Wednesday, January 1, 2014

Black Locust Paradox

Black Locust
Robinia pseudoacacia

A broad paradox exists around this tree; reviled by some as invasive and toxic, others clamor over its beauty and wood. Let’s examine the pros and cons surrounding Black Locust.

Black Locust, Robinia pseudoacacia is of the pea family Fabaceae. its native to the southeastern United States growing in groves through the central Appalachians and Ozarks. Scorned for its invasive behavior it has been planted for reforestation and naturalized throughout temperate North America. In my region Black Locust is expanding its range up the Mississippi valley throughout the Eastern Deciduous Forest ecosystem. In Europe, southern Africa and Asia it’s planted on boulevards for its ornamental beauty and reforested for timber where native pests are non-existent.

Black Locust is a large slender tree with narrow crown and thick, deeply furrowed gray bark. The leaves are long, pinnate with 9–19 oval leaflets, Each leaf usually has a pair of short spines at the base. At night leaflets fold up and droop.

The fragrant flowers are white with yellow-green signal. Borne in pendulous racemes, they are edible. The seed pod is a legume, producing 2–10 seeds which are shed in the Fall the empty pods can persist through the winter.

Black Locust prefers sandy or rocky neutral soil, most often found colonizing old fields, forest borders and openings. Black Locust does not compete well with other trees and does not tolerate shade, so it often gets crowded out of thick forests. It grows very fast, but does not live long due to diseases and insects. It rarely lives to be 100 years old in North America. Black Locust can survive drought, salt and poor soil.



Robinia pseudoacacia an invasive species in some areas has a habit of freely suckering from roots near the surface. This is especially aggravated by cutting. It has been known to crowd out native species.

A number of insects and diseases can severely damage Black Locust.The value of Black Locust has been practically destroyed in nearly all parts of the United States by Locust Borers which riddle the trunk and branches. Heart Rot Fungi enters through the tunnels created by borers. Attacks by Locust Leafminer occur almost yearly. Attacks by the Locust Twig Borer can also play havoc with young saplings and seedlings. Young trees grow vigorously for a number of years, until overtaken, soon becoming stunted and diseased, and rarely live long enough to attain great commercial value. They are best harvested early for posts or timbers.

The bark and leaves are toxic, when consumed by domestic animals they generally require immediate veterinary care.

Some woodworkers complain the wood is hard to work due to it density.


It’s a major honey tree the source of monofloral honey from France were it’s widely planted as a boulevard tree since it tolerates air pollution.

Black locust have nitrogen-fixing bacteria on its root system; for this reason it can grow on poor soils and is an early colonizer of disturbed areas.

The wood is extremely hard, resistant to rot and extremely durable. It is highly valued as firewood for wood-burning stoves; it burns slowly, with little visible flame or smoke when dry. Moisture in logs can be converted to steam during heating, which cannot escape through dense tissues so it can suddenly pop and sparks fly all over!

The wood is used for boat building, furniture, flooring, decks, landscape timbers and fence posts. Posts made from locust are good for grapevine supports in vineyards. Locust wood lasts longer than pressure treated wood, lasting 70 years in ground without painting or staining. The wood has the grain of oak, golden with a glow that shifts in sunlight. Outdoors the sun bleaches the wood gray.

Seeds and the young pods of the black locust can be edible when cooked. Robinia pseudoacacia flowers are eaten coated in batter and fried in oil.

Black Locust is the host plant for Silver-spotted Skippers and Clouded Sulphur butterflies.


Black locust is a highly variable species. Many cultural varieties have been recognized, especially in Europe. 

Common ornamental cultivars found in American nurseries are: 'Frisia', a selection with bright yellow-green leaves, ‘Purple Robe’ has lavender pink flowers, ‘Erecta’ has upright form.

I find this paradoxical tree a source of great beauty each spring. Blooming groves accent the Mississippi River valley, a non-native amongst the eastern broadleaf forests.