Illinois Natural History Survey - University of Illinois

VEGETATION MANAGEMENT GUIDELINE
Black Locust (Robinia pseudoacacia L.)

blocust.gif


SPECIES CHARACTER

Description

Black locust has ladder-like (pinnately compound) leaves that have an odd number of leaflets, with 1 leaflet at the tip. A pair of short, sharp thorns occur where the leaf is attached to the strong, zigzag stem. The alternate branches lack a terminal bud. The fragrant, drooping, white, pea-like flowers occur in clusters that appear in May and June and develop into smooth fruit pods (legumes) up to 4 inches (10.2 cm) long, containing 4-8 seeds. Black locust is a fast growing tree that attains heights over 100 feet. Seedlings and sprouts exhibit rapid growth and heavy thorns that occur in pairs. The seeds are toxic and children have been reportedly poisoned from chewing the licorice-like roots and inner bark.

Similar Species

This member of the legume family (Fabaceae) is distinguished by its pinnately compound leaves with up to 21 oval, smooth-edged leaflets, together with the pairs of spines where the leaf is attached to the stem. Black locust should be accurately identified before attempting any control measures. If identification of the species is in doubt, the plant's identity should be confirmed by a knowledgeable individual and/or by consulting appropriate books.

Distribution

The natural distribution of black locust originally centered on the lower Appalachian Mountain slopes of the southeastern United States with outliers north along the slopes and forest margins of southern Illinois, Indiana, and Missouri. Planted extensively for its nitrogen-fixing capability and hardwood qualities, black locust has been reported to be the most widely planted North American tree. Also, it is planted extensively to provide nectar for honeybees and to provide wooden fence posts. Due to successful reproduction by root suckering, black locust (including many cultivated forms) has become naturalized throughout much of the New and Old Worlds. In Illinois, it has been recorded from nearly every county.

Habitat

This tree occurs in a variety of disturbed habitats including pastures, degraded woods, thickets, old fields, roadsides and other rights-of-way. It has become naturalized in upland forests, prairies, and savannas.

Life History

Black locust grows best in humid climates, although it has been introduced in many parts of the world where the climate is much drier. Black locust is a prolific seed-producer but seedlings are not common; few seeds germinate because of the impermeable seed coat. Most natural reproduction is vegetative by means of root suckering and stump sprouting. Root suckers arise spontaneously from the extensive root system of trees as young as 4-5 years old. Productivity of root suckers increases in full sun, in open areas, and in sandy loamy soils. They are interconnected by fibrous roots to form groves of trees with oldest plants in the center and youngest on the periphery.

Effects Upon Natural Areas

In Illinois, this aggressive plant poses a potential threat to all upland natural areas and is an especially serious management problem on hill prairies, sand prairies, and savannas.


CONTROL RECOMMENDATIONS

Black locust is difficult to control due to its rapid growth and clonal spread. Mowing and burning largely have proven only temporarily effective due to the tree's ability to spread vegetatively. As a result, management has concentrated on chemical control with variable success. Whatever control measure is adopted, a follow-up treatment is usually necessary.

Cutting

Spread of black locust can be hindered by repeated cutting during the growing season. All stems should be cut, and new stems that appear subsequently should also be removed in the same growing season. This treatment will probably need to be repeated for several years to achieve adequate control.

Herbicides

Krenite (a formulation of fosamine ammonium) is a non-volatile, contact, brush herbicide, applied as a spray to leaves usually during the 2-month period before fall coloration. Krenite should be applied only in July-September. In northern Illinois, Krenite should be applied before September 15, and is most effective when applied in August. Thorough coverage with soft water carrier is required and a nonionic surfactant will improve results. A 1% solution applied as a foliar spray is effective. Krenite inhibits bud expansion in the spring, and control effects are not seen until the following spring. Slight regrowth may occur the following season but saplings will die during summer. Follow label recommendations to obtain best results; minimize drift. Care should be taken to avoid contacting nontarget species.

Garlon 3A (a formulation of triclopyr) is a selective translocated herbicide that can be applied as a foliar or cut-surface treatment. Cut-surface treatment provides high level of control of tree root systems, especially for suckering species such as black locust. Cut-surface application can be made during any season of the year, but application during the dormant season reduces the potential for drift injury. Undiluted or diluted Garlon 3A at a rate of 50% water can either be sprayed on the cut surface using a hand sprayer or else wiped on the cut surface using a sponge applicator (sponge-type paint applicators can be used). Either a stump or girdle can be used for the cut surface. Girdles around the stem can be made quickly, using a chainsaw. Application should be within a few hours of cutting, adhering closely to label precautions and directions.

Basal bark treatment with Garlon 4 can be effective, although resprouting has occurred in at least one instance with this treatment. Two to 2 1/2 oz. of Garlon 4 is added to one gallon of diesel fuel. Spray this mixture, using a hand sprayer, to the basal portion of the black locust trunk. Spray to a height of 12-15 inches (30.5-38.1 cm). A thorough spraying that includes spraying until run-off at the ground line is noticed is necessary to hinder resprouting. This treatment should not be used in high quality natural areas because the diesel fuel may kill vegetation around the tree.

Use of triclopyr is best done in the dormant season to lessen damage to nontarget species. Great care should be exercised to avoid getting any of the mixtures on the ground near the target plant since some nontarget species may be harmed. Avoid using triclopyr if rain is forecast for the following 1-4 days; otherwise runoff will harm nontarget species.

Glyphosate (trade name Roundup) can be foliar-sprayed on black locust leaves as a control. For good control, all leaves on all shoots should be treated. Roundup should be applied by hand sprayer at a 1/2 to 1 1/2% solution (0.6 to 2 oz. of Roundup/gallon of clean water). Spray coverage should be uniform and complete. Do not spray so heavily that herbicide drips off the target species.

Black locust stems can be cut at the base with brushcutters, chainsaws or hand tools, followed by treating the stump with a 20% solution of Roundup. While the Roundup label recommends a 50-100% concentration of herbicide for stump treatment, a 20% concentration has proven effective.

The herbicide should be applied either by spraying individual stumps using a hand held sprayer or else by wiping each stump with a sponge applicator. Treatment should occur immediately after cutting for best results. Application in late summer, early fall, or the dormant season has proven effective.

Glyphosate is a nonselective herbicide, so care should be taken to not let it come in contact with nontarget species. Foliar spray of glyphosate should not be used in high quality areas because of problems with spraying nontarget species.

In general, foliar spray application of herbicides should not be used in high-quality areas because of potential damage to nontarget plants. Herbicide application to cut stumps or cut surfaces is preferred in high-quality natural areas because this minimizes damage to nontarget plants.

Any herbicide should be applied while backing away from the treated area to avoid walking through the wet herbicide. By law, herbicides only may be applied according to label directions and by licensed herbicide applicators or operators when working on public properties. As mentioned earlier, follow-up treatments are usually necessary because of black locust's prolific sprouting and rapid growth.


FAILED OR INEFFECTIVE PRACTICES

Tordon RTU (picloram) is a premixed general use herbicide labeled for cut-surface application only. This herbicide kills treated black locust stems, but vigorous sprouts develop from roots. Tordon RTU has high soil mobility and persistence, and is no longer labelled for use on sandy soils. 
Girdling kills the black locust stem that is girdled, but it does not prevent the formation of suckers.

Mowing areas around mature trees where seed pods have dropped seems to promote seed germination.


ACKNOWLEDGEMENTS

Dr. Robert M. Mohlenbrock and the Illinois Department of Conservation generously permitted use of illustrations from their Forest Trees of Illinois.


REFERENCES

Converse, C.K. 1985. Robinia pseudoacacia. The Nature Conservancy Element Steward ship Abstract, 31 January. (12:34:05).

Fernald, M. L. 1950. Gray's manual of botany. 8th edition. American Book Co., New York. 1632 pp.

Liegel, K., R. Marty, and J. Lyon 1984. Black locust control with several herbicides, techniques tested (Wisconsin). Restoration and Management Notes 2(2):87.


PERSONAL COMMUNICATIONS

Glass, Bill. 1989. Division of Natural Heritage, Illinois Department of Conservation, Springfield, Illinois.

Laurie, Dennis. 1989. Lake County Forest Preserve District, Libertyville, Illinois.

McClain, Bill. 1989. Division of Natural Heritage, Illinois Department of Conservation, Springfield, Illinois.

Packard, Steve. 1989. The Nature Conservancy, Chicago, Illinois.


Written for the Illinois Nature Preserves Commission by:
Jim Heim
Illinois Department of Conservation
Castle Rock State Park
R.R. 2
Oregon, Illinois 61061



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