The Honey Locust and the Black Locust are both considered invasive species of trees. The Honey locust, or Gleditsia triacanthos, is native to central North America, while the Black Locust, or Robinia pseudoacacia, is from a few small regions in the US. Both the Honey Locust and the Black Locust are members of the legume family, or Fabaceae, which also includes peas and beans.
The Honey Locust has sweet, nutritious seedpods that wildlife and livestock love. The pods of the Black Locust are highly toxic. The Black Locust puts out large, showy scented flowers while those of the Honey Locust are small and insignificant. The pods and thorns of the Honey Locust are longer.
Both are deciduous trees that adapt easily to different environments and now occur in many places around the world. Another name for the Black Locust is the “false acacia”, but it is not related to true acacias, which belong to the Mimosa family. They can also be distinguished from acacias because the latter occurs only in subtropical and tropical areas, while the Black Locust is found at higher and cooler altitudes.
The Difference In Toxicity Between The Black Locust and Honey Locust
The pods of the Honey Locust are highly sought after by wildlife and domestic livestock as the pulp of the legume is sweet tasting. It was used in traditional medicine and for food and tea by the indigenous people of North America. Native Americans also used the dried pulp from Honey Locust pods as a sweetener.
White-tailed deer, hogs, opossums, raccoons, rabbits, and hogs love the seed pods, and so do goats, sheep, and cattle. Browsers and grazers also like the tender spring shoots and the bark of young trees.
By contrast, the pulp of the mature pods of the Black Locust are toxic to animals and people.
All parts of the Black Locust are poisonous, but the bark and seeds contain the highest concentrations of the main toxin called Robinin. Its properties are similar to ricin and abrin and cause several alarming symptoms when ingested. These include –
- Muscle weakness and horses who have eaten it may get laminitis
- Rapid breathing
- Dilated pupils
- Colic and abdominal pain
- Constipation and the diarrhea
Horses are sometimes attracted to the Black Locust’s shoots and bark, but it is terrible for them. Consuming even 0.04% of their body weight in this plant can be fatal. If you see wildlife eating the pods and other parts of the tree, it is most probably a Honey Locust, not a Black Locust. Although it rarely causes death in humans, it can take quite a while to recover from Black Locust poisoning.
It is safe to plant Honey Locust trees near livestock enclosures and grazing areas, but Black Locust trees should never be located in these areas.
Invasiveness of Honey Locust v Black Locust
The Black Locust originates from Illinois, but it is considered an invasive species in many other parts of North America, such as the Midwest, New England, and northern California. It converts grassland to a forest and has been outlawed in Massachusetts. In Australia and South Africa, it is regarded as a weed.
The Black Locust proliferates profusely by self-seeding and sending out suckers. Thanks to its popularity as an ornamental tree in Europe and other places, it is now the most widespread American tree in the world.
The Honey Locust is also considered invasive in many regions, and the same control methods are used for both locust trees. Removing the suckers, cutting the stems and branches back during the growing season, and spreading hay around them are recommended ways to curb them. Burning and bulldozing have also been used to try and keep Locust trees under control.
The Black Locust is more invasive than the Honey Locust, although they can both be problem trees that need strict management. If the Honey Locust trunks are cut off, they will sprout from the roots and the stump, which compounds the problem.
Black Locust trees fix the nitrogen in the soil and grow in dense colonies that deprive native plants of sunlight and nutrients. According to some studies, the trees can even release their toxins into the surrounding soil to suppress the growth of native plants. They prefer dry, well-drained soils, disturbed places, and sunny spaces and are intolerant of shade.
Bulldozing and cutting of Black Locust sprouts causes the formation of new growth, and the trees are tough to eliminate once they have taken root.
Honey Locust v Black Locust Pods
The pods of both trees have a thin, smooth, shiny appearance but those of the Honey Locust are much larger. They grow to between twelve and eighteen inches long, while those of the Black Locust are only between two and four inches long.
The pods of the Black Locust are flat and pea-like and typically hold four to eight seeds that are much smaller than those of the Honey Locust. The Honey Locust seed pods usually have between twelve and fourteen seeds and start to curl and spiral as they age.
Color-wise, the Honey Locust pods start out as lime green and turn reddish-brown in the Fall. The pods of the Black Locust are dark brown.
Honey Locust v Black Locust: Wood
The wood of the Black Locust is resistant to many pests and diseases because of its toxicity and so is rated as very durable by woodworkers. It is therefore popular for fence posts, flooring, boat building, and furniture. The color ranges from darkish brown to pale, greenish-yellow and can sometimes be confused with the wood of the Honey Locust.
However, Black Locust wood is a bit harder and heavier than Honey Locust wood and has more of a greenish-yellow hue compared to the warm red or orange tints of the Honey Locust. The latter’s heartwood is medium or light reddish-brown, while the sapwood is pale yellow. Black Locust wood can cause skin and eye irritation, while that of the Honey Locust does not.
The pores of Black Locust wood are full of tyloses which are outgrowths on the xylem vessels of the heartwood. Honey locust pores don’t have these. When the wood of the Black Locust is freshly cut, it smells foul, but the odor disappears when it ages.
Flowers Of The Honey Locust v The Black Locust
Pollinating insects love the flowers of the Honey Locust, which are strongly scented. The cream-colored flowers appear in clusters at the base of the leaf axils in late spring. The Honey Locust’s flowers are tiny compared to those of the Black Locust and not showy at all, while the Black Locust flowers put on a spectacular show.
The flowers of the Black Locust occur in large clusters and have an intense fragrance resembling that of orange blossom. They are white and around two to two-and-a-half centimeters long. Depending on the location, they appear in late April to early June. The upper petal has a yellow spot. Honey bees are drawn to the flowers of the Black Locust, but the honey yield may vary significantly from one year to another.
Leaves Of The Honey Locust v The Black Locust
The leaves of the Honey Locust become full in the late spring, earlier than those of the Black Locust, which continues to have bare branches for a few more weeks. Honey Locust leaves are bright green and small when new and gradually fade to yellow. By contrast, the leaves of the Black Locust are noticeably larger, oval-shaped, and green with a blueish tinge.
The Black Locust leaves are simple and compound, while those of the Honey Locust are feathery and pinnately compound. The Black Locust has rounded leaves that alternate along the stems. They fold closed in wet weather and at night, and the leaflets are much broader than the Honey Locust’s.
The Honey Locust’s leaves have fine-toothed margins with dark green upper surfaces and dull yellow-green undersides. The Black Locust’s leaves are a darker green than the leaves of the Honey Locust. Unlike Black Locusts, Honey Locust leaves do not have a leaflet at the tip of the leaf stem.
Honey Locust v Black Locust: Bark
The trees are easily distinguished just by looking at the bark. The Honey Locust has a lot of sharp, four-inch thorns around the base of the leaves and branches, while the Black Locust has much fewer, shorter spines, mainly at the base. Black Locust branches higher up in the tree rarely have spines.
The thorns of the Honey Locust start out green and soft but turn red as they harden and then fade to an ash-gray, while the spines of the Black Locust are dark purple.
The Black Locust’s bark feels slightly hairy and turns a dark greyish brown with multiple ridges and furrows along its length. The bark sometimes has a criss-cross appearance where the ridges intersect, causing diamond shapes. The dark-colored bark often has a reddish-orange tinge, and young trees may have white down that disappears as they mature.
The thorns of the Black Locust can be nasty because they are poisonous and grow up to two inches long. Although being pricked doesn’t result in poisoning, if the bark is ingested, it can cause severe stomach pain and even death. The thorns are low enough to poke a person in the eye and can also inflict painful scratches. If a horse consumes just half a pound of bark, it can be fatal.
The bark of the Honey Locust is reddish-brown or dark gray on older trees and is divided into thin, tight scales. Unlike the Honey Locust’s bark, that of the Black Locust is not thorny and sharp.
Honey Locust v Black Locust: Height and Growth Habits
Black Locusts are extremely fast-growing trees that can get to between fifty and one hundred feet tall. They have irregular, contorted branches and a narrow crown. Black Locusts have been found to attract crows which are a nuisance in their own right, scaring other birds away, eating their babies, and destroying vegetable gardens.
Honey Locusts are also fast-growers and can get to an age of one hundred to one hundred and fifty years. They like hot sunny places and are drought and cold resistant. The Honey Locust can grow to a height of between fifty and seventy feet and provides excellent shade in summer. It has an upright arching habit, shaped like an inverted vase.
The Black Locust is typically hardier than the Honey Locust and can grow in less hospitable environments. It is endemic to the southeast United States, while the Honey Locust is native from Pennsylvania to Nebraska. However, they are so widespread that these distinctions are hardly relevant.
The leaves of the Honey Locust resemble those of ferns, unlike the leaves of the Black Locust, and the Black Locust usually grows a bit taller and narrower than the Honey Locust, which has a spreading habit. In Nebraska, Honey Locusts can be more than sixty feet wide. The Honey Locust prefers slightly acidic soils, and they are popular in landscaping.
The Black Locust is a tall upright tree with a narrow crown that becomes scraggly as it ages. Its canopy can grow to around twenty feet wide. In unusual circumstances, it has been known to grow up to one hundred and seventy-one feet tall.
Honey locusts can be distinguished from Black Locusts by their longer, widespread thorns and longer seedpods. The bark of the two trees is also quite different in color and shape, and the Black Locust’s flowers are large, showy white clusters, while those of the Honey Locust are creamy and insignificant. Honey locusts are sweet tasting and attract wildlife and livestock, while all parts of the Black Locust are poisonous to humans and animals.
Both Honey Locust and Black Locust trees can serve beneficial purposes in permaculture design. However, much thought and care needs to be put into the planning of the location of these trees.
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