Unhulled sweet-clover seed is sown principally in Kentucky, Alabama, and Mississippi. On the limestone soils of regions, which appear to be naturally adapted to sweet clover, very good results are obtained by using unhulled seed. It is not because southern-grown unhulled seed germinates better than northern-grown unhulled seed that better stands are obtained in the South from it, but it is mainly because southern farmers better understand the somewhat exacting conditions necessary for obtaining a stand with this kind of seed. Unhulled sweet clover contains a large percentage of hard seeds which will not germinate until they have been in the soil for some time and have been subjected to varying temperatures.
Seeding experiments have been conducted at Arlington, Va., where unhulled seed which contained 90 per cent of hard seed was sown during each month of the winter. Good stands were obtained on those plats seeded at the rate of 24 pounds (3 pecks) of seed to the acre during December and January, and fair stands on the plats seeded at this rate in February. Later seedings failed to produce a stand.
A large percentage of the unhulled seed sown in the South is seeded during January and the first part of February. Good stands are seldom obtained from unhulled seed south of the latitude of Washington, D. C., when the seed is sown later than the middle of February.
The use of unhulled seed has usually been attended with failure in the northern portion of the United States, although occasionally good stands have been obtained the following spring from late fall seeding. This failure is in part due to the fact that the seed has been sown in the spring and at a tine when only seed germinating well should be used. When unhulled seed is to be sown north of the latitude of Washington, D. C., it should be sown not later than February 15, and preferably earlier. Observations show that fairly good stands may be obtained by seeding during the winter, but care should be taken not to sow seed earlier than necessary on land which is subject to washing. Farmers should have no trouble in purchasing hulled seed, and therefore it is recommended that only hulled seed which germinates well be sown.
The rate at which sweet clover should be seeded varies with the germination of the seed, the condition of the seed bed, the climatic conditions of the region, and the method of seeding. Throughout the humid sections of the eastern United States sweet clover ordinarily is seeded at the rate of 15 to 20 pounds of hulled seed to the acre. From 12 to 15 pounds should be ample where the seed bed is in good condition and the seed germinates 75 per cent or more. In Illinois, the western North-Central States, the Mountain States, and the Pacific Coast States good stands are generally obtained by sowing 10 to 12 pounds of hulled seed to the acre. In eastern Washington it is claimed that from 5 to 8 pounds to the acre are sufficient for good stands.
When sweet clover is grown under irrigation, 8 to 10 pounds of hulled seed usually are sufficient, and from 2 to 4 pounds per acre are enough when seeded in rows from 2 to 4 feet apart. Of unhulled seed 3 to 6 pecks (24 to 48 pounds) or 20 pounds of hulled seed are usually sown in the South for pasturage or hay. In any region at least 10 pounds more of the unhulled than of hulled seed should be sown to an acre. Unless annual yellow sweet-clover seed is thoroughly cleaned it should be sown at the rate of 25 to 30 pounds to the acre.