I want to mention that
@rockm most clearly mentioned the fact that different techniques are used for bonsai in different phases of development.
Starting from seed, take many years, easily more than 5 years, during this "nurseryman's phase" of raising bonsai, you learn and use very few bonsai techniques. It is mostly tree nursery horticulture.
Taking nursery stock that may have 5 to 25 years of growth, and bringing it thru the steps to become pre-bonsai ready for it's first real styling, again is a narrow, limited number of bonsai techniques. Different from the techniques for seedlings, but no where near the full repertoire of bonsai techniques. Nursery stock can only teach you so much. Landscape trees harvested from an urban or suburban setting still fall into the nursery stock category. "Urban-yamadori" are still just older nursery stock, they are not the equivalent of wild collected trees.
Yamadori, by which I literally mean trees collected from the mountains. And forest collected trees need a different set of bonsai related skills. Handing century old, wild collected stock is a specialized skill set that is very much a part of bonsai.
Trees that have been grown for bonsai, then go through a series of being styled, then allowed to grow out. Then styled again, then allowed to grow out. This is the transition from pre- bonsai to bonsai. When I use the term bonsai, I mean the tree is ready to display at a local club level show. Here the largest number of bonsai techniques comes in to play. There are many techniques that don't get used until a tree is near show ready..
Then there are exhibition quality trees, these are at the higher level of refinement. Good enough to take ribbons and or prides at the local level and on the way toward becoming a top level tree. There are a series of techniques that get used here that would never be seen used on a less developed tree.
Finally there are "post exhibition quality" trees. Here the trees have matured past the effectiveness of the style they had been trained toward. Here the trees are in need of complete restyling, a new and different vision need to be imposed. This is actually one of the areas of expertise that Kimura was known for.
Now if you, as a beginner want to learn all bonsai has to offer, you need one or more trees in each of these phases of development. You will never learn the advanced techniques if you only start with inexpensive nursery stock. True given enough time, nursery stock can move through all these phases, but it will take decades to get there.
As a beginner, make a 5 year plan. Learn the horticulture. Learn about initial training of nursery stock. Then plan for the purchase of advanced materials by the time you have been in the hobby 5 years. You don't need a bunch of advanced trees, but you should get at least one tree that is near exhibition quality. This will be the tree that teaches you the "real art" of bonsai techniques. You will need to learn display techniques. You will have to learn the refinement techniques. Owning an advanced tree will force you to stretch your skills.
Don't doom yourself to being a perpetual beginner, by sticking with only nursey stock. Branch out get at least one yamadori, at least one more advanced tree, and by watching for club auctions, look for "post exhibition" quality trees. Bonsai hobbyist "of a certain age" may auction or sell off their collection. Be ready to jump on a sale. This is an excellent time to get an advanced tree without spending a fortune.
So diversify your collection. Not with a large number of species, but rather with trees of a range of stages of development. Only this will teach you the "whole Art of Bonsai".
