Well after providing germination instructions for the seeds, I felt the need to add some information on how to proceed once your seeds have germinated. Now, the following instructions will need adjustments depending on your situation.
First, let’s assume the tree seeds have been sown and germinated indoor. This is not the best method, but can be the only way for some people because of their urban situation. Usually, because of the lack of light, temperature variations and natures obstacles, such a wind, a seedling grown indoor will have bigger leaves, long internodes and be thinner.
The explanation is simple, the plant instead of bulking up on light, will grow lengthened in order to reach for the maximum light as possible. For this, the sunflower plant is fun to watch, It require alot of lights, and the plant, will grow very tall, in order to be the tallest among the plants, and will also turn itself wherever the sun is. This is called phototropism. So in conclusion, you want to have the lightest possible for your seedlings.
If needs be, use neon tube, mixing red and blue color. After, a while, and supposing you are at the right time of year, you can bring your seedling outside. The best time to do that, is to bring them outside, when your indoor and outdoor temperature are similar. You will want to keep your seedling in the shade for 2-3 weeks, for them to acclimatize to the surplus of light.
Second, if you have sown and germinated seeds outside in a container. As they are already outside, the young seedlings will benefit of the sunlight from the start. The disadvantage of growing in a container, is that the roots have limited space to grow. This will result in a long tap root all curled up in the pot. A basic principle of tree growing, is that the foliage of the tree will often mimic the roots development. So if your roots are all curly, the tree will grow curly.
Another disadvantage is that growth will be limited because root developments is limted. To prevent that, it is often suggested to cut the tap root at a very young age, forcing the seedling to grow lateral roots. This method is often used to shorten the trunk and have the first branch lower to the root system. Altough, it is efficient, I prefer the next method of growing seedlings.
Third, wich is the best way, is to sow tree seeds, germinate them, and grow the seedlings in a field. This method will mimic nature, but often do it better. In nature, tree seeds germinate under a layer of leaves and dead branches, forcing the seedling to make it’s way to the top by any means, even if it grows curbed. But if you grow your seeds in a field/garden, without obstacles, the seedling will grow with a straight trunk and benefit from the lights and wind.
Plus, the roots will have all the space they want, benefiting the foliage and crown development. If the seedlings are getting all the light they can get, the plant will grow bigger and trunk and branches will have short internodes. With this method, you dont have to cut the tap roots because you’ll have sufficient branching near the root’s (nebari) of the seedling. You should leave the seedlings alone for at least a year, and next spring you should proceed to dig them, cut the excessive roots, create your nebari by combing the roots, and plant them in a container. This will yield the best result for young seedlings and be on a good start for nice bonsai.