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