Richard Taylor, Extension Agronomist; rtaylor@udel.edu
Frost crack seeding is an inexpensive means of establishing or reestablishing legumes in pasture or hay fields. This method is ideally suited to fields too small or irregularly shaped in which to use large equipment. It is most appropriate for use with small-seeded legumes such as white, ladino, or red clover, although it has been used with grasses and, on rare occasion, with larger-seeded legumes such as hairy vetch and alfalfa (low probability of success). If done at the correct time and managed properly to improve the success rate, frost crack seeding can be a very inexpensive tool available to all forage producers regardless of size and level of technology.
The principle involved in frost crack seeding is to seed during late winter or very early spring when freezing and thawing of the soil is producing frost action with ice crystals coming out of the ground and opening cracks in the soil. If the legume or grass is broadcast over the field surface the freezing, thawing, and refreezing will incorporate the seed to some degree into the soil to enhance germination. Also, the seed is present during a cool, moist time that favors legume germination before summer annual weeds germinate. Usually, this occurs between late January and late February although this year we’re still having favorable conditions even this late in March.
In frost seedings, either the frost action or livestock activity is used to control competing vegetation, prepare a seedbed, cover the seeds, and provide seed to soil contact for germination. If there is not sufficient frost action after applying seed and animals are available, allow animals to walk the pastures to tread seed into the soil surface. This should be done only when the soil is firm enough so that the cattle or horses will not punch through the sod and push the seed too deep into the soil.
Grass seedings may not be successful with the frost crack method. It is likely that the smaller seeded grasses (timothy, Kentucky bluegrass, and orchardgrass) will be the most easily established with this method. The success rate can be increased if the field is mowed or grazed very short in the fall or winter before seeding. Results of the seeding often are not evident until at least midsummer. The legume or grass plants are very small early and will need as much sunlight and as little competition as possible to allow them to become established. Fields grazed down to 3 or 4-inch stubble will have a higher probability of success. Hay fields where existing vegetation can not be controlled to reduce its competition with the new seedlings will see far fewer surviving seedlings than in pasture situations.
Of the legumes that have been successfully over-seeded into cool-season grasses, red clover is the most productive, ladino clover is the easiest and least expensive to establish, and annual lespedeza is the most versatile. Red clover generally will not survive more than two years due to the buildup of disease organisms in the soil. White or ladino clover are shallow rooted with surface stolons but can survive for many years if adequate soil moisture is available and harvest management is favorable. Under the best conditions, frost seedings succeed in only three or four years out of five but remain a less expensive option for producers who must rent or otherwise obtain no-till seeders. The lack of good soil-to-seed contact increases the risk of failure and even in successful years it often means that although the percentage of legume is increased the legume stand is not uniform. This often is acceptable to grazers but not to hay producers.