Friday, April 29, 2016

Black Locust

     Black Locust (Robinia pseudoacacia) is a medium sized tree that you may notice in bloom right about now. Whether due to its profusion of white flowers or their sweet perfume, this tree stands out at this time of year. Sometimes also called Yellow Locust or False Acacia, it rarely gets taller than 80'. It is a short lived (generally under 100 years) member of the Pea Family (Fabaceae). Its Genus name was given in honor of a father and son team, Vespasian Robin and his father Jean, who served as botanists in French King Henry IV's court. Indeed, the oldest living tree in Paris is said to have been planted by Jean Robin in 1601 and is an ancient Black Locust. The Robins helped make locust popular throughout Europe where it was planted extensively. Here in North America, it has now been introduced into every state.
     A pioneer tree, locust grow fast and can take very poor soil conditions. Because it is a legume, enriching the soil due to the nitrogen fixing bacteria that dwell within their roots, it is often planted in strip mined locations to reclaim the land and to prevent erosion. It can bloom in 6-12 years after planting.

Black Locust have compound leaves and paired thorns. Because this is a young sapling, it has larger leaves and thorns than a mature tree.

     This tree has deciduous, compound leaves and paired thorns. The leaves are interesting in that they fold up and droop at night and during very cloudy weather. The flowers consist of hanging racemes of very fragrant, white flowers. These are extremely attractive to some bees. Black locust is considered a major honey plant, and honey made from its flowers is sometimes sold as "acacia honey." The flowers themselves are edible and sweet. In fact, I find them quite enjoyable myself, though I've learned to make sure ants are not sharing my enjoyment of them. In Romania, perfumed jam is supposedly made out of the blooms from introduced trees. 

Black Locust have fragrant, white flowers that hang in racemes, often with yellow centers.

     Pollinated flowers result in 4-14 seeds contained in bean-like pods. Although a pioneer species, this tree does not compete well with other plants. It is one of the trees most affected by the allelopathic chemicals that goldenrods produce and thus are not often found germinating in meadows near where the goldenrods are dominant. Most new growth results from root suckering, with new trees popping up away from the parent tree. These are all clones of the parent plant. Locust do not tolerate shade, so give way to other longer lived trees later on, having in some cases enriched the soil before their demise.

Black Locust seed pod

     Various native tribes made use of Black Locust. The Chickasaw used the roots to treat headaches. The Menominee flavored bitter medicines using locust bark. The Cherokee made the most use of it however. They chewed on it for toothaches and to induce vomiting. They made a bark tea from it and created both bows and blow gun darts from it as well. They found the wood useful for fences and as pegs in log cabins. They even gave it as a tonic to their cows.
     During the Civil War, Confederate Dr. Francis Porcher, charged with finding alternatives for goods no longer available due to Union blockades, listed several uses for Black Locust. He suggested that an antispasmodic syrup could be made from the blooms. He related that the flowers could also cause drowsiness and vomiting in infants, but that they could be fried and eaten. The inner bark could be used to make cordage and other rope. Porcher considered the wood to not only be useful for fence posts, but as wood tree nails in ship building.
     The durability of the wood means that Black Locust has often been used for fence posts and rails. The lumber is very rot resistant even when in contact with wet soil. Abraham Lincoln is said to have split many a locust log. It also burns very hot, but is prone to shooting coals out due to imperfections in the wood.

Locust Borer Beetles are wasp mimics who feed on goldenrod flowers as adults, but bore inside Black Locust as larvae.

     Many of these imperfections in the wood are due to the work of outside organisms, the most prominent of which are the Long Horned Locust Borer Beetles (Megacyllene robiniae) which even bear the treesname as part of theirs. These drill through the wood as grubs, weakening the wood and causing splitting and dropping of limbs. Interestingly, these beetles are Yellow-jacket Wasp mimics who love to feed on pollen as adults. Ironically, their favorite flowers are those from goldenrods, the very plant who exudes chemicals that inhibit the growth of the locust trees. The galleries the grubs make open up the tree to the growth of fungi. The most apparent of these is a bracket fungus, Phellinus rimosus, which are easy to find on older trees. This, along with their intolerance of shade, may limit their age in North America, but they can live for a very longtime when freed from these limitations. The oldest tree in Paris I mentioned earlier is a good example, since the locust borers I believe are not found in France.  

Locust Bracket Fungus is often found on older trees and can result in heart rot in the trunk. 

      Many other creatures make use of Black Locust. In fact, over 72 caterpillar species feed on it. This includes the larvae of a common butterfly, the Silver Spotted-skipper (Epargyreus clarus) which folds the leaves over itself to make its shelter. Although not a favorite, numerous animals feed on the seeds and browse the foliage as well.  

Silver-spotted Skipper caterpillars often fold leaves on the legume plants they feed on, using multiple leaves when they get to big to just fold one over themselves.
Silver-spotted Skipper caterpillars will try and scare you off with their fake eyes if their shelter is disturbed. 

     As a child, I had two large Black Locust growing in my yard. I played under their shade, ate the sweet flowers, and occasionally cursed them when I stepped barefoot onto a fallen thorn. I even fell and was knocked unconscious once when I fell from high in its canopy, resulting in an ambulance ride and much concern to my family. So seeing these trees in bloom every year brings back all sorts of memories from my youth, usually made even more vivid since I can't resist but taste the flowers again every year. 
Enjoying some Black Locust flower buds again this year...


  1. Hi,We are in the process of creating new interpretive signage for the Park Department of Brownsburg, Indiana. We would like your permission to include the image Locust Borer Beetles. If you require any additional information, or have question, please do not hesitate to contact me at

  2. I know this is an older post but thought I would add that Black Locust makes excellent decking. It is really hard, nearly as hard as ipe and lasts for a very long time - well past pressure treated pine or the popular cedar. It's local and can be responsibly harvested. The one drawback is that the lengths of board are generally shorter so you need to make some design changes if covering a large, wide surface. Maintenance is easy and it silvers beautifully.