Crimson clover is exceedingly variable both in color of flower and in time of maturity. These variations are particularly noticeable in fields planted from a mixed lot of seed, the flowers presenting a range in color from nearly pure white to a deep purplish red and the seeds a difference in date of ripening of more than a month. Since crimson clover is thought to be mainly self-pollinated, it is easy to fix these qualities by selection and to establish definite varieties.

In Europe six or seven different varieties of crimson clover are recognized and sold by seedsmen, varying from extra early crimson flowered to, extra late white flowered and from very hardy to non-hardy. By the use of a succession of these varieties the European farmer is able to spread his harvest over six or seven weeks instead of having it concentrated within a few days, as in America. Similarly, the culture of the plant has been extended northward from Italy to Sweden by means of hardy strains. A wild form of crimson clover having yellowish flowers and hairy foliage occurs in southern and eastern Europe and in England, but it is not of economic value.

In America no sharply defined varieties of crimson clover are recognized, except a white-blooming variety which is sold in the South and is two weeks later than the ordinary crimson-flowered sort. Hardy strains have been developed and used in a small way in Massachusetts and Ohio, but these are not commercially available.