Yes, that is always a concern of mine. Some of the neighbors and others around town have beautiful full grown maples in their yards which to my novice eye appear to be relatively old(ish) and healthy, so I'm optimistic. I don't doubt modifications need be made to deal either the brutal mid-summer sun here.
I will start learning more about how to identify grafts. I will also take an even closer look at the nursery for smaller maples as another poster graciously suggested. I may seem odd but I might just go to someone working there and ask do they have smaller, crappier versions of these trees somewhere in the nursery.
The short answer to the question of chopping them is it's done all the time. That's how we make bonsai for the most part. Chopping a trunk typically produces a large flush of growth below the chop, Canopies on newly bought trees are a distraction, they don't really matter, neither does the original height of the tree for the most part unless it offers something like decent taper or considerable visual "movement" that can be incorporated into a final design. Those characteristics usually don't exist in stock landscape saplings though. What you're looking for is a decent root spread at the surface and the initial six to 12 inches of lower trunk. Everything else is going to be replaced. Don't get hung up on existing canopies. Look at the lower trunk and initial root spread.
That said, in Texas Japanese Maple as bonsai can be problematic--heat, sun and dryness are the issues. While there may be landscape JMs in Dallas, it depends on WHAT VARIETY they are and WHERE THEY'RE planted. My aunt and uncle have an old JM in the their front yard, but it's in open shade from tall bull pines all day and has been since they planted it. Full Texas sun in August can fry many varieties of JM. Heck some varieties here in Va. get cooked by August. You can find dozens of them in the sale areas comes August because they look awful with dried out leaves. People think they're dead, so the nursery puts them on sale to get rid of them so they won't have to overwinter them. They're not dead, though. Threadleaf and laceleaf cultivars, since they have extremely thin leaves, are notorious for burning up in the summer sun.
It is critical to give JMs at least afternoon shade in hotter climates. Unlike big dominant forest trees like oak, beech, etc. JMs are mostly understory trees, used to growing in open shade in their original habitat in Japan. As bonsai all that becomes even more important, since pots can get very hot in summer and roots suffer, leaves turn brown and crispy.
Having a heat tolerant cultivar can help things along. There are several --here's a
list.
As for the graft, those are usually two to four inches above the root crown in most landscape maples. Sometimes they are quite noticeable and produce odd-looking swelling, a sudden "neck up" in trunk diameter a few inches above the soil line is a typical give-away, as are differing bark color/texture. Those differences get worse over time, which is not really a big deal for landscape trees, but since bonsai relies on the lower trunk as a visual anchor, it can make a huge difference.
Bottom line, if you can't determine the exact variety, get one and see what you can do and how it performs in your yard. It's a leap of faith you sometimes have to make. Experience is the best teacher. I wouldn't do much of anything to it for a year except keep it alive and healthy. Next year you might start working on it. BTW, if it's not labeled, you're probably not going to find out exactly what it is, unless the nursery owner is knowledgeable about what they're selling and can recall what they are.
