Table of Contents
Hanover, J.W. 1993. Black locust: An excellent fiber crop. p. 432-435. In:
J. Janick and J.E. Simon (eds.), New crops. Wiley, New York.
Black Locust: An Excellent Fiber Crop*
James W. Hanover
- ATTRIBUTES OF BLACK LOCUST
- BLACK LOCUST FOR FIBER AND BIOMASS
- CULTURAL SYSTEMS
- Table 1
- Table 2
Black locust, (Robinia pseudoacacia L., Fagaceae), is a remarkable yet
relatively neglected tree species with untapped potential which much of the
rest of the world already appreciates. The native range is east- and
west-central United States (Hanover 1992), but the species now grows wild in
all of the contiguous states. It is ironic that one of our common native
species may now be considered as a new crop for fiber. An intensive research
and development effort for black locust has the goal of exploiting one of our
most valuable plant resources with a multitude of uses including fiber
Black locust is a nitrogen-fixing legume (Hanover and Mebrahtu 1991). When
mature, it can reach 35 m in height and 1.0 m in diameter and such trees are
usually found on upland sites in hardwood forests with black oak, red oak,
chestnut oak, pignut hickory, yellow poplar, maple, and ashes. It tolerates a
wide range of soil pH (4.6 to 8.2), but grows best in calcareous, well-drained
loams. The species is intolerant of water logged soils and shading. Black
locust reproduces prolifically from root sprouts and dominates early forest
regeneration in native forest stands and areas disturbed by man. Black locust
possesses virtually all of the characteristics that define a typical weed
(Hanover 1992; Keeler 1989).
Other important attributes of black locust are its extremely rapid early growth
rate, very high density wood, high resistance to wood decay fungi, tolerance to
low fertility sites, drought resistance, abundance of natural product chemicals
in its wood, bark and leaves, and large amount of genetic variation in most
attributes (Barrett et al. 1988). On the negative side, black locust has been
plagued by the locust borer, Megacyllene robiniae Forster, which attacks
the stem and causes deformation or breakage. This single factor has been
primarily responsible for the lack of attention to the species for lumber
production. In other areas of the world where black locust has been introduced
and the borer does not exist, the species is attracting wide interest and
utilization (Keresztesi 1988). The tree also tends to have a crooked stem but
this defect has been remedied by selection and breeding (Keresztesi 1983;
Hanover et al. 1989). For the fiber production systems proposed here both
borer attack and crooked stems would be of little consequence due to the short
There are at least six ways in which black locust can be used as a fiber crop
or to generate large amounts of biomass at relatively low energy inputs. These
include pulp for paper, leaves and young stems for fodder, leaves and young
stems for solid, liquid or gaseous fuels, and extraction of specialty chemicals
such as natural wood preservatives. There now exists a substantial natural
resource of black locust timber and poles of fence post size growing throughout
its natural range and now appearing in colonized areas outside that range. The
species is prized for making fencing and post products and paper companies
harvest it for pulp. Due to its dispersion or lack of concentration in pure
stands and usually poor stem form, there is virtually no black locust lumber
used in commerce. This situation could change when new, improved genetic
materials become widely planted and commercially available because the wood is
of very high quality; in many respects superior to species like teakwood or
What are the specific characteristics of black locust which make it a desirable
fiber source? Black locust wood has a large portion of uniformly distributed
libriform fibers which confers great strength to wood. It has an average
specific gravity of 0.68 compared to other North American hardwoods which
average 0.51. Its average fiber length is 1.05 mm, slightly shorter than other
hardwoods which average 1.13 mm. The central stem or pith of young black
locust has a fiber length of 0.75 mm and specific gravity of 0.57, still well
above the average mature hardwood values.
Some other important characteristics of the wood of black locust are listed in
Table 1 and 2. Of particular interest are: (1) very young plant material
contains no heartwood with all of its associated extractive chemicals; (2) the
caloric content of young material is high and unchanged with age; and (3) the
moisture content is very low relative to other species.
Less information is available regarding the chemical and physical
characteristics of black locust leaves compared with the wood. Leaves are very
high in nitrogen and have been used in animal feeding trials with mixed success
(Baertsche et al. 1986; Cheeke et al. 1983). Baertsche et al. (1986) compared
leave-stem mixtures of ten woody plants and alfalfa for chemical compositions
and found black locust to be superior to all species in crude protein content
(22.3% of dry matter).
Black locust also has potential to serve as a source of energy. According to
Abelson (1991) there is great potential for energy crops in the United States
in the future, and this species should be considered along with a wide array of
other species. The wood is also a veritable chemical storehouse with
extractives comprising 11% of dry weight (Table 1). Thus, black locust should
be considered as a potential source of natural products just as other crops are
being developed for their unique extractable natural products (Hanover 1990;
Simon et al. 1990; Turick et al. 1991).
A critical consideration in evaluating the potential of a new crop for fiber
production and commercial utilization of its components is the efficiency and
economy of producing the new crop usually under quite different or
non-conventional cultural conditions. It is in this context that we are
focusing on black locust, i.e., development of very efficient, large scale
cultural systems for furnishing the raw material to be used for any of the
purposes stated above.
Black locust lends itself admirably to direct seeding much as is now done
conventionally with agricultural row crops. The seed must be pretreated with
acid (H2SO4 for 50 min.) to allow it to germinate and germination rates are
very high. We have successfully drill-sown several plantations and achieved
good stands rather rapidly. Because black locust is one of the fastest growing
species in North America, it literally appears to outcompete weed competition
early in development. Individual trees can reach 3 m in height in one year,
but the average is closer to 1 to 2 m, depending on soils and other conditions.
Genetic selection and breeding efforts now underway should further enhance
yields (Hanover et al. 1989).
Because black locust sprouts readily from the roots and regrows from cut stems,
we have several options for regenerating another crop either in one growing
season or over several seasons. Thus, the need to reestablish by seed is
eliminated at considerable reduction in energy inputs for many years. A
drill-sown, vigorous stand of closely spaced (30 cm) black locust can be
harvested in July, in Michigan, and by early September another crop will have
regrown. Alternatively, the initial crop can be allowed to grow the entire
season and be harvested before or after leaf fall in October, depending upon
the product to be extracted. Each of the six potential uses of the material
generated now need to be closely examined to determine the best methods for
harvesting and overall economic feasibility.
Black locust has physical and biological characteristics that make it a prime
candidate for fiber production. These include: very rapid early growth; the
ability to fix N2; reproduction from root sprouts and coppice; wide climatic
and edaphic adaptation; good fiber quality; high density wood; high caloric
content; low moisture content; high protein content; high genetic variation;
potentially useful chemical extractives; amenable to intensive culture
management. Research should now focus on the genetic improvement and cloning,
the utilization of leaves and stems, and the development of high yield
- Abelson, P.H. 1991. Improved yields of biomass. Science 252:1469.
- Baertsche, S., M.T. Yokoyama, and J.W. Hanover. 1986. Short rotation,
hardwood tree biomass as potential ruminant feed-chemical composition, nylon
bag ruminal degradation and ensilement of selected species. J. Anim. Sci.
- Barrett, R.P., T. Mebrahtu, and J.W. Hanover. 1988. Black locust: a
multi-purpose tree species for temperate climates, p. 278-283. In: J. Janick
and J.E. Simon (eds.). Advances in new crops. Timber Press, Portland, OR.
- Cheeke, P.R., M.P. Geoeger, and G.H. Arscott. 1983. Utilization of black
locust (Robinia pseudoacacia) leaf meal by chicks. Nitrogen Fixing Tree
Res. Rpt. 1:41.
- Hanover, J.W. 1990. Chemical basis for resistance of black locust to decay.
In: K. Miller (ed.). Advanced technology applications to eastern hardwood
utilization. Prog. Rpt. No. 3, Michigan State Univ. Agr. Expt. Sta., East
- Hanover, J.W. 1992. Robinia pseudoacacia: A versatile legume tree for
temperate/subtropical regions. Proc. Int. Conf. Black Locust: Biology, Culture
and Utilization. June 17-21, 1991, Michigan State Univ., East Lansing.
- Hanover, J.W., and T. Mebrahtu. 1991. Robinia pseudoacacia: temperate
legume tree with worldwide potential. Nitrogen Fixing Tree Highlights
- Hanover, J.W., T. Mebrahtu, and P. Bloese. 1989. Genetic improvement of black
locust: a prime agroforestry species. In: P. Williams (ed.). Proc. First
Conf. on Agroforestry in North America. August 1989, Guelph, Ontario, Canada.
- Keeler, K.H. 1989. Can genetically engineered crops become weeds?
- Keresztesi, B. 1983. Breeding and cultivation of black locust, Robinia
pseudoacacia, in Hungary. Forest Ecol. Mgt. 6:217-244.
- Keresztesi, B.. 1988. Black locust: the tree of agriculture. Outlook on Agr.
- Simon, J.E., D. Charles, E. Cebert, L. Grant, J. Janick, and A. Whipkey. 1990.
Artemisia annua L.: A promising aromatic and medicinal, p. 522-526. In:
J. Janick and J.E. Simon (eds.). Advances in new crops. Timber Press,
- Stringer, J.W. 1981. Factors affecting the variation in heat content of black
locust biomass. MSc. Thesis, Univ. of Kentucky, Lexington.
- Stringer, J.W. and S.B. Carpenter. 1986. Energy yield of black locust fuel.
Forest Sci. 32:1049-1057.
- Stringer, J.W. and J.R. Olson. 1987. Radial and vertical variation in stem
properties of juvenile black locust (Robinia pseudoacacia). Wood Fiber
- Turick, C.E., M.W. Peck, D.P. Chynoweth, and D.E. Jerger. 1991. Methane
fermentation of woody biomass. Bioresource Technol. 37:141-147.
*This research was supported by the Michigan State University/USDA/CSRS Eastern
Hardwood Utilization Research Special Grant Program (Grant No.
Table 1. Main-stem wood properties of ten- to twelve-year-old black
zModified from Stringer and Olson (1987). Main-stem defined from
groundline to 80% total tree height. Mean dbh (diameter at breast height) and
height of the 10 trees sampled was 12.5 cm and 10.5 m, respectively.
|Property ||Average ||Range|
|Stem volume (m3) ||0.043 ||0.03-0.05|
|Wood (%) ||84.7 ||80.0-86.3|
|Heartwood (%) ||54.1 ||34.8-60.2|
|Specific gravity ||0.68 ||0.65-0.71|
|Ash content (% dry mass) ||0.62 ||0.47-0.74|
|Fiber length (mm) ||1.05 ||0.94-1.11|
|Extractives (% dry mass)|
|Benzene-EtOH ||3.5 ||2.7-3.9|
|EtOH ||1.1 ||0.7-1.6|
|H2O (hot) ||2.8 ||2.4-3.1|
|Total ||7.4 ||6.2-8.3|
Table 2. Stem diameter variation in wood propertiesz.
zIncludes mainstem and branch material from 2- to 10-year-old black
|Diameter class |
|Caloric content |
|Moisture content |
|0.1-2.5 ||0.549 ||4641±52 ||41.1±16.5 ||absent|
|2.6-5.0 ||0.588 ||4644±58 ||38.0±16.1 ||3.4|
|5.1-7.5 ||0.644 ||4637±34 ||33.2±9.7 ||28.2|
|7.6-10.0 ||0.658 ||4665±42 ||26.7±6.4 ||38.0|
|Mean ||0.609 || ||33.1|
yFrom Stringer (1981).
xFrom Stringer and Carpenter (1986).
w% wet-weight basis.
Last update April 23, 1997