I don't think you have to move right to questioning his competence. I would check first what assumptions each of you is operating under. If you have given him instructions to produce a certain kind of environment in the yard, shade may be an essential part of that and perhaps he can't produce any other source of shade in the short term and is concerned you'll not be happy with the outcome if there isn't enough shade. Or something. He may also be making some assumptions about how much you want to water your plants. I de-shaded the north side of my house with the removal of a tree and it does get sunny as the sun veers around in the afternoon. New trees are just growing in enough to make an impact 4 years later. If there is a chance you will be removing the big tree at some point within 5 years, the pot is a very good idea for another reason - you'll have to get the little maple out of the way of falling tree parts. They do grow well in pots, especially if the pots are big enough. KarinL

I have 1 large Silver maple, 1 large Sugar maple, and used to have 5 large Norway maples in this yard. In my experience, there is a significant difference amongst the 3. The Silver maple is very greedy about water and has large gnarly surface roots that grow aggressively into the gardens that are irrigated. These trees have a high anaerobic tolerance, can tolerate seasonal flooding, and I wouldn't worry in the slightest about piling mulch or soil in the root zone (1 foot of soil is a bit much - on the other hand, the roots might be gleeful to have more soil to grow into). I also regularly hack away at large roots to cut them back from the perennials and septic system, with little apparent effect. It's not easy to grow under these trees, but drought tolerant plants do okay. The Sugar maple has much more well-behaved roots. Only, these trees don't tolerate root disturbance, salt, suffocation, or compaction. I dug out 2 large Burning bush from one side of this tree and within months it suffered die back on the trunk on that side and a strip of bark peeled off. So I only dig gently around this tree or not at all. Norways are near impossible. They have dense shade, allelopathic roots, and they suck the soil dry. Not to mention they are extremely invasive. I had 4 of them cut down. The only thing growing happily in the root zone of the remaining tree is Vinca minor. I love the native maples, but they aren't easy to garden under, and I'm not interested in pampering plants to be able to do so. But I have other places to garden.

Your "landscapers" are sorely misinformed. Pretty much anything can grow under a Japanese maple, as their root system does not have the same aggressive, water-guzzling, nutrient-hogging character that other, native maples display. And a hot, dry summer will require regular periodic irrigation of any JM for it to survive and thrive - these are not drought tolerant trees. My only concern when underplanting a taller growing JM is that you select plants that will not need frequent division to maintain them, as excessive cultivation in the root zone or any significant disturbance can damage the delicate root system, sometimes to fatal results. Selecting something to contrast with the dark red foliage is a good idea, like bboy's suggestion of blue leaved hostas. You could add Japanese painted ferns as well to pick up the coloring of both the hostas and the tree foliage. I like the contrast of gold tones myself, so golden leaved hostas, Japanese forest grass (Hakonechloa 'Aureola' or 'All Gold'), Campanula 'Dickson's Gold', spotted London Pride (Saxifraga x urbium 'Aureopunctata) or Lamium 'Anne Greenaway' or 'Aureum' and autumn fern are all suitable choices. Epimediums are an excellent choice as well, as is any moisture preferring, shade tolerant groundcover or part shade perennial that doesn't require frequent division. FWIW, my JM's are planted in the middle of mixed borders with an assortment of a wide variety of part shade shrubs, perennials/grasses and groundcovers surrounding them. So the notion that "nothing" will grow under them is very misplaced.

The espaliered witch hazel at CUH is impressive. My pruning education (formal and practical) said to tread lightly with this plant family, so I was surprised by what I saw there. My winterhazel confirmed its dislike of pruning by sending up a bunch of water sprouts, tall straight growth through the middle of the plant, when I took off a couple of damaged stems. I imagine the gardeners at CUH are successful because the plants are very large, albeit 2-D, and the pruning has surely been done slowly and carefully. Keeping it in a shrub shape, if that means shortening most of the branches, may offer disappointing results. You may want to consider disanthus a for a nice full shrub shape, although the fall color will be less spectacular in shade. Anything planted under a maple is going to be water-challenged, however. Maybe a native (oceanspray comes to mind) would do well.