It may just look like a volunteer from the larger tree that you were too lazy to pull out. What kind of maple is the big one? Perhaps you could build a little stacked stone circle or u-shaped wall to set it apart from the bigger maple.

You can under plant as long as you make sure it gets adequate water and nutrients and keep the large roots of the big tree at bay.

I don't understand your first paragraph. Was that in response to someone else?

Planting under an existing, mature maple and "keep the large roots at bay?" How would one do that? I can't even picture it, given the fact they're existing.

Water and nutrients aren't an issue.. at this point! :)

The first paragraph was in reference to the estecics of planting a small tree under a similar larger one...nevermind.

Maples have very shallow root systems. Have you ever seen a maple tree planted too close to the sidewalk. As the roots grow larger they will push up the concrete. If a large root is near a smaller plant it could girdle the root system of the smaller tree. If you water the small tree, the larger trees roots will grow very fast in that area to get every last drop of water.

Roots do not grow at random, they grow where conditions are favorable. If you make a favorable location for your Hupps Dwarf, the other tree's root system WILL eventually encroach.

Water and nutrients will most probably be a problem, eventually.

Now that's information I can use. Thank you.

When you said, >>It may just look like a volunteer from the larger tree that you were too lazy to pull out. What kind of maple is the big one? Perhaps you could build a little stacked stone circle or u-shaped wall to set it apart from the bigger maple.I was confused about the volunteer thing, since I'd clearly said "plant," and building a wall or something to set it apart didn't apply, either. That is why I asked if you'd intended that for another thread.

Big maples are atrocious root zone "hogs". If it's anything like the ones I have (had, got rid of several) you will need an axe to get even the smallest planting hole dug.
For ones I needed to retain for shade, I ran a Cat 1 middleplow about 20' out from the trunk and severed the roots. Probably not applicable to your situation - although you could do the same thing with a rented ditch witch - but it does stop the roots from robbing water and nutrients beyond the line. Easy to see in summer where the side near the tree has brown grass and the side past the cut has green grass. Of course you only want to do this to 50% or less of the tree's root zone.
Alternately you can just do what you've done, which is to water whatever you plant under them scrupulously. For me that wasn't really an option because that area of my garden is over 200' away from any hose taps.

I've found that silver maples roots (weeds that they are), can be so thick in beds under them, that you'd think you were digging in a mushroom compost bed with all the mycorrhizae there. Root competition is intense and things are almost always bone dry.

Type of maple may affect that greatly (for your larger tree). I've never seen plantings under mature silver that don't look pretty anemic.

I do wish I knew what type of maples these mature ones are.

Years ago, I had raised beds against the house, maybe 8' from these trees. Those beds were full of maple roots. I still have a 24' x 4' raised bed along the other side of this same tree that doesn't have a root issue. Can't figure that out, but I'm glad!

Ironically, in 12 years, the beds actually under the trees are lush and healthy, without the thick root problem!

They were also planted 20-30 years ago by the original owner, very close to the house. These beds under the trees receive timed waterings which cover my JM collection, hostas and heucheras. So they're close enough to the spigots to receive a steady source.

I'll have to think about this.
Thanks for the insight!