Black locust (robinia pseudoacacia) is a tree that is both hated and loved. On one side of the fence you will find people who say that black locust is a horrible invasive species. On the other side of the fence, folks will say black locust is a miracle tree with endless uses and ecological services. In the middle of the fence, you will find black locust itself, as a post holding up the fence.
Originally black locust is native to central Appalachia and the Ozark mountains. It has spread from these locations, and is now found naturalized in every continental state, in several Canadian provinces, and in parts of Europe. It is most often found growing in disturbed sites, old fields, vacant lots, and roadsides.
Black locust is an incredibly fast growing tree. It is able to form relationships in the soil with certain bacteria that allows it to fix nitrogen right out of the atmosphere. This ability allows locust to grow in very poor soils, so long as they are out in the sunshine and the ground is not water logged.
I have seen black locusts grow 6 feet in their first year of life (though 2-3 feet is more typical for starting from seed). Established locusts cut down, can put on as much as 10 feet of re-growth the following year. Within a 20 year span, black locusts are often big enough for small diameter lumber. This is a tree that doesn’t take a lot of time to grow.
This is also a tree that will spread and form large colonies. Rarely do we see a single black locust trunk by itself. They send out runners and sprout up endlessly until they reach shade or a barrier (road, water, lawn, etc.). Many black locust colonies are several acres large.
Black locusts will stampede out into a field or old gravel pit, or anywhere that things have been opened up for them. They are a pioneer species that will not become established in a forest.
Locust casts a very light shade. The leaves are made up of small round leaflets that allow a tremendous amount of light to pass through. The shade created by black locusts is so weak that undergrowth is always rampant underneath them. Most stands of black locust are tangles of honeysuckle and multiflora rose. Where exotic shrubs do not dominate the understory, hardwood tree seedlings find an excellent place to become established. The light shade offers protection, while the locust trees improve the soil through their nitrogen fixation and easily compostable leaf litter.
Black locusts are a short lived tree. Because of their shallow root system, they typically start falling over by the time they reach 60 plus years old. By this time, an abundance of hardwood seedlings have become established in the understory and will be ready to take over.
Seed production of black locusts begins early and can be heavy. The trees produce pea shaped pods containing a row of small hard seeds. These edible seeds, with their hard seed coat, can remain dormant in the soil for decades. Perhaps, they are waiting for the next forest disturbance to sprout again. The seedpods flutter off in the wind, but they do not travel very far at all.
Strings of white and lavender pea like flowers hang over the trees as if they are covered in robes of blossoms in late spring. Stands of black locust can be seen from very far away during this time of year. The flowers are edible and delicious, I think they taste just like peas. They are a special treat for only about a week every year.
Honeybees as well as many other pollinating insects will flock to these fragrant blossoms in great numbers. Black locust is considered an important nectar flow by most beekeepers.
This is where black locust really stands out and makes a name for itself. Despite it’s rapid growth rate, black locust produces a very dense hardwood. Black locust is an excellent firewood, it burns hotter than oak or maple. It can be cut over and over because of it’s ability to grow back from the stump. Many people have been experimenting with putting black locust on coppice rotations for fuel wood, in which the trees are harvested every seven years.
Locust is not just an excellent wood for burning, it is also incredibly rot resistant. It will easily outlast white oak, and triple the lifespan of pressure treated lumber. Pressure treated lumber (p.t.) is wood that is infused with chemicals at high pressure to keep it from rotting. The resultant wood is used for picnic tables, playgrounds, fence posts, and decks. P.T. lumber is considered highly toxic and has been linked to cancers and environmental pollution.
Black locust is a natural superior alternative to p.t. lumber. Every outbuilding on my farm (including the porch) is built with black locust posts for footers.
The wood is not only strong enough to use for vehicle bridges (as is done in some parts of Europe), but it is also a beautiful hardwood with a dark orange-brown color. It has found it’s way into furniture, cutting boards, and musical instruments.
It is important to note that it is the inner heartwood of black locust that is rot resistant, not the outer ring of white sapwood.
Black locust can be started either from seed or from root cuttings.
The seeds have a very tough coat. This allows them to remain dormant in the soil for several years until conditions are ripe for growth (usually after a massive disturbance). This coat must be weakened for the seeds to sprout. You can either abrade each seed with a file or use a hot water treatment. Bring a pot of water to a boil and then take it off the heat. Drop the seeds into the hot water and let them soak for 12-24 hours.
Growing locust from root cuttings is used to propagate superior clones. There are breeding programs in the USDA, in Hungary, and among individuals to select outstanding timber type locusts. Typical wild black locusts will often have twisty trunks. People have been searching for and breeding trees with arrow straight growth. To clone these trees, root cuttings are the preferred method. Start at the base of the tree, look for a root flare, and carefully follow it until roots as thick as a thumb are found. These roots can be cut into 2” sections and planted in pots or in a nursery bed. Root cuttings are best taken when the trees are dormant.
Is Black Locust Native or Invasive?
If the definition of invasive is a plant that is able to naturalize outside of it’s native range then, yes, black locust is certainly invasive. However, if we look at the ecological effects of black locust we see a different story.
For the most part, invasive plants have negative effects on the environment for two reasons. They crowd out native species, thereby limiting diversity, and they are not fed on by native insects, creating biological dead zones. Black locust fits into neither of these roles.
It does not crowd out other native trees the way Norway maple or Tree of Heaven does. In fact, black locust actually improves conditions for native hardwoods to grow in a similar fashion to another pioneer species, quaking aspen.
Black locust leaves are fed upon by a lot of insects, the flowers are used by many native pollinators, and even the wood is bored into by the native black locust borer.
Here in upstate NY, black locust has not traveled that far to find a home. Other species that are regularly considered native here from a similar distance include Eastern redbud, Carolina silver bell, pawpaw, persimmon, vernal witch hazel, fringe tree, and sourwood. Redbuds escape cultivation almost everywhere they are planted in central NY, but they have not been received the same negative attention that black locust has.
Rather than simply looking at the native range of a plant to determine if it is beneficial or harmful, I prefer to look at the ecological impact of a species. From what I have seen, black locust is a wonderful tree able to grow in the most abused landscapes, such as abandoned gravel pits and strip mines. It is both useful to wildlife, native plant communities, and people.
If we are to pursue ecological solutions to the problems we face, then black locust can be a key player. It is an outstanding soil improver, biomass producer, source of nectar, and high quality, renewable timber. Black locust belongs in the hedgerow of every farm that uses fence posts or beams. It can be cut again and again. Black locust will never complain about abuse either to itself or to the land base, it will always respond with rapid growth and curtains of white blossoms. We can label black locust as an invasive plant, or we can recognize it as an ally on our path to a healthier world.