Some seeds have such thick coats that moisture can’t
easily penetrate, and so the seed is very slow to germinate. Germination
can be hurried by thinning or scratching the seed coat. This can be most
easily done with a file, sandpaper, or a knife. The seed coat should be
scuffed, not deeply sanded or cut. Seed should be planted immediately
after scarification. Soaking seed overnight is usually not a good idea,
and may actually reduce survival rates.
Two excellent guides to plant propagation