Acer negundo seedling

Warning: this is an invasive species in southern Ontario.

Acer negundo, known in English as "Manitoba maple", "box elder", "boxelder maple", or "maple ash", and in French as érable du Manitoba or érable negundo, is a deciduous tree native to eastern and central United states and southern Alberta, Saskatchewan and Manitoba.

I like finding volunteer tree seedlings in the yard and was disappointed to find out that this was Manitoba maple, which the Invasive Exotic Species Ranking for Southern Ontario (PDF) ranks as a category 1 invasive, "Aggressive invasive exotic species that can dominate a site to exclude all other species and remain dominant on the site indefinitely." At least this one is small enough to be removed fairly easily (I'm assuming).