I would like to plant an oranmental Japaneese maple in a partial sun site at my office. How tall will it get? Is this the right time of the year to plant it?

Best time of year to plant all trees and shrubs is Fall. Spring would be second. I think I'd place Summer last in line, so I'd say hold on to it until late September or middle October- whenever the hot days have cooled significantly.

Most Japanese maples tend to be smaller understory trees so think in terms of 15 feet max, and they grow slowly.

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Shepherd of the Trees
It is not our part to master all the tides of the world, but to do what is in us for the succor of those years wherein we are set, uprooting the evil in the fields we know so that those who live after may have clean earth to till.