How to Germinate Japanese Maple Seed When planting Japanese maple seeds, it?s important to pretreat them before sowing them in the ground. If you plan on planting your seeds outdoors in the spring, place them in a paper bag and store them in a cool, dark place through the winter. If you plan on starting them indoors in a pot, you can skip the winter storage and begin treating the seeds right away. First, break off the wings of the seeds. Next, fill a container with water that?s very warm but not too hot to put your hand in it, and soak your seeds for 24 hours. Then mix the seeds in a small amount of potting soil and put it all in a sealable plastic bag. Poke a couple holes in the bag for ventilation and put it in your refrigerator for 90 days to stratify. Once the 90 days are up, you can plant the seeds in a container or directly in the ground. If you live somewhere with cold winters, you can skip the fridge and simply sow your seeds outdoors after they soak. The cold of winter will stratify the seeds just as well.