Mammoth clover may also be utilized with advantage in lessening the numbers of certain noxious weeds, and in some instances of eradicating them altogether. This it does in some instances by smothering them, through the rankness of the growth. In other instances it is brought about through the setback which is given to the weeds by first pasturing the crop and then cutting it later for seed.
Distribution.—Mammoth clover has long been grown in several of the countries of Europe and Western Asia. It is also grown in certain parts of Siberia. It was doubtless introduced into the United States from Europe by emigrants from that continent, but when exactly is not known. It has probably been many years since its introduction into America, but it is only within the more recent of the decades that it has attracted general notice. In some areas in this country it grows with great luxuriance, fully equaling, if not exceeding, the crops grown in any part of Europe.
Mammoth clover calls for climatic conditions about the same as those for medium red clover. (See [page 61].) It flourishes best in moist climates of moderate temperature, and it will endure more drought than the medium red variety and possibly more cold.
The distribution of mammoth clover covers nearly all the States of the Union, but as with medium red clover the adaptation for it is relatively higher in the Northern than in the Southern States of the Union. The highest adaptation for mammoth clover is probably found in certain parts of Michigan, Wisconsin, Minnesota, the northern valleys of the Rocky Mountain States, the elevated portions of those further south and the country around Puget Sound. The adaptation is also high in much of New York, Ohio, Indiana, Illinois, Iowa, Missouri, Kansas and Nebraska. In the Southern States that lie northward, good crops may be grown in some locations, but not in all. As the semi-arid belt is approached, mammoth clover will grow further west than the medium red, but in the greater portion of this region it will not succeed. The adaptation of the North Atlantic States, including those of New England, is not of a high order, but rather more so, probably, than for the medium red.
In Canada also the adaptation of medium and mammoth clover is much the same as for the medium red. In some parts of Ontario, especially Western Ontario, it grows remarkably well; but in the maritime provinces it does not grow so well; nor does it thrive in the provinces of the Canadian Northwest as it does in Ontario.
As with medium red clover, the distribution of this variety has not been fully determined in either the United States or Canada, more especially on soils of the prairie, where it does not succeed well at present. It is probable that under some conditions on these soils, and also in the South, the absence of the requisite bacteria in the soil may account, in part, at least, for failure in attempts made to grow it. With the introduction of these, the area of successful cultivation may be considerably extended.
Soils.—Mammoth clover may usually be successfully grown in soils well adapted to the growth of the medium red variety. (See [page 65].) This means that it will usually grow with much luxuriance in all areas which produce hardwood timber, and are usually covered with a clay or muddy loam soil underlaid with clay. It will also grow with great luxuriance in the volcanic ash soils of the irrigated valley lands of the Rocky Mountain States, and in the loam and light loam soils of the Puget Sound country. It has greater power than the common red variety to grow in stiff clays, in sandy soils underlaid with clay, and in areas where moisture is insufficient near the surface soil. In stiff clays the roots penetrate to a greater distance than those of the medium red variety and gather more food. Consequently, a stiff clay soil that would only furnish a light crop of the medium red variety in a dry season may furnish an excellent crop of the mammoth. The quality of the hay is likely to be superior to that grown on soils altogether congenial, since it is not likely to be over-rank or coarse.
On sandy soils underlaid with clay, and especially where the clay is some distance from the surface, this clover is more certain to make a stand, since the vigor of the plants enables them to gather food until the roots go down into the clay.
In areas where the moisture is more or less deficient, the other conditions being favorable, this clover can send its roots down into the subsoil, where moisture is more abundant than on the surface. Because of this power, it is better adapted than the medium red to much of the area of Southwestern Minnesota, Western Iowa, Western Kansas and Nebraska, and, in fact, much of the area bordering on the semi-arid country.
On clay soils that are so saturated with water that in the winter or spring the clover is much liable to heave, there is conflict in opinion as to whether the mammoth or the common red variety will heave the more readily, but the preponderance of the evidence favors the view that the roots of the mammoth variety can better resist such influences than those of the common red.