I'm extremely fond of Viburnum prunifolium--a handsome small tree native to my woods.

Hawthorn (Crataegus crus-galli)has persistent red berries that would look good with the fall color of the maple.American Mountain Ash (Sorbus americana) also has good berries. Sassafras (Sassafras albidum)?

Blue beech (Carpinus caroliniana - Plant Finder)

Is a great suggestion. Like the sassafras it would have good fall color that would look great with the sugar maple in the fall.

Eventually those sugar maples will get huge and shade this tree. You either want a fast growing tree that will give you a show then die before the sugar maples mature, or a shade tolerant underisory tree that can survive in their shade.

For those not familiar with the Ontario climate do you know your USDA zone?

you might want to go to the nursery ..... at fall color time ... to get a chance to pick your true color ... if they cant dig after leaf fall ... the perfect time.. maybe they can have it ready in spring ...

the whole process would be best on a dormant tree ...

ken

Going to the nursery like Ken said ...... and finding an Acer triflorum with yellow color would look great between the sugar maples. I see Leamington, CA. is USDA zone 6b/7a. Whatever you choose you might look for color contrast.

Dax

Leamington is north west shore Lake Erie. Point Pelee. Well south for Canada. I did wonder about maturing size of the maples squeezing the tree out in future. That is 30yrs + of course. I like the sound of Sassafras (this is bigger though, isn't it?), and Blue Beech. A. Triflorum too. My only chance to get up to the property to plant is 2nd week October. Leaves still on, likely. I'd rather not wait til spring.

What size are those sugar maples? Naturally you understand that putting something between them will crowd the three together at some point. Additionally, those two sugar maples at appx. 45' apart would touch someday, probably 30-40 years down the road.

Sassafras is a very large tree like a sugar maple. You're correct. The way I see things is you need a medium fast to fast growing tree to compete with the sugar maples so when all three do come together, the middle tree you select isn't completely crowded from the sugars where it has no where (upright) to go. All this is relevant to the sizes of the sugar maples presently.

There's still quite a bit unknown to all of us.

Dax

That far north is zone 6? That gives you more options. If you can find a native nursery that carries redbud from a local seed source that could work. (The fancy cultivars are way less tolerant of snow then the label suggests). It's vulnerable to wind, but American yellowwood would be gorgeous...flowers and nice yellow foliage. Sourwood is finicky but beautiful.

Sourwood and sassafras are tricky to transplant and might be hard to establish in a place you can't be there regularly the first year.

Ed, z6 is pushing it, but it is wine and peach country. Average temp is 2 to 3 celcius warmer than London Ontario, 100 miles north east, where I am most of the year.

Dax, the maples are new plantings, one still a whip, the other 9ft. I understand your reasoning about the Sassafras now. I'll have to do some more thinking. With the new tree pushed ahead to the southwest I thought it would have its own space a few years longer. That is a good point about all large trees being equal competitors.

cornus kousa

cercis canadensis

hibiscus syriacus (crown raised)

nyssa sylvatica

I kind of disagree with Dax. As I said, lots of understory trees tolerate the shade of older trees. Also, sugar maples grow slow. A fast growing tree can reach a good size while you wait for the maples, and then die. I always advise people to mix fast short lived trees for now with slow growing/long lived/shade tolerant trees for later. (Although I'm usually trying to talk people into adding the long lived trees...you have those already)

Another thought...American persimmon. Good yellow fall color, shade tolerance, tolerates moisture and clay. Maybe a bit big.

Is there a smaller variety of persimmon? I thought Serviceberry as well. When I am down at the property this weekend I'll measure out the distances and have a good look again. Might have to put this spot off till next year. I appreciate everyone's thoughts to help me get a better focus. I've never done landscape composition before.

European persimmon are smaller and more reliable fruit producers but less tolerant of the cold...be careful with them in Canada. There is almost certainly a dwarf variety of American Persimmon...dwarf fruit trees are a thing.

Serviceberry is a fine option. Amelechair Laevis is native to New England and one of the more "tree form" varieties. I find it transplants pretty easily and tolerates neglect in it's first year. (Not all trees do...)

And I agree with Ed that my surmise of the situation may be off. What it's going to come down to is calculated growth rates of all three as we all can see. When you've done your math you'll know what kind of look the end result will become.

As for persimmons which I love btw, their bark is knockout cool and... you should consider only a grafted tree that's female. There is never a need for a male for pollination. All that males do is put unwanted seeds in your fruit. 'Rosseyanka' is a cultivar that's most-excellent for your zone. There isn't a reason for a dwarf persimmon and I can't even say there are dwarfs.

Dax

My understanding was that most American Persimmon varieties need a male for pollination (unlike the European version). And I've always thought male persimmon are an underused option for locations where you want a non-messy tree...

After the maples develop enough the smaller-growing tree is going to look squeezed. And I certainly wouldn't plant anything choice and expensive where it was expected that companion trees were going to overwhelm and spoil it later. Probably better to plant shrubs that will not try to grow up into the realm of the maples, read visually therefore as a third leg. And multiple specimens, of multiple kinds so it isn't just the one thing looking as though being hustled by two thugs: "Come with us, buddy!"

Another thing to consider is adding more sugar maples later, to produce a grove effect with different age classes, as in a wild stand. This would look less stiff than having just two of them by themselves, spaced well apart. Since in grove planting the grove of multiple examples of one kind of tree is the feature - and not the individual trees - the spacing is closer.

Embothrium, I did have a third Sugar so they made a triangle, two to the east closer together, but the one was a crummy specimen I did not have patience with, garbage sent from a marginal nursery, when I wasn't there to inspect delivery -- got planted just to plant it, I pulled the tree out in July of this second season. It was under the canopy of an Elm I don't think is long term. It's partner Elm croaked over the winter.

The grove effect --- how should they go in relation to each other, is 4 better than 3? I mean do you need more than 3. Uneven number 5 or 3 would read more nonsymetrical and natural, yes? I would actually have room for that, if I change the plan. Thinking it over I suspect the space I want for the smaller tree is probably too close to the Maple at the west. I am not good at remembering space/distances and will have to check when I am there.

Once you get past 3 any number even or odd enables a flowing grouping to be made.

I really, really like the way crepe myrtle 'natchez' looks around sugar maples. Serviceberry is a nice tree and should work for the spot, I also think cornus florida would look good. If you want something less common, maybe cornus alterifolia.

>I really, really like the way crepe myrtle 'natchez' looks around sugar maples<

OP states location is in Ontario Canada.

Hi Ed, regarding persimmons... it would be futile to re-write what I already stated so I hope this covers your questions:

No males = no seed in fruit or the rare/very few that may happen on occasion.

A solitary female or multiple females (as many as the eye can count) will produce heavy crops of fruit with zero to very few seeds when no male is within pollination range.

Jerry Lehman of Indiana is the USA expert on persimmons and this information he told to me during a telephone call which I (also) took notes from/during.

Dax

Embothrium, I was well aware that the OP couldn't grow crepes in their climate. Otherwise I would have suggested it in my first comment.

I measured, and the Maples are 55ft apart. Some more space. Doesn't change the scenario I don't think.