This is the kind of thing where the site has to be looked at in person and assessed. Too many variables to make arbitrary determinations from afar.

To throw out a ballpark observation for what it's worth, I've walked through a lot of old cemeteries where Japanese Maples had been planted and seemed to be thriving in some pretty heavy shade. So I would be inclined to say go for it. In fact, if there are any sizeable old cemeteries in your area (especially the late 19th century garden era type)I would suggest a walk through - Japanese Maples seem to be a popular planting. Take a digital camera along to document your observations. If nothing else you will have an interesting walk and are sure to see some notable specimens.

Actually, it's not the shade I'm concerned about. A planting hole within the larger tree's dripline will have roots in the way - (which can be chopped out I guess) - and the larger trees will compete for moisture in the soil as well. But looking at forests, large trees are very close together so I guess it could work out. I haven't seen any Japanese maples anywhere here except Bloodgoods for the most part - but thanx for the info - I'll keep alert for them!

It may sound a bit morbid, but I think cemeteries are a good classroom for observing how exotics behave under different conditions - like shading. You wont see these species in our native forests, and most residential gardens aren't large and shaded enough to provide this information either. Likewise cemeteries tend to have a lot of exotics in them which basically get left alone, sometimes completely neglected, for long periods of time. Root disturbance, storm damage, suckering, seedling production, it's all recorded here over the decades.

i am planting them that way in the woods of my lot. actually, i would likely not do this with "store bought" expensive cultivars but have done it with seedlings that i did not particularly want or weren't special enough to warrant a prominent spot in the garden. i have several that are about 3.5' tall and are planted within 3' of the trunks of mature oaks. i have been removing the smaller oaks with an open, high-shade area canopy in mind, underplanted with japanese maples, camellias, native and evergreen azaleas and other shade tolerant plants. the maples that are planted close to the mature trees are doing just fine. my method is based on developing paths through the woods and the areas next to the trees will not interfere with this. there seem to be areas between the major root buttresses where the fine roots are absent and the soil is deep enough to do this if you probe the areas with your shovel.
as i said, these are sort of dispensable subjects but have surprised me with good growth and the ability to compete with the larger trees.

Well, that's quite interesting, Jeff! I don't know about oak roots, but the large trees in my yard are a 40' and a 25' sugar & silver maple that have several large surface roots. The j.m. I am planting are cheapo seedlings of bene maiko, oridono nishiki, and shindeshojo, so if they croaked -no big deal. I could plant them near the edge of the tree canopy (8-10 feet away from the trunk). I don't really want these trees to get very large so maybe that would work out OK. Thanks for the info!!

The problem, as everybody knows, is root competititon. I have a rather large collection (>150) planted as understory trees in a mixed oak (Q. robur)/hornbeam (Carpinus betulus) forest. The trees are tall and between 20 to 100 years old.
The Japanese maples can be planted as close as desired to the oaks, they thrive fine even when planted relatively young (2-3 years old grafts).
On the other hand, it is basically impossible to plant any maples within the drip line of the hornbeanms, and even outside only well established maples can live with the ferocious competition of the hornbeam roots. As a matter of fact I have removed many of the hornbeams to make room for additional maples.

Regards,

Gomero

Thanx for the reply. I did some research and found out oaks don't have surface roots like my sugar and silver maple so they can be underplanted. I have a feeling I won't be able to plant my j.m. anywhere under the canopy of my larger maples. But perhaps somewhat further out will be OK with dwarves like Beni Maiko where they will still get shade in the afternoon. I wish I could wiggle my nose like Samantha and turn my maples into oaks!!!!! The roots are really making a mess of the lawn, too.

There is a nursery near my house that has a weeping JM maple planted underneath a taller JM and it gorgeous!

Crapapooey?..ha ha,I laughed out loud...luv it :)