Plant classification is mostly based on reproductive qualities, and in that regard, Acer negundo is very much maple like, the flowers are similar, and they produce double samaras like their maple cousins. Another giveaway is that the leaves are opposite on the stems, and not that many tree species do that. They are compound leaves, which is quite unique in the Acer genus. Still, boxelder is indeed a junky tree, but so is the silver maple. Both are pioneer species, so they grow fast in an open area, but don’t do well with competition.