Most Japanese Maple seeds ripen in the fall. Watch the tree and wait for the seeds to turn brown. The seeds are ready to be harvested when they are brown and can be easily removed from the tree.
The seeds are attached to a wing, it’s best to break the wing off before storing or planting the seeds. Japanese Maple seeds have a very hard outer coating as do many ornamental plants. Under natural conditions the seeds would have to be on the ground for almost two years before they would germinate. All that happens the first winter is the moisture softens the hard outer shell, and the second winter germination is beginning to take place.
In order for all of this to happen in the proper sequence so the seedlings actually sprout at a time of the year when freezing temperatures or hot summer sun doesn’t kill them, takes a tremendous amount of luck.
You can improve the odds by controlling some of these conditions, and shorten the cycle. Once you have picked the seeds and removed the wing just place them in a paper bag and store them in a cool dry place until you are ready for them. You don’t want to plant your seeds out in the spring until the danger of frost has past. Here in the north May 15th is a safe bet.
If May 15th is your target date you should count backwards on the calendar 100 days. That will take you to about February 5th if my math is correct. On or about the 100th day prior to your target planting date, take the seeds and place them in a Styrofoam cup or other container that will withstand some hot water. Draw warm to hot water from your kitchen faucet and pour it over the seeds. Most of the seeds will float, just leave them in the water overnight as the water cools down. 24 hours later most of the seeds will have settled to the bottom of the cup.
Drain off the water. Place the seeds in a plastic bag with a mixture of sand and peat or other suitable growing mix. Even light potting soil will work. The peat or soil should be moist, but not soaking wet. Poke some holes in the bag so there is some air circulation, and place the bag in your refrigerator for a period of 100 days.
After 100 days you can plant the seeds outside. If you have timed it correctly, you should be at or close to your target planting date.
To plant the seeds just sow them on top of a bed of well drained topsoil or sterilized potting soil, and cover with approximately 3/8” of soil. Water them thoroughly, but allow the soil to dry out completely before watering thoroughly again. If you water them frequently, not only do you stand a chance of the seeds rotting from being too wet, but you will also keep them cool, which will slow down the germination process.
Once they start to germinate provide about 50% shade to keep the sun from burning them. Snow fence suspended about 30” above the bed will provide about 50% shade. Japanese Maples will tolerate some shade so it isn’t too important to transplant them too quickly. Depending on how close together they are, you might be able to leave them in the same bed for one or two growing seasons. Don’t transplant until they are completely dormant.