Answer: This is a very interesting question. There are some special considerations when planting in a large above ground planter, even quite a large one such as this. There are extra stresses on plants growing there, including somewhat restricted root space, the artificial nature of the soil or substrate, the enhanced drainage due to it being raised, and wider than normal soil temperature fluctuations.
Plant selection is somewhat limited in that you must use plants that can handle those special considerations in addition to being suited to the climate (winter cold hardiness, summer heat and humidity) and the specific site (such as wind exposure, amount of sun and at what time of day, heat reflecting off a building or pavement, overall space available, soil type and moisture levels, and so on.)
Based on your zip code you are gardening in USDA winter hardiness zone 5A or the coldest part of zone 5. Depending on your microclimate this could be as cold as zone 4. Since the planter is raised, I would suggest you work with plants considered hardy to zone 4 or colder.
Unfortunately, this is colder than Japanese maples can withstand -- only the hardiest are considered hardy into zone 5 with 5A being unreliably so.
Additionally, these trees require an evenly moist yet well drained, organic and humusy soil that is acidic. The raised planter will most likely dry out faster than would be suited to this tree. Overall, I would not recommend that type of somewhat delicate tree for this application.
Instead, you might consider other very small trees such as a small cultivar of crabapple perhaps with a weeping form, a deciduous magnolia such as Magnolia x 'Ann' with lovely branch structure and blooms, or perhaps the small maple Acer ginnala if you prefer the look of a small maple. These are all tolerant of a wide range of growing conditions and suited to use in planters such as yours.
Your local professional nursery staff and/or county extension may have additional (or different) suggestions based on a more detailed understanding of the growing conditions in the planter and your overall design goals. Good luck with your planting!
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