To germinate plum (or any prunus really) reliably, place the seed in a vice and crack the seed coat. You should just hear the "crack." If you do it right, it does not damage the seed inside. Then, remove the seed coat and you will see that the seed inside looks like a tiny almond. Place the seeds in moist pearlite (or damp paper towel if you are cheap like me) in a Ziploc bag in the fridge and plant in the soil when they develop an 1/8" root (about a month or so later). I have never tried without stratification. Germination is almost 100% with this method.