Any type of tree can be used for bonsai however, in order to create an impressive specimen one looks for certain characteristics in the material used. This information should help a little.

Trunk thickening is essential to develop a trunk which has proper taper meaning it goes from thick at the bottom to thin at the top.

How do you thicken the trunk of a bonsai tree?

To achieve the proper taper of the trunk which gives the tree proper proportion and gives an older look to your bonsai tree you will need to strategically cut the trunk. The more weight the trunk supports the thicker the trunk will grow. Trunk thickening is in a direct relation with old cells laid down in the cambium of the tree. The cambium grows from outside outward and allows the tree to cover up minor wounds and wall off rotted wood. More light and foliage result in more food production which leads to mores cells being laid down in the cambium and a thicker trunk as a result. It is generally accepted that to build an impressive bonsai tree you must create a good root structure, then a thick strong trunk and finally the branch structure.

http://www.best-bonsai.com/bonsai-knowledge-base/trunk-thickening/

Thank you SOOO much for all the advice. I've never tried this before and I think I can do it with the tips you've given me! I'll get them started in the spring.. I didn't know citrus could still make fruit even when "bonsai'd". That's awesome :O

I'll check out that link for thickening the trunk! I'll definitely read up on it an consider it, especially since their trunks are so tiny right now. It sounds like this takes a looooong time, but I'm excited to try it c:

If it were me, I'd utilize the clip and grow method. Let the trees grow as tall as they can for the growing season (or as tall as you have space for), then cut it down alllll the way back to 1-2 leaves. New leaves and branches with sprout everywhere! Choose one in best position to be the next section of the vertical trunk, and prune off the rest. Follow the same cycle for next growing season, but cut back to 1-2 leaves of that season's growth. Keep this up for a number of years and you will end up with a trunk with nice thickness, taper, curvy movement and character from healed pruning scars. And you will be keeping the tree compact! 5-10 years from now when you have a nice trunk, you can start developing your main branches (as mentioned above) and use the the same cut and grow method on them to get character, taper, movement, and to keep it compact. Once you have your main branches looking nice after a number of years, do the same for your secondary branches, etc. You will end up with a thick trunked but compact clementine bonsai!

This video shows and explains exactly what I am talking about on a lemon tree https://www.youtube.com/watch?v=YJYlStda7SQ

The word means plant/tree on a tray in Japanese but has come to include pots. Thats the idea anyway. I think bon is tree and sai is tray. Thats what I read anyway. Hope that helps with your ideas about what bonsai is or can be. :)

Good luck with the citrus. I'd like to see WIPs. One thing to note is that even if you reduce the tree's size the flowers and fruits will be normal size. Most will leave the flowers but if fruits develop, but remove them all except for one as it takes energy away from the tree. Good luck with it! :)

Flip that around, actually :-)

bonsai 1914, from Japanese bon "basin, pot" + sai "to plant."