Mimosa can refer to a couple of different plants. Is it Albizia julibrissen you have? I have experience with this tree. I have no experience with Calliandra.

I grow Albizia in containers. It grows fast, and needs top and root pruning to keep it containerized. The compound leaves are quite large, and reportedly do not reduce very well, making it unsuitable for small bonsai. I have seen some nice Albizia bonsai, though, so it can be done.

What you need to understand before you begin bonsai; small-trunked bonsai do not become thick-trunked bonsai. Before you can start to train your tree as a bonsai, you need to have the thick trunk first. So, you will need to grow your tree in the ground (for fastest growth) or in a container (slower growth) in order to develop a thick trunk. Only then will you begin training. Developing the thick trunk will take some years.

This quote from bonsai4me.com: "A common misconception amongst newcomers to the art of bonsai is that trees (bonsai) with large, thick trunks must have had decades of training to become the size they are and that a thin-trunked seedling will one day acquire a thick mature trunk even though it is planted in a bonsai pot.

Unfortunately, once a tree is growing in the confines of a small pot, with its roots restricted and upper growth regularly pruned, the trunk and branches of the tree will only thicken very slowly.

Large bonsai with thick trunks are nearly always developed in the ground prior to being planted into a pot; some are purposely field grown, some are collected mature trees."

Do a lot of research, and you may find you would be better off finding a bigger tree to begin with.

Cheers!

Thanks for your response, it was very informative! The mimosas around here are all at least 5ft to 20ft tall and have the pink to reddish "plumage". I live in an apartment, so I can't grow one in the ground. If I grow it in a large pot, could that give a similar "environment" to "the ground", and should it be semi-exposed in winter to get the right type of "environmental exposure" for substantial growth?

I will assume that you are growing Albizia julibrissen, also called mimosa (just realize there are other plants called mimosa).

You can grow Albizia in a large container (I do), but it will grow slower than in the ground. However, Albizia is a vigorous species, and it still grows pretty fast in the container. It can tolerate some shade, but is best in sun. Albizia is a somewhat-hardy deciduous tree. In your zone 8, it should be left outside during the winter to satisfy its dormancy requirements. It will be just fine outside during the winter.

Growing trees in containers requires proper soil, water and nutrition to optimize its growth and realize the tree's genetic vigor. You will also need to root prune periodically (which you will also do once your tree has achieved bonsai status).

As a start, read this excellent post: Trees in Containers

I looked at pictures on-line and my "parent tree" is an Albizia. Thanks for all the information and the link to Trees in Containers. My seedlings started poking through yesterday (2nd week of 100+ weather)(they're in a plastic take-out container in the shade) and I'm looking forward to working with them. Patience is a virtue I need to acquire... I have a 2yr old euonymous fortunei I was hoping would be a good candidate for bonsai while I wait for the mimosas, but I've read that it isn't.
Thanks again!
Kimberly