Here is what I know about mulberry:
Red mulberry (Morus rubra) is native to North America. White mulberry (Moris alba) is native to Asia. White mulberry is considered an invasive species in North America and hybridizes easily with red mulberry. A study of DNA (by Purdue?) showed that most if not all wild/feral mulberries in USA have some level of hybridization. Black mulberry has much less presence in North America.
The leaves on red and white mulberries are highly variable from juvenile throughout the life of the tree, identification cannot be presumed from the shape or size of the leaves. Because of the hybridization the amount hairs on the leaves is also quite variable from tree to tree. Black, red or white berries are possible on either species.
Mulberry trees are easily layered or rooted from cuttings. Root vigor is comparable to that of elms and trident maples. The trees grow quickly under favorable conditions, I have experienced caliper increase of up to 1" in a growing season on ground growing trees. They back-bud easily but not as excessively as Chinese elm. The leaves reduce somewhat in pot culture. Flowers are inconspicuous, fruit is small and in scale for bonsai. The bark is fairly smooth and does not quickly roughen up to convey the feeling of age. The roots are an interesting orange color (similar to osage orange), I do not know yet if they will bark up to a more reasonable hue on a root-over-rock.
Layering (ground or air) would be my preferred method of collection as wild trees typically have one or two roots about 400 feet long. Layering can be done throughout the growing season but should not be left too long as they can overdo themselves putting out roots and make a mess.
I would not hesitate to recommend a mulberry as a bonsai subject for any level of hobbyist.
