It is against the law to sell a black locust tree in the Commonwealth of Massachusetts. Yet, millions of these “invasive” trees can be seen thriving along highways and in thickets on Cape Cod and of course, at the Arnold Arboretum (left image, 23173*C). Right now, these trees are covered with beautiful chains (racemes) of white flowers (right image, 22838*A) whose form provides clear evidence that they are members of the pea family (Fabaceae). Robinia pseudoacacia is not “technically” a native tree of New England, with its natural northern range limited to Pennsylvania. But, that does not mean that of its own accord, it was not going to eventually become a natural citizen of New England.
20,000 years ago, there were no trees in Massachusetts. After all, it is hard to be a tree with a thick glacial sheet of ice covering the entire region. But, with the retreat of the glaciers, and warming temperatures, all manner of tree species that had been “pushed” south with the onset of glaciation and colder temperatures began migrating north again to reinhabit their old stomping grounds. Some species galloped ahead of others and reached New England in a mere few thousand years, while others migrated at a more leisurely pace. Black locust is one species that took its time, getting only as far north in the Appalachians as Pennsylvania. Given a few thousand more years, who is to say that it would not have arrived on its own and spread throughout New England? Perhaps the human intervention of introducing black locust trees into New England only accelerated the process.
So, enjoy the peak flowering of black locusts at the Arnold Arboretum or along the interstate. They are outlaws worth loving. And don’t take just my word for it. Here is what Charles Sprague Sargent, the founding director of the Arnold Arboretum, wrote in his Silva of North America in 1892: “Robinia pseudoacacia is surpassed in beauty by few American trees. In no other are lightness and grace of foliage combined with such massiveness of trunk and spread of branches. Few trees produce more abundant, beautiful, or fragrant flowers, or afford more pleasing contrasts of color in the light green of the youngest leaves with the darker hues of those of the earlier part of the season, and between the different shades of color of the upper and lower surfaces of the leaflets as they rise and fall with the slightest breath of air.”