I agree that germination is controlled by temperature. I thought frost seeding is to get good seed to soil contact for small seeds so that there's good germination when the temps are right? What yoderjac
said in the second post pretty much sums up my thinking on frost seeding. Although unlike what he said, I have had success frost seeding oats, maybe it was just a freak occurence but I had close to 100% germination seeding oats on bare dirt.
Not that it makes any practical difference, I doubt the heaving of soil with the freeze thaw cycle made much difference with the oats. Most cereal grains can be surface broadcast. How well they germinate depends on a number of factors including your soil type. You are spot on that seed/soil contact is key. A cultipacker pressing seed into the ground or a nice heavy rain pounding the seed and splashing soil on them can give good germination rates. My soil is heavy clay and the OM content is finally starting to come back. In my soil, WR provides me the best germination rates when surface broadcast. That does not mean I can't surface broadcast other cereal grains, I just need to increase the seeding rate to compensate.
My guess is that if you frost seeded clover and had just waited until freezing was over to broadcast the clover, you're germination rates would be similar. In general, frost seeding works best when overseeding an existing perennial clover field that is patchy. I don't get good results frost seeding larger seed annual clovers like Crimson. I never plant perennial clover in the spring. I find you spend too much time and energy fighting weeds. I think the best practice is to broadcast perennial clover with a nurse crop in the fall. Perennial clover spends quite a bit of time putting down a root system. When planted in the fall, it has time to germinate but depending on when you plant, it may not do much more in the fall. The cereal acts as the fall attractant. This gives the clover an early jump on weeds. If you use WR as the nurse crop, it not only takes up space, but also has an allopathic effect on weeds. Timely mowing that first spring releases the clover from the WR so it is taking over when the WR dies naturally in the summer before weeds can take hold.
I find that frost seeding perennial clover does not give it as much jump on weeds. You end up with more weeds and a shorter-lived clover field.
Don't get me wrong, I've very weed tolerant when it comes to clover. I like to start off with as weed free field as possible. After that first spring of mowing the WR, I just mow once a year just before our season. After a couple years, you would not even know it is a clover field in the summer because of the weeds unless you get on your hands and knees and look under the weeds. Yet after the fall mowing and when nights turn cool an our fall rains come, the clover bounces back and takes over the field. I no longer use herbicides to control weeds in clover for the most part.