastern Minnesota and northeastern Iowa, where mustard and wild oats were, at one time, as abundant as in the small grain-growing section of Minnesota and the Dakotas; but by converting the tillable lands largely into pastures and meadows these weeds have ceased to be so troublesome. Small grain fields where rotation has been practiced are almost entirely free from weed seeds. So far as annual weeds are concerned there is no better way of treating them than by a systematic rotation of crops. Treatment of Annual Weeds.-The most important point in connection with annual weeds is to prevent the formation of seeds. Cultivation of a field should be thorough at all times. The best time to kill these annual weeds is when they are young. The roots of all seedling plants are readily killed by being exposed for a short time to the sun. A field or garden should be plowed and then brought into a good state of tilth by using the harrow. When planted with small grain or any other crop, a little more labor in the spring may mean the saving of much labor later. All of the annual weeds mentioned in the list of most troublesome weeds should be treated in this way. It may be difficult to destroy older plants of purslane, for example, but young seedlings are easily destroyed. Many of the annual weeds may be destroyed with a solution of iron sulphate or copper sulphate. Treatment for Biennial Weeds.-The biennial weeds are not nearly so numerous in kind as the annual or perennial and are much more readily destroyed than the latter. The bull thistle, burdock and other biennial weeds of this character are readily destroyed by cutting the plants off a few inches below the surface of the ground. A small spade will do this easily. In cutting, it is, of course, essential to cut the root below the buds. The bull thistle produces a mat of leaves close to the ground the first season; by cutting these off, as stated above, the plant will not shoot up the second season. Treatment of Perennial Weeds.-Perennial weeds are nearly always difficult to destroy, much depending upon the character of the “roots” produced by them. The first and most important consideration is clean cultivation. At no time should leaves be permitted to appear, as these are the organs which make plant food. As an example, nimble will may be cited, which is so common in the Mississippi valley. The “roots” of these plants (there are several different kinds) are clustered. By plowing the field, and running a harrow through it, then later giving it thorough cultivation, the weeds can nearly always be destroyed. They do not persist long in a pasture in competition with blue grass. Dr. C. E. Bessey has well emphasized the importance of cultural methods in the extermination of perennial weeds. The horse nettle is quite as difficult to destroy as any of the perennial weeds. Like milkweed, it produces a long root, sometimes three to four feet long and when the root is severed, it produces buds that form new plants. The “smothering” method has proven efficacious. This is probably the most effective and at the same time the least expensive method of removing horse nettle. Rape is probably the most suitable crop to employ for this purpose. If the soil is not already rich, a liberal dressing of barnyard manure should be applied during the winter or spring and the soil should be harrowed or cultivated frequently until the time of seeding, which may be at any time during the months of May or June. This cultivation will prevent weed growth and at the same time will assist in the retention of moisture. If the rape is sown in drills, about two pounds of seed per acre is a sufficient quantity, and three pounds if sown broadcast. When the crop has attained a rank growth it may be pastured or removed and fed to stock. Where land is lacking in vegetable matter, it is a good practice to plow the crop under when it is properly matured. The latter is not necessary when the object is merely to destroy the nettle, as the rank growth of the crop is very effective in completely smothering the weed. Other Methods.-Planting to corn or roots is a method much in vogue for the destruction of horse nettle. As in the method described above, the plant should be kept down before seeding time. When the crop appears above ground, the use of the horse and hand hoes should be unsparing. When the welfare of the crop prohibits the use of the horse hoe, the hand hoe should be used at intervals until the crop is removed, and even then it may be necessary to give attention to this pest. There is no question about this mode of treatment being effective if properly