Fall-plowed ground ordinarily makes an ideal seed bed for spring seeding. Soil which has been previously planted to a cultivated crop, such as corn, is usually put in sufficiently good condition for sweet clover by disking. Good success has been attained by merely broadcasting the seed on sandy soil and scratching it in with a harrow. Such a seed bed appears to be ideal when the seed can be covered sufficiently to insure plenty of moisture. It must be remembered that young sweet-clover plants are not drought resistant and that every precaution should be taken in seasons of drought or on laud which drought affects badly to so prepare the seed bed that the largest quantity of moisture will be conserved.
Excellent stands have been obtained at times by double-disking native prairie sod and either covering the seed with a harrow or sowing it with a drill.
The proper time to seed sweet clover should be determined by the germination of the seed, the climatic conditions of the region, and the condition of the seed bed at the time of sowing. When growing under natural conditions, seed which has lain in the ground over winter germinates in sufficient quantity during the following spring to produce a stand. It is therefore assumed that since this seed has passed the winter on or in the ground and has produced a good stand the following spring, sweet clover may be sown at any time of the year and a satisfactory stand obtained. Little is thought of the enormous number of seeds which shatter from a single plant and fall on an area not exceeding 5 or 6 feet in diameter. Single plants have produced as many as 350,000 seeds (the approximate number in 1½ pounds), or about 10,000 seeds for each square foot of ground covered. It matters little how many of these seeds germinate in the fall they mature or during the following winter, when the seedlings will be killed by freezing, for there will be enough viable seeds left in the ground to germinate when conditions are favorable in the spring. Conditions are very different when sweet clover is sown on cultivated soil at the rate of 5 to 20 pounds of seed to the acre—25 to 100 seeds to the square foot. When this quantity is sown, it is necessary that it be planted at such a time that the greatest number of seeds will germinate and produce plants.
Hulled seed makes up a large percentage of the sweet-clover seed sown. The germination of hulled seed varies considerably, although ordinarily it is higher than that of unhulled seed. Seeding experiments conducted at Arlington, Va., with seed which germinated 80 per cent show clearly that seed which germinates well should not be sown during the winter months in those sections of the country where midwinter thaws are likely to occur, and especially in sections south of the latitude of southern Ohio. In these experiments seed was sown during each month of the winter. Good stands were obtained only on those plats which were sown in the latter part of February and during March and April. At least 75 per cent of the seed sown during November, December, and January germinated on warm days during winter thaws and was killed by later cold weather. Notwithstanding the fact that sweet-clover seedlings will endure fairly low temperatures, seed germinating more than 50 per cent should not be sown during the winter months, and preferably not more than a week previous to the average date for the last severe freeze.
No data have been secured on winter seeding in those portions of the United States where open winters do not occur. It is probable that in those sections the winters are sufficiently cold to prevent germination before spring. Good results may be obtained by winter seeding, but as usually no trouble is experienced in those sections in obtaining a stand by seeding as soon as the soil can be worked in the spring, it is strongly recommended that seeding be done with hulled seed, which germinates well at this time of the year.
Many excellent stands have been obtained by seeding late in the spring, but in most sections seeding at this time is not as certain to produce a good stand as earlier seeding. Late spring seeding may be preferable when the ground is weedy and the clover is to be seeded without nurse crop. Under these circumstances a crop of weeds may be destroyed before seeding.
Very good success has been obtained in the Southern and Central States, and in some of the Northern States, by seeding sweet clover in the late summer or early autumn. When there is sufficient moisture in the soil for germination and when good seed is used, better stands have been obtained by seeding about eight weeks before severe frosts are to be expected than from spring sowing. This is particularly true in regions where late spring droughts or severe summer droughts are likely to occur. Seeding at this time may be done after an early crop has been harvested and when weeds are not likely to be troublesome. Plants from fall seeding mature from 10 days to two weeks later the following season than plants from spring sowing of the same year. The later time of maturing is an advantage, in that the plants will be ready to cut during better haying weather. The root growth is not as large from fall seeding as from spring seeding, and therefore not quite as much humus is added to the soil. Late fall seedings are very likely to be injured from heaving on wet clay soils.