Trees with hard seed coats, such as Robinia should be scarified to allow in moisture. Scarification can be carried out using sandpaper or a file to abrade the seed coat, or using a knife to nick the coat. Some seed can be softened in warm water by soaking for 24 hours; care should be taken when soaking seed, as rotting may be the end result.

I Place seeds in a clear plastic bag filled with moist but not wet coir, composted bark, or a mix of equal parts of the above with coarse sand, perlite or vermiculite and seal the bag.
Chill seed in a refrigerator, kept below 5°C (40oF) for four to 20 weeks, depending on the species.
Shake the bag periodically, and sow seed immediately if it germinates in the bag.

This works for me, hope this helps you.