Frost seeding can be a very effective way of seeding. It can be used if you do not have equipment to seed, or if you wish to thicken or add to a perennial without working what exists. Frost seeding works best with clovers, however, can be very successful with brassica and chicory as well. The important thing to remember when frost seeding brassica is that you have to be careful in selecting a site that will not be under water for any extended period of time during the spring (more than a day or two). Clover and chicory will tolerate standing water a little better. Alfalfa does not work with frost seeding.
If you are starting a new plot and you wish to frost seed, you will need to plan one year ahead. Spray the area with roundup as many times as needed to kill all the existing vegetation the previous year. This may require you to spray two or three times during the summer. This step can be eliminated, but you will not have as good of results. The next year, you will have a very clean area to work with. In the spring, when there is still snow on the ground and before everything starts to grow, simply spread seed in the area. (This can be done as early as you want, even mid winter, however, birds may get to your seed if too early). The best time to do this would be when the ground is consistently freezing at night and thawing during the day. You want to spread the seeds early in the day before the ground thaws. Those are ideal conditions, but anytime from mid-winter to early spring will work. With the melting snow, frost coming out of the ground, and rainfall there will be plenty moisture to germinate.
Seeding into existing stand
You can also frost seed into an existing perennial stand. You do this in the same way, at the same time of year. There are two reasons you would want to frost seed into an existing stand. First, you may want to seed into an area that has died from flood, drought, or freezing, or simply to improve a thinning stand. A second use for seeding into an existing stand would be to get brassicas into a clover, alfalfa or chicory stand or to get clover into a biennial stand. This would give you an annual growing in a perennial plot and it is easy to do.