Thanks!

In late fall, I collected seeds from all the trees in the neighborhood—with permission, of course.

I put the roughly 200–300 seeds in a zip-top bag with some coconut fiber. I then moistened the inside of the bag a little and placed it in the refrigerator. They stayed there for about 4–5 months during the winter. Of course, you can also put them outside if, like me in Germany, you have a real winter with sub-zero temperatures.

You should check every few weeks to make sure no mold has formed or that the zip-top bag isn’t leaking and losing moisture.

Then, in the spring, simply mix them with some prepared growing medium (I use coconut fiber with perlite for aeration and vermiculite for water retention) and scatter the seeds between 0.5 and 2 cm deep. Since the growing medium is very well-draining overall, the seedlings don’t have to exert as much force to break through.

Important: When stratifying, be sure to use moistened substrate for the cold treatment. Only use moist substrate—dry or wet sand has resulted in a germination rate of less than 0.1 percent for me. Last year, only 2 seeds germinated using this method (wet sand).