Clover in some of its varieties is frequently grown from year to year in orchards and for the two-fold purpose of gathering food for the trees and providing for them a cover crop in winter. The medium red and crimson varieties are preferred for such a use. The latter is the more suitable of the two, since it does not draw on soil moisture needed by the trees, owing to the season at which it is grown. Enough of the seed of these crops may be allowed to mature to re-seed the land from year to year, and thus keep it producing. The clover plants not only gather nitrogen for the fruit trees, but in their decay they increase the power of the soil to retain moisture for the benefit of the trees. Some varieties of clover may be grown as catch crops, that is, as crops which are grown in addition to some other crop produced the same season. When thus grown, it is usually for purposes of soil improvement rather than to furnish food. The varieties best adapted for this purpose in the Northern States and Canada are the medium red and the crimson, the latter being much more circumscribed in the area where it will grow successfully than the former. When medium red clover is thus grown, it is commonly sown along with one of the small cereal grains, and is buried in the autumn or in the following spring. (See page 75.) The extent of the advantage is dependent chiefly on the amount of the growth made, and this in turn is influenced by the character of the soil, the season, and the nurse crop. In certain areas favorable to the growth of clover some good farmers sow clover along with all the small cereal grains which they grow. Crimson clover is usually sown in the late summer after some crop has been reaped and it is plowed under the following spring. (See page 250.) In the Southern States Japan clover and burr clover will serve the purpose of catch crops better than the other varieties. The former will follow a winter crop (see page 284), and the latter a summer crop. (See page 294.) Although alfalfa is not usually looked upon as a rotation crop in the Rocky Mountain valleys, it may be made such a crop. In these it grows so vigorously as to fill the soil with its roots in one or two seasons, hence it may be made to rotate profitably with other crops. (See page 135.) In such instances, however, medium red clover would probably answer the purpose quite as well, and possibly better, since the labor of burying it with the plow would be less difficult.