Hello all. Last June I found these two trees growing too close to our house and instead of just killing them I decided to see if I could make something of them. I dug them up and planted them in two pots we had and put them back where they were. I watered them for a few months and they looked like they were having a hard time. They went through the winter outside and just started to grow again a few weeks ago. I am not sure how old they are but they are about 8 inches and 14 inches tall. What should I do next? Should I let them grow some more, trim them, etc. I am very new to Bonsai and any advice would be appreciated.
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Your first tree looks like a maple, probably Acer platanoides, the Norway maple. Might be Acer rubrum, red maple. The second does look a bit like sweet gum, Liquidambar styraciflua, If the leaves are in alternate pattern, then it is Liquidambar, if the leaves are opposite like the maple, then it is a maple.
Filling in your location helps with giving you climate related advise. Almost all timing issues with bonsai are based on local climate, not some "universal calendar". Everything is related to expected growing temperatures, and to expected first frost and last frost dates.
Most bonsai spend a good number of years, usually 5 to 10 years growing out to many times the size they will finish as bonsai. If you want a bonsai tree 12 inches tall, usually somewhere between seedling and the bonsai display bench a decade later, the tree will have to grow to be 5 to 15 feet tall in order to thicken up the trunk. Then the tree is cut DOWN to bonsai size. It is very rare that bonsai are created only by growing the tree UP to bonsai size.
Maples and Sweet gum *if that is what your second tree is?* usually work best as bonsai in the 18 to 36 inch tall range. They are difficult to work with as little trees less than 8 inches tall. So plan on growing these out for a few years. Set them up to grow out. Wide shallow containers will create a better spreading root base. Don't expose the surface roots on young trees in training, but you do shape the root system every time you repot. I use 16 x 16 x 5 inch flats with mesh bottoms to grow out young material. They hold about 3 or so gallons of potting media. THey are good for growing trunks up to 2 inches in diameter. maybe a bit bigger.
So your trees have a bit of growing to do. Nothing to do this season except let them grow, read and learn more about bonsai. The "New to Bonsai section is loaded with good advice. The "maples" section is also a forum you should read back into. And Liquidambar you should use the search function on BNut to pull up tricks and tips for sweetgum. It will appear in "general", "Miscellaneous" and "Other Deciduous".