@River's Edge and
@Mike Westervelt have answered the direct questions well so I will add a note on specifically learning cottonwood and deciduous techniques for the first time simultaneously, just in case you find yourself muttering "
cottonwood doesn't work for bonsai" a couple years from now. Brace yourself for a steeper learning curve especially as you begin to develop branches or experiment with defoliation etc. Cottonwood is not Japanese Maple or Hornbeam, vigor is easy, control is more challenging.
Cottonwoods, poplars, aspens and willows (i.e. salicaceae) are more firmly in the "
you need to know, not guess" department and this is not immediately obvious when you start growing them right up until you lose your first really big section of growth. So, if you can, I wholeheartedly suggest finding someone who grows a species in this family to study with / talk to on a season-by-season basis so you can pick up the ins and outs of willow-family species in addition to learning all the deciduous basics first. I want to be as respectful as possible in saying this, but the original question and followup questions of this thread are day 1 deciduous 101 questions whereas cottonwoods/willows are decidous 301 with a prerequisite of 101 and 201. For cottonwoods, IMO it is not sufficient to merely say "
cut at the safe times, which are <such and such>". You can lose anything from a whole branch to the entire upper half of a tree by ignoring sucker growth, for example. Disjoint tips & tricks and google searches and random questions on forums aren't a great way to navigate all the way through from 101 to 301, if you catch my drift.
Another wise bnutter (I forget who, if you are out there please ID yourself) eloquently said something about willows akin to "
you need to decide for the willow or it will decide for you" and this is even more aggressively true for cottonwood. After messing around with other populus (eg: p. nigra) and then also various salix species, I believe this applies to all of salicaceae. If you find yourself frustrated with the species later on, dwell on that bnutter's statement.
@pandacular there are also some larger-sized black cottonwoods at Rakyuo bonsai now (perhaps you spotted these when you were there recently). These will hopefully help answer some questions about branching stability / ramification faster than what I'm doing.
