I usually introduce it in November for several reasons:
1. They make great gifts for the holidays!
2. If students want to buy special items for leaves, Black Friday sales make them cheap.
3. If you do it too soon, kids might expect every project to be "like this" and they are not.
As for those Core Content or STEAM connections, trees are a great way to connect with fractals (https://goo.gl/C1VfwO) and are also a great way to connect with Asian history. (https://goo.gl/CqBnvb)
I have done 4 different kinds of bases.
The first two require no roots and are easier:
1. A block of wood with a 1/2 in to 3/4 inch hole filled with wire and a 1/4 inch dowel hammered in to tighten it.
2. A half inch dowel in a block of wood, covered with a plaster environmental base with the dowel exposed. We then wrap the wires to that dowel and create a tree.
3. Make and attach roots to an object students bring in for an expressive quality.
4. Attach roots to a rock. We get soapstone in bulk from Nasco when the budget allows. Found rocks will work too.
I am sure you could make something with clay too that could be fired and would work well too. (Raku Maybe?)
If you use wire, pliers, and cutting tools are important, as well as eye safety equipment. We have a wood shop and are able to borrow from them from time to time. Cutting wire can be done quickly by wrapping it around some cardboard and using a tin snip or other durable cutter. Students can also cut one by one with a ruler nearby, but that takes a lot of time. I find 16 to 18 inch wires are a good size to work with. About 12 inches if you do not need roots.
Once wire is cut, you'll need to even out one end, and begin twisting, separating in half, then twisting again, separating in half and so on. They twist best when you part them like a "Y" and put your hand in the middle to twist. The BEST THING about this project is that even "ugly" trees still look like trees. Branches should end in pairs or threes. If someone has a short single wire, just pull it out sideways, and add a leaf to it later. All ends will need about 1 to 2 inches of wire to add on a leaf or bead.
When trees are near complete, we review traditional Bonsai forms, and show a video about it. They are required to shape their Bonsai to show the art principle of movement. I try to have them pick a movement that is self expressive. If they are an active student, pick and active pose. A Shy student might like a willow pose. I remind them that art should always be tied to the maker, the artist, or expressive in a way that would be appreciated by the recipient.