Black Locust

There’s a line in E.B. White’s book Stuart Little that I love “…orchards so old they have forgotten where the farmhouse is”. I often think of that when I come across abandoned orchards or long untended Lilacs and Roses that mark old farmsteads no longer standing. It came to mind when I saw this stand of Black Locust (Robinia pseudo-acacia) in western PEI recently.

Photo 1: Part of a stand of Black Locust (Robinia pseudo-acacia) on PEI.

Black Locust is native to the Appalachian Mountain region from Pennsylvania south to Alabama and Georgia, with a second, separate native range further west in Missouri, Arkansas, and Oklahoma. It’s been widely planted as an ornamental elsewhere and so isn’t unusual to see as a street tree in urban and suburban areas. It’s less common to see it in rural areas, and while I know of a few sites in central and eastern PEI, this is the first I’ve found further west.

“Found” is a generous term, as I’ve passed this site hundreds of times and never noticed this stand before. In late June the trees were in full bloom and unmistakeable even from highway speed (Photo 1). 

Photo 2: The distinctive flowers of Black Locust.

Black Locust is a member of the Pea Family (Fabaceae) and has distinctive, irregular flowers that will remind you of those on garden peas or beans (Photo 2). Just like those familiar garden plants, Black Locust’s flowers turn to bean-like pods, something not commonly seen on PEI trees!  As is common with this family, Black Locust is able to partner with bacteria to convert atmospheric nitrogen into a form that plants can use, such as ammonia. This not only benefits Black Locust itself but also enriches the soil for other plants around it.

Photo 3: Thorns on the young growth of Black Locust.

In addition to having distinctive flowers and leaves, Black Locust has thorns on its young growth (Photo 3). Despite rumours to the contrary, these thorns are sharp but not venomous; a poke should be no more serious than that from a Hawthorn or Rose. That said, all parts of the tree are toxic if eaten and there’s a very interesting case study of a tree feller whose arm was punctured by a three-inch long Black Locust splinter.

The splinter worked its way out after four days, but six days later the person went to hospital for emergency treatment. Black Locust toxin (robin, a natural toxalbumin) in the wood caused significant cell death necessitating several surgeries, time in ICU, and skin grafts. The person lost most of the use of his hand as a result. That’s a rare accident, and I think the message isn’t ‘be afraid of Black Locust” but rather ‘remove Black Locust splinters promptly’ (and do not consume any part of this tree).

Black Locust spreads by both seed and suckers, forming dense stands which prevent most other plants from growing.  It has the potential to be an invasive species – and is definitely problematic in some areas – but isn’t high on my list of species of concern for PEI. It doesn’t tolerate shade well and so is unlikely to invade natural forests. The places I’ve found it locally are all hemmed in by farmland, roads, and/or development, limiting opportunity for spread.  It is salt-tolerant and not something I’d want to see in our coastal areas, but other than that I don’t expect it to cause too much trouble locally.

Black Locust is an interesting ornamental tree that has escaped cultivation and is now part of PEI untamed!