Advice - The propagation of maples

Large numbers of Japanese maples can be propagated from seed, especially in order to produce grafting understock.
Because many seeds lose the ability to germinate quite rapidly, the seeds are sown in the spring following their harvest.

You can choose some variety here :

Stratifying seeds allows them to rehydrate slowly so that the process of hormonal activation can get started.
Prepare a blend of fine sand and peat (50/50 or 1/3+2/3).
Add the seeds to this blend and spray to dampen it slightly.
Vacuum pack the mixture in a plastic bag, seal it well and chill (3-4°C). (Refrigerator).
Pretreating A. palmatum with Thirame (fungicide, 4g per kg of seed) is recommended.
During this time the sprouts will start to develop. When 50% of the seeds have germinated it is time to plant them. This can be done in seedbeds outdoors in the month of April, or in trays or pots at a temperature of approximately 18°C in akadama soil with a medium grain size. The seeds should be covered with 1 to 2 mm of fine akadama or a substrate made up of half potting soil and half river sand.
Stratification will take from three to six months depending on the quality and freshness of the seeds.

This is done by cutting small branches (10 - 15 cm) and planting them in trays during spring in a closed system propagation greenhouse. They will take root in 3 to 4 weeks.
Not all varieties can be propagated from cuttings. The easiest ones include: A. P. kiyohime, A. P.deshojo, A. P. arakawa, A. palmatum. Note: Plants grown from cuttings grow more slowly.

Grafting is an operation that consists of attaching one plant or piece of plant material (called the scion) to another plant (called the understock). It’s a kind of association to which the understock and the scion bring their own characteristics. Their lives are then closely intertwined but their characteristics do not mix. 

ADVANTAGES

Better growth resulting from use of understock that has a more vigorous root system.
Varieties that produce sterile seeds and those that do not propagate easily by cuttings can be produced by grafting - this includes 95% of palmatum varieties and cultivars.
{You can graft onto a young plant (called the understock) that comes from a one-year-old seedbed and has a trunk diameter at its base on the order of 2 to 3 mm, or at least the same size as the scion.
There are two periods for greenhouse propagation: the beginning of the year, i.e., from January through mid-March, or during the summer from 15 to 30 July.
This technique is called side-veneer grafting.
There are other possible grafting techniques, but this is the one most commonly used by professionals.
For springtime grafting, the understock must be placed in a greenhouse to start it and the scion should be kept dormant in a cold environment.
Choose a small branch on the mother plant (at least 10 to 15 cm, depending on how vigorous the understock is).
At the base of the stem, make a solid scarf cut with a budding knife.
The understock is cut by making a vertical slit in the trunk to loosen the bark. The scion which has a corresponding notch is then inset, and everything is kept in place with grafting tape. All that’s left is to wax the graft union to keep the scion from drying out too much. The plants are then placed in a closed system propagation greenhouse. Watch out for fungal attacks.
(Temperature between 18 and 26°C, 80% humidity).

DRAWBACKS

Some plants may develop swelling at the graft union, which can be unsightly if it is too high up. This is why the graft union must be located as close to the collar as possible. This avoids differences in the bark for certain varieties with decorative wood
(Acer P. sango kaki and Acer P. arakawa). Personally, I take great care to make grafts quite low on the plant.

It is possible to improve the nebari quickly and produce fine plants using the technique of air layering, especially for mini-bonsais and styles with multiple trunks.
This involves planting the cutting or young graft in the ground, leaving it to grow for several years, and when the trunk is thick enough, air-layers can be produced. Select a part of the trunk with an interesting shape, or layer under branches to create multiple trunks.
This should be done in April or May, when the plant is actively growing.
This procedure involves removing a strip of bark (annular decortication) around the trunk, at the place where you want roots to grow.
Then cover it with moist sphagnum and hold it in place with a piece of plastic attached at the top and bottom so that it forms a pocket.
Make sure that the sphagnum does not dry out too much by periodically injecting water through the plastic using a syringe.
After a few weeks, roots will start to grow where the bark was stripped.

The layered plant should be separated either in the fall if you can see enough roots through the plastic sheet, or the following spring. In this case, the layered plant will need to be protected through the winter, to keep the fine roots from freezing. The plant must be cut away just below the new roots with pruning shears or a saw, leaving one or two centimetres of the old plant to raise the roots above the bottom of the pot.
Then plant the new tree in a pot, attaching it securely to the pot with twine wrapped around the outside of the pot and over the branches so as to prevent the wind from snapping the young roots.
The tree can then be grown normally, so long as it is protected from the wind, then planted in the ground or a planter.