AGRICULTURAL EXTENSION SERVICE
UNIVERSITY OF FLORIDA
Hairy Indigo as a Cover Crop in
(A report of grower demonstrations)
R. E. NORRIS
FRED P. LAWRENCE
Agricultural Extension Service
Grower demonstrations on which
this production guide has been
developed were carried out in
1949, 1950 and 1951-and orig-
inally reported in a mimeo-
graphed report. This revision,
Cover picture courtesy U. S. Soil Conservation Service.
COOPERATIVE EXTENSION WORK IN AGRICULTURE AND HOME ECONOMICS
(Acts of May 8 and June 30, 1914)
Agricultural Extension Service, University of Florida,
Florida State University and United States Department of Agriculture, Cooperating
M. O. Watkins, Director
HAIRY INDIGO AS A COVER CROP IN
R. E. NORRIS AND FRED P. LAWRENCE
Cover crops have always played an important role in the
maintenance of fertility in Florida's sandy soils, and they are
just as important today as ever.
In the past, non-leguminous volunteer cover crops were grown
extensively and until the late 40's perhaps occupied 75 percent
of the state citrus groves. The need for leguminous cover crops
on sandy soils has been recognized for many years. They not
only furnish organic matter that is so badly needed, but when
they decompose they furnish a readily available form of nitro-
gen to other plants. In addition, they moderate soil temperature
during the hot summer months by shading.
Beggarweed, cowpeas, and velvet beans were the earliest
legumes used in citrus groves. It soon became evident, however,
that pumpkin bugs and grasshoppers made the growing of cow-
peas and velvet beans impractical under most conditions, and
beggarweed could not be relied upon to reseed itself, particularly
Figure 1.-Representative stand of mature Crotalaria spectabilis
in dry years. In the late 30's crotalaria was introduced and was
thought to be the answer to the leguminous cover crop question
in citrus groves. It did remarkably well the first few years
after it was introduced. This was true of both Crotalaria spec-
tabilis and striata. However, crotalaria grown rapidly under
grove conditions soon became exhausted and did not reseed it-
self; even when new seeds were broadcast it did not make a sat-
isfactory stand on land where it had formerly grown. As a re-
sult, crotalaria is confined largely to newly planted groves where
it usually succeeds for one to three years.
Little research has been conducted on the role or value of
legumes in citrus since 1932, when the Florida Agricultural Ex-
periment Stations published Bulletin 253, "Studies on Summer
Cover Crops in A Pineapple Orange Grove," by Stokes, Barnette,
Jones, and Jefferies.
Figure 2.-Native cover
In the mid 1940's a new legume, hairy indigo (Indigofera
hirsuta L.) was introduced into Florida by the Agricultural Ex-
periment Stations. Researchers decided to try establishing this
new legume in citrus groves and comparing it with other cover
DESCRIPTION OF HAIRY INDIGO
An annual summer legume, hairy indigo (Indigofera hirsuta
L.) is sensitive to cold and is killed by the first hard frost. It
grows into a shrubby plant having medium to fine stems that
become woody as the plant approaches maturity. The leaves are
compound, being made up of several small leaflets, and resemble
those of vetches, except that they are covered with short hairs
and are larger in size. The seeds are crowded into cylindrical
pods, which causes them to develop into small cubes. Seeds of
hairy indigo are small-averaging about 200,000 to a pound and
weighing about 55 pounds per bushel. An abundance of "hard"
seed is produced when hairy indigo matures its seed crop. It
does not germinate for several months after falling to the ground.
Under normal growing conditions the "hard" seed provide a seed
source for a volunteer cover crop the following season. The
plants when not crowded grow from 4 to 7 feet in height and
often 5 to 7 feet in diameter. When seeded to a heavy stand
they grow into a dense thicket that is difficult for a person to
pass through. Heavier seed yields are obtained from thinner
stands, and less woody cover develops from the more dense
Figure 3.-County agent, Bob Norris, practically obstructed from
sight by representative stand of hairy indigo
CLIMATIC AND SOIL ADAPTATION
Hairy indigo is adapted to the warm areas of the world. The
variety most prevalently grown in this country is adapted to cen-
tral Florida and southward because it requires a long growing
season. The earlier maturing strain, which has been developed
since 1945, extended the limit of adaptation northward into
Georgia and other southern states. Both early and late matur-
ing strains are sensitive to cold and are killed by the first hard
The crop is adapted to a wide range of soils but does espe-
cially well in the open sandy soils of central Florida.
SEED-BED AND SEEDING
Best results are obtained when indigo is planted on a well
prepared seed-bed although this is not absolutely necessary.
If the ground is rough and covered with heavy growth, it should
be plowed under and disked level prior to seeding.
The seeds may be planted as soon as the danger of frost has
passed and when soil moisture conditions are apt to be favor-
able. The earlier the plants can be successfully grown, the more
certain will be the possibility of a good seed crop, and the earlier
the plants will shade the soil. Late seedings make poorer growth
and produce a poorer quality of seed. A cultipacker with a seed
attachment is an excellent planting tool. A hard rain or irriga-
tion water applied soon after seeding also effectively packs the
soil around the seeds and is most effective. Inasmuch as the
seeds are small, they should not be planted more than 1/2 inch
deep. Three to 5 pounds of seed per acre should be used when
drilling and 4 to 8 pounds when sown broadcast. The thinner
seedings are recommended when the crop is grown for seed and
heavier seedings when grown for a cover crop.
As the result of an observation made in Indian River County
by the late County Agent Marcel A. Boudet, it was thought that
seed inoculation with certain strains of bacteria might prove ben-
eficial. Dr. George D. Thornton, now assistant dean of the Col-
lege of Agriculture, became interested in the problem and pre-
pared a number of cultures which were used in a series of dem-
onstrations in Lake and Indian River counties. J. R. Hender-
son, agronomist, Agricultural Extension Service, supervised the
establishment of these demonstrations.
These demonstrations were followed very closely, but no
noticeable differences between the inoculated seed and the check
plots were observed. Because of this, inoculation of indigo seeds
was judged to be unnecessary.
FERTILIZING AND LIMING
Extensive fertilizer studies have not been made with indigo,
but the plant appears to respond to liming and fertilizing when
grown on Lakeland sandy soil. On poor soils 300 to 500 pounds
of an 0-10-10 or 0-14-10 fertilizer per acre should be applied
before the crop is seeded. Lime (or dolomite) at the rate of
1,000 pounds per acre will be beneficial on acid soils, although
the plant is quite tolerant of the slightly acid soils of the state.
The current pH range for citrus (5.5 to 6.2) has been found
quite satisfactory for indigo. When grown in citrus, additional
fertilizer is not necessary.
Figure 4.-Same stand as shown in figure 3 with a section cut to
show the density of the growth
INSECTS AND DISEASES
No diseases are known to affect indigo. It is highly resistant
to root-knot, although some root-knot has been reported in a
few plantings on heavily infested soils. Subsequent to the work
with these demonstrations hairy indigo was determined a host
to the burrowing nematode. This should not condemn its use
in citrus groves, since hairy indigo is not vegetatively propagated
and nematodes are not transmitted through the seed. Army
worms have been known to attack hairy indigo although it is not
a preferred host. It is free of grasshoppers and pumpkin bugs.
APPROACH TO STUDY
An attempt was made to demonstrate the value of indigo in
comparison with native cover crops in citrus groves.
The plan consisted of getting as many growers as possible
to plant indigo in citrus groves to see if it would continue to re-
seed itself, and then to compare its yield or tonnage per acre
with that of native cover crops produced on the same grove or
groves with comparable soil types and management programs.
After this was accomplished the next step was to work out some
standards to follow. Since the primary concern was yields, a
representative stand of the entire area was chosen by measuring
a 4-foot square, cutting all plants growing within the area (at
ground level) and weighing them immediately. The plants were
then bagged and tagged and taken to the main Experiment Sta-
tion in Gainesville, where they were dried in an oven and their
dry weight was determined.
The late Professor G. E. Ritchey was of tremendous help,
and at the end of the first year's work he furnished the follow-
ing conversion factors:
28% of green weight equals dry weight of plant
2.1% of dry weight equals N content of plant
2722.5 times weight of 4 x 4 plot equals green weight
per acre, on a solid acre basis
.0059 times green weight equals N per acre
1/5 to 1/6 of the total N is in the roots of the plant
Dr. George K. Davis of the Nutrition Laboratory furnished
the following analysis of hairy indigo (plant in early bud stage):
HOH Protein ASH Ca Mg P Fiber
Tender tops (bud stage) 6.44 20.71 5.85 2.64 0.56 0.31 10.53
Leaves (same plant) 7.46 20.93 6.09 2.67 0.52 2.61 8.85
Whole plant (bud stage) 6.05 10.82 3.86 1.79 0.40 1.52 32.80
From Table III of Agricultural Experiment Stations Bulle-
tin 253 (out of print) we found the following information on
different plants that had been averaged for seven consecutive
Cover Crop Yield Per Acre, Air Dried Nitrogen Return Per Acre
Crotalaria striata ..
Velvet beans ...........
Natal grass ............
In these demonstration plots it was found that indigo varied
from 7,872 pounds green weight per acre on very poor stands up
to 45,921 pounds in exceptionally heavy stands. An average of
all weights taken during the four years was not determined but
it would be between 10 and 15 thousand pounds green weight
WEIGHTS AND NITROGEN RETURN
(per acre on five of the groves checked)
1949 1950 1951
Grove Dry Wt. N Dry Wt. N Dry Wt. N
Grove #1 .............. 2,625 55.31 4,368 91.7 3,864 81.4
Grove #2 ............ 11,200 236.00 7,000 147.0 7,840 164.6
Grove #3 .............
Late strain ...... 5,600 118.00 7,560 158.7 6,580 138.1
Early strain ...... 5,600 118.00 6,650 139.6 5,488 115.2
Grove #4* ............ 3,500 73.73 2,450 51.4 2,688 56.4
In grove No. 4 grower follows the practice of cutting the crop down in the summer.
Hence these weights are of the second growth crop.
Hairy indigo is recommended as a leguminous cover crop
for citrus in Florida. It grows well on light sandy soils and is
highly resistant to root-knot. Although the plant is a host of
the burrowing nematode, the nematodes cannot be spread through
the seeds and it is not an acceptable host plant for the pumpkin
or stink bug. It gives comparatively high tonnage of green
manure per acre; yields have been checked up to slightly over 22
tons (green weight) per acre. It appears that under average
conditions the yield will be between 5 and 71/ tons green weight
per acre. The plant reseeds itself and apparently requires no
inoculation or special fertilizers, although both may be of some
small benefit. Its chief drawback as a cover crop in citrus is that
it does not produce seed until late in the year. Usually it is
mid-October before mature seed spikes are produced.
When to Plant:
March to June when good moisture is present is recom-
mended. Later plantings usually result in lighter yields
and may preclude seeding in instances of early frost.
Rate to Seed:
Four to 8 pounds per acre (on solid acre basis) depending
on method of seeding.
How to Seed:
Sow on well prepared seed-bed. Cover not more than one-
half inch. It is not imperative but it helps to pack the soil
with a cultipacker, roller, or drag.
Green Manure Crops:
To assure reseeding for the following year chop, disc, or
plow after seed matures in the fall before the first killing