Along with blue grass, white clover plants aid in choking out weeds. This result follows largely as the outcome of the close sod formed by the two. But in some soils, plants of large growth and bushes and young trees will not thus be crowded out.
Distribution.—White clover is certainly indigenous to Europe and to the Northern States, and probably Western Asia. It grows in every country in Europe, but with greatest luxuriance in those countries which border on the North Sea, the climates of which are very humid, and more especially in the Netherlands and Great Britain. It stands in high favor in Holland, but is not regarded so highly in England, owing, probably, to the great variety of grasses grown there in permanent pastures. It is generally thought that it was not indigenous to the Southern States, but has reached these from those farther north. It would seem to be capable of growing in all countries well adapted to the keeping of cattle; hence, it follows in the wake of successful live-stock husbandry.
White clover seems able to adapt itself to a great variety of climatic conditions. Nevertheless, it is certainly better adapted to a moderately cool climate than to one that is hot, and to a moist, humid climate than to one that is dry. It has much power to live through dry seasons, but it will not thrive in a climate in which the rainfall is too little for the successful growth of small cereal grains. Where snow covers it in winter, this clover will grow on timber soils as far north as any kind of cereal can be made to mature; and it will also grow as far south as the Mexican boundary on the higher grounds, when there is enough moisture present to sustain it.
It would probably be correct to say that this plant is found in every State in the Union, and that it succeeds well in nearly all the Northern States, from sea to sea. Although it grows well in certain parts of the Southern States, especially in those that lie northward, the general adaptation in these is not so high as in those further north. The highest adaptation in the United States is probably found in the Puget Sound region and in the hardwood timber producing areas of the States which lie south from the Great Lakes and in proximity to them, as Northeastern Minnesota, Northern Wisconsin, Michigan, Ohio and New York. But the adaptation is also high in the more elevated of the mountain valleys of the Northwestern States when irrigated waters may be led on to these lands. The areas lowest in adaptation are those that lie within the semi-arid belt. The low-lying lands of the South, where hot weather is prolonged in summer, are likewise low in their adaptation, but not so low as the former. The prairie areas of the Northern Mississippi basin have an adaptation for growing white clover that may be termed intermediate, but where hardwood forests grow naturally on these the adaptation is high. In New England the climatic conditions are very favorable, much more so than the soil conditions.
In Canada, conditions are found highly favorable to the growth of this plant in the country lying eastward from Lake Huron, north of Lakes Erie and Ontario and also on both sides of the St. Lawrence River. Adaptation is also high along the Pacific and in the mountain valleys not distant from the Pacific. In all the areas of Canada, which once produced forests, this plant will grow well. But north from Lakes Huron and Superior, the soil conditions are against it, because of their rocky character. Certain forest areas west from Lake Superior, and also in other parts, the sandy soils of which sustain a growth of Jack Pine (Pinus murrayana) trees, do not grow white clover with much vigor. The prairie areas of Canada, westward from Lake Superior to the mountains, do not grow white clover with much success, and the adaptation for its growth would seem to lessen gradually until the Rocky Mountains are approached.
Soils.—Small white clover will grow on almost any kind of soil, but by no means equally well. Highest, probably, in adaptation, especially when climatic conditions are considered along with those of soil, are the clay loams west of the Cascade Mountains and northward from California to Alaska. During the moist months of early summer, this plant turns the pastures in these areas into a flower garden. Almost equally high in adaptation are the volcanic ash soils of the Rocky Mountain valleys. When amply supplied with water, the finest crops of white clover can be grown even superior to those grown on the lands described above. Almost the same may be said of what are termed the hardwood timber soils, which are usually made up of clay loam lying upon clay. Such areas abound in Minnesota, Wisconsin, Michigan, Ontario and some States further south. In these soils it grows with much luxuriance, more especially when lime and potash are abundant. Similar luxuriance may be looked for in the deposit soils of river basins in which the clay element predominates, but not in those that are largely made up of sand. It will also grow well on the stiffest clays, whether white or red, when moisture is present. On prairie soils, the success attending it is dependent largely on their texture, composition and the moisture which they contain under normal conditions. The more firm these soils are, the better will the clover grow in them, and vice versa. This is equivalent to saying that the more clay they contain, the better will the white clover grow in them.
Where the humus soils of the prairies are deep and are underlaid with clay, white clover will grow much better in the subsoil, if laid bare, than in the surface soil. Prairie soils which lift with the wind are ill adapted to the growth of this plant, whatsoever may be their composition. Much of the soil in the semi-arid belt would grow this plant in fine form, but want of moisture, where irrigation is absent makes its growth prohibitory in a large portion of this area. On ordinary slough soils, this clover finds a congenial home, but it will not grow quite so well, relatively, in these as alsike clover. On sandy soils, such as those on which Jack pine and Norway pine (Pinus resinosa) grow, this plant will maintain itself, and in wet seasons will make considerable showing on these; but in very dry seasons the plants will die, the growth the following season coming from seeds already in the soil. In the soils of the extreme South, the inability of white clover to make a good showing is probably more the result of summer heat than of want of power in the plants to gather food. In those of the Southwest, want of moisture and excessive heat render its growth, in a sense, prohibitory.
Place in the Rotation.—Since white clover is usually not sown for meadow, but is rather sown for pasture, it can scarcely be called a rotation plant in the strict sense of the term; and yet, because of the extent to which it grows when it has once obtained a footing in soils, it is more or less frequent in all rotations in which grass or clover is one of the factors. As it usually comes into the grass pastures, when these have become established, it will occupy about the same place as blue grass in rotations; that is to say, whatever would be proper to sow after the blue grass would be proper to sow after this plant; since the two usually unite in making the same sod. It will, therefore, be in order to follow this plant with corn to feed upon the nitrogen furnished by the clover. The same will be true of any small cereal that has special adaptation for being grown on overturned sod, as for instance, flax or oats, or of any crop that revels in the decay of vegetable matter, more especially in the early stages of such decay, as, for instance, potatoes and rape. When white clover is sown on land that is cultivated, though only sown as a factor in a pasture crop, as with all other clovers it may best be sown on land that is clean; that is, on land on which the preceding crop has been cultivated to the extent of securing a clean surface on the same. If, however, this crop must needs be sown on land that has not been thus cleaned, its great inherent hardihood will enable it to establish itself where some clovers and grasses would fail.
It is common to sow white clover on land from which the forest has recently been removed, also on natural prairie, where it has not previously grown. In these instances it simply follows the crop of forest in the one case and of native prairie in the other. But it will not take possession of the land in either case to the exclusion of other grasses.
Preparing the Soil.—The preparation of the soil for growing this plant is much the same as for growing other plants of the clover or grass family. Fineness, firmness, cleanness and moistness are the chief essentials to be looked for in making the seed-bed. For the same reason that it has much power to grow among weeds for so small a plant, it has also much power to grow on surfaces not in the best condition of preparation for receiving so small a seed. But when sown to provide a seed crop, it is specially necessary to make the land thoroughly clean before sowing the seed. This is necessary for the reason, first, that small white clover, because of its tardiness in growing in the spring, and because of its comparatively small growth has not much power to crowd weeds; and second, because of the labor involved in preventing weed seeds from maturing in a crop that ripens its seeds somewhat late in the season. While it is advantageous to burn off the grass from a natural meadow where white clover, is to be sown, it is not so essential, nevertheless, as when preparing such land for being sown with some other varieties of grass or clover. The young plants will endure under conditions which would cause those of many other varieties to fail.