Acer palmatum seed is notorious for germinating sporadically. Germination of Japanese maple seeds (Acer palmatum) is not difficult, but it can be a bit tricky if your wife or husband doesn't let you take over part of the refrigerator!
First of all we soak the seeds in lukewarm (not hot!) water for 48 hours. This softens the seed coat a little. Next we mix the seed with a little damp (not too wet) sphagnum moss or peat and put it in a plastic zipper bag. Leave the seeds in this humid atmosphere for 4-6 weeks at room temperature.
Now comes the tough part - leave the zipper bag in the refrigerator for 60 to 90 days. This is to mimic the natural action of seeds going through a winter season. Technically it is called 'stratification'.
After the 90 days sow in trays or pots filled with a good seed compost. If you have a seed propagator use it or find a warm place to maintain a steady temperature of 65-70F (18-20C)
Cover the seeds to their own depth: ¼ Inch (6 mm) with seed compost.
Sowing Time: Because of the quantities we grow we either sow in the late fall directly after new season seeds are available or we start stratification in the December/January period. For smaller amounts of seed you can try this anytime!
After the chill period, gradually raise the temperature to 55F (13C). Seed should germinate!
Transplant seedlings when large enough to handle into 2-3 inch pots.
Also, don't throw your seed pans away! Keep them for another year in an out-of-the-way place as more seeds may germinate in the second year!
Grow seedlings on in a cold frame or shady area and plant out the following spring or autumn in a sheltered spot. Grow on in pots for 1-3 years, transplanting to bigger posts as the tree grows before planting into final position in a slightly shady, sheltered spot in moist, but free draining, neutral to acid soil.
Don't forget that only grafts or cuttings taken from 'named variants' of Acer palmatum can be totally ‘true to type’. Seeds do not reproduce exact replicas of the cultivar from which they developed. Usually strong resemblance is found among the seedlings and such variation should be celebrated! In the end, you may just get the seed that produces the next beautiful, winning cultivar! If you do, you can treasure it in your own garden or consider starting your own backyard garden business to propagate and offer it for sale!