Q&A: Acer seedling care
A. If you have these seedlings indoors, put them in 10 cm (4") pots for now, using a potting mix that has some nutrients. Give them plenty of light each day, but not direct sunlight, and water them as needed. Start around the beginning of May, harden them off. Do that by taking them outside for longer and longer periods each day, exposing the plant to outdoor conditions. After about a week, you should be able to leave them permanently outside. Pick a reasonably sheltered location — give them morning sun, but some protection from the hot afternoon sun, especially important for young trees. Red-leaved cultivars need more sun than green cultivars in order to develop good colour.
Fertilize with a high nitrogen fertilizer — follow the instructions on the container, and do not over-fertilize. Water as needed, preferably early in the day.
They can stay in a pot until they are three to four years old, but will need to be potted into a 15 cm (1 gallon) and maybe eventually 22 cm (2 gallon) container, depending on how quickly they grow and how tall they become. When the pot seems full of fine roots, it is time to go to a larger pot.
Japanese maples don't like temperatures much below -10 C (14 F) so, for the winter, either sink the whole pot into the ground, or move the pot into an unheated shed after the tree is dormant.
It will be interesting to see what form and colour your seedlings develop as they mature. Each of these seedlings is a genetic individual, so will have inherited characteristics from both parents. It is a good idea to keep as many seedlings as you have space — it takes quite a bit of time for the characteristics to emerge — and then you can keep the prettiest ones; the ones with the best form and colour, for yourself and give the others away.
When you finally pick the location you are going to plant them, they prefer a nice loamy soil with high organic content and even moisture. Don't plant them in a site with too much exposure to the sun or drying winds.