Canada thistle (Carduus arvensis (L.) Robs.)


Material Information

Canada thistle (Carduus arvensis (L.) Robs.)
Series Title:
Circular / United States Department of Agriculture, Division of Botany ;
Physical Description:
14 p. : ill., map ; 23 cm.
Dewey, Lyster H ( Lyster Hoxie ), 1865-1944
United States -- Division of Botany
United States Department of Agriculture, Division of Botany
Place of Publication:
Publication Date:


Subjects / Keywords:
Canada thistle   ( lcsh )
federal government publication   ( marcgt )
non-fiction   ( marcgt )


General Note:
Caption title.
General Note:
Signed: Lyster H. Dewey, p. 14.
General Note:
"Washington, D.C., June 17, 1901."--P. 14.

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 029958452
oclc - 94923268
System ID:

Full Text

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and New Zealand it is often called California thistle. The names boar
thistle, cursed thistle, dog thistle; field thistle, and waste thistle have
also been applied to it.
In different botanical handbooks published during the past twenty-
five years this plant has been described under three different technical
names, Carduus arvensis, Cirsium arvense, and Cnicus artensis.
Recent studies upon the synonymy confirm Carduus arvensis as the
correct name.
Canada thistle is usually first introduced into new localities by the
seed. The seed germinates and a rosette of leaves lying almost flat on
the ground is first formed.
These leaves are prickly on the
-- margins, somewhat woolly on
. the under surface, but green
r and nearly smooth on the
upper. The following year a
.I flowering stalk branching at the
E K top grows up to a height from
|% one to three feet(20 to 100cm.),
i rarely higher. The stalk is
more slender than that of most
other thistles and bears very
l few spines (fig. 2, a). The
Earlier lower leaves are four to
eight inches (10 to 20 cm.)
-" d long and about one-fifth as
-1 wide. The later leaves on the
|' upper parts of the stalks and
branches are smaller. The
Fz;. '2. Canalda thktef. Detail: a. main stem andI
leaf: b, flower hi-ad : c. seed with pappus. natural size: lower surfaces of the earlier
it. ueed, enlarge,, with pappus (itLtathiel. leaves are at first somewhat
woolly, but, the upper surfaces of all the leaves are bright green and
smooth or slightly hairy. They are uneven or ruffled and the margins
are irregularly toothed and very spiny, the longer spines being one-fifth
to three-eighths of an inch (5 to 8 mm.) long. The flowers are rose-
purple, rarely white, in heads one-half to seven-eighths of an inch (1.5
to 2 cm.) in diameter, clustered at tlhe ends of the branches. The
green bracts surrounding the flower heads (fig. 2, b) are entirely devoid
of hard, stiff prickles or spines. Usually a comparatively small number _
of the flowers produce seeds (strictly, akenes) and in many localities
no perfect seeds have been found. The abundant white, feathered
pappus or down (fig. 2, r) is usually formed, however, even though the
seeds are not developed. The seeds are smooth, brown, about an

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eighth of an inch (3 mm.) in length, nearly cylindrical, pointed at the
lower end and with a slight projection from the center of the truncate
circular apex (fig. 2, d).
When plants grow from the running roots in the fall they send up
shoots with rosettes of leaves smaller than those produced by the seed-
lings, but when they grow during the spring and summer the rosette
stage is usually omitted. The running root is light yellow or nearly
white, smooth, cylindrical, and about a quarter of an inch (7 mm.) in
diameter (fig. 1). Numerous rootlets are scattered along its whole
length, but there are no nodes, scales, or buds, such as are found on
the underground portions of the shoots. It is, therefore, technically
a root, and not a rhizome or rootstock, as it has often been called.
The running root extends horizontally at a distance of three inches
to three feet below the surface of the ground, the lower depths being
reached usually where the soil is deep and porous. It will send shoots
to the surface through at least three feet of hard-packed soil. This
fact has been repeatedly noted where the plants on vacant lots in
Washington have been covered by the brick-like soil drawn from exca-
vations and thoroughly packed by the successive cart-loads hauled
over it.

Several other thistles are often mistaken for Canada thistles. As
these are chiefly annuals or biennials, differing very much from the
Canada thistle in their injurious characters, and requiring very different
treatment for their eradication, it is important that they be distinguished.
The most common and most widely distributed of these is the bull
thistle, Carduus lanceolatus (fig. 3). This is an introduced biennial
species. It is distributed exclusively by the seeds, but these are pro-
duced in such great numbers that the plant multiplies very rapidly.
It often forms patches several acres in extent on newly cleared land,
but in old fields the plants are usually more scattered. It seldom per-
sists in any great quantity and is readily destroyed by cultivation. It
may be easily distinguished by its greater size and much larger heads.
with spine-pointed scales, and by its leaves the upper surfaces of which
are always rough, like a cat's tongue.
In the South Atlantic States bird's nest thistle, Carduns spinosissi-
Imus, is often mistaken for Canada thistle. This is a native perennial
thistle, which blooms early in spring and is sometimes troublesome in
early crops and in meadows. It is not regarded as a serious pest,
however, and is easily eradicated by cultivation or by increased fertil-
ization and thick seeding. It spreads by the seeds alone, as it has no
running roots. The bird's nest thistle may be recognized by its short,
thick stem, only 10 to 20 inches high, bearing at the top two to five
large heads, surrounded by very spiny pinnate scales.

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On the Pacific coast milk thistle, Silybum marianum, has been
mistaken for Canada thistle. Milk thistle is an introduced annual
plant that has become abundant in some places in California and in
waste ground about eastern cities. It may be recognized by its coarse,
thistle-like habit, large leaves, mottled with white, and milky juice.
The curled thistle,
~\ Carduus c r i s p u s,
introduced at a few
S"' "/ points from Vermont
B, ., ^'' ,- to Pennsylvania, and
in ballast about sea-
ports, has been mis-
- ~ taken for the Canada
".A thistle, and in this
V:. *'r, case the resemblance
S\is very close. It has
SJ the same slender
i habit, small heads,
S.B and light-colored foli-
age. The best char-
i,/,7 acters for distinguish-
ing between them are
the prickly winged
stems, the sp in e-
a pointed scales, and
the plumeless or non-
feathered pappus bris-
% -tles of the curled this-
tle, as compared with
j ..the feathered pappus
S- '" \.S d (f the Canada thistle.
': In case of doubt as
to any plant supposed
F :'. ;. liull thistle. a, JiecL o(f main stem. within leaf: b.flower t(o be Canada thistle1
heali: e. setil. with ipajus., natural size: fel, seed. eilargedIl. with t m i i
iU ,.IJLIS iletact.hd. te best method is to
destroy it so as to pre-
vent seed pr( luction-atter mailing to tlhe Department of Agriculture
for identification some of tlhe leaves and flower heads. Specimens of
(C'anadla thistle, (r ()f plants SUp)posed to be Canada thistle, are especially
ldesiredl from localities where its presence is not indicated by maIrks on
tle L. a(ccmipanying iiap.
Thie (Can ia thistle was a troublesome weed in the fields of southern
Europe as nearly at least as the beginning of the sixteenth century. By
the middle of tlhe eighteenth century it had spread throughout the

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greater part of Europe, and now it is found in western Asia, northern
India, Australia, New Zealand, and North America. In all regions
where it has become naturalized it has the same reputation as an
aggressive and pernicious weed.
Some of the earlier American botanists held the opinion that while
the Canada thistle had doubtless been introduced into New England
from Europe, it was probably indigenous in western Canada. It now
appears very improbable that it is indigenous anywhere on the Amer-
ican Continent. It was evidently introduced into the French" settle-
ments in Canada earlier than into the English and Dutch colonies of
New England and the Middle States. It is reported to have been
found about the residences of French missionaries in Canada early in
the seventeeth century. There is a tradition that it was purposely intro-
duced into Canada by the French for feeding swine; but there appears
to be no just ground for this tradition, as there is no record that thistles
were ever used to any considerable extent as food for swine in Europe.
It is said to have been introduced into eastern New York with the
hay and camp equipage of Burgoyne's army in 1777. It probably
reached Vermont at the same time or previously, as it was recognized
as a troublesome weed in that State earlier than in New York. The
farmers of Vermont had become so greatly alarmed by its progress
that a law was passed by the legislature in 1795 directing its destruc-
tion on all lands within the State. In 1813 the legislature of New
York passed a law authorizing certain towns to pay rewards for its
destruction. These laws were well enforced at first, as they were
passed at the request of the farmers directly interested. But the
farmers soon learned that the thistle could be controlled, and then
their alarm ceased and with it their interest in the complete extermi-
nation of the plant. Since then successive generations have attacked
the thistle spasmodically, soon becoming satisfied of their ability to
keep it in check, and relaxing their efforts. The thistle has never
relaxed, however. It has continued to grow and to spread each year,
although it has been exterminated on many individual farms. As early
as 1850 it had spread throughout New England, Pennsylvania, and
eastern Ohio, and had gained a foothold in scattering localities in many
other States. During the war its range was extended southward all
along the line east of the Missouri River, and in more recent years it
has been extending westward.

The Canada thistle is now found from Maine to Virginia and west-
ward to North Dakota and Kansas, and on the Pacific coast from
Washington to northern California. The States south of Virginia.




Kentucky, Missouri, and Kansas are practically free from it, and it has
become established in only a few localities in the Rocky Mountain
region and Great Basin. It is abundant and troublesome from southern
Maine to Maryland and westward to Indiana and Wisconsin, and in
restricted localities in some of the States bordering this area, also in
the Pacific coast States.


Very few reports of Canada thistle have been received from the
Southern States, and when these have been investigated it has nearly
always proved that other thistles were mistaken for Canada thistle.
The true Canada thistle was introduced several years ago into Apa-

Fit;. 4. MIap hliowing distribution of Canada thistle in the United States a- indicated by reports
received at the Department of Agriculture. + Record or report, 0 specimens seen.

lachicola, Fla., but it soon died out, although no efforts were made
to dlestroy it. In 1893 it was introduced on ballast ground at Mobile,
Ali., but none of the plants bore perfect seeds and only one plant
appeared in 1894. Canada thistle seeds were undoubtedly carried
to many parts of the South in hay during the war, and during recent.
years the increased traffic between the Northern and Southern States
ljas given abundant opportunity for the introduction of thistle seeds.
Tljese facts indicate that it is not likely ever to become such a pest
in the South as it is now in the North. It may be expected, how-
ever, in the rich valleys in the Piedmont regions, where the climatic
cMnditions are similar to those of the mountain' regions of Pennsyl-

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The Canada thistle has long been abundant and troublesome in
Manitoba and is now found at many points along the Canadian Pacific
Railway from Winnipeg to the Pacific coast. In most of the localities
where it has been introduced in Minnesota, North and South Dakota,
Colorado, and Idaho, it thrives and shows a disposition to spread. The
climatic and agricultural conditions throughout a large part of this area
preclude the employment of many methods for combating the thistle
which are in common use in the East, and at the same time the prac-
tice of irrigation will aid in disseminating the seeds. Therefore, if the
plant is allowed to become widely established here, it promises to be
even more troublesome than it has been in other parts of the country.

The Canada thistle spreads over large areas or travels long distances
by means of its seeds. It spreads into patches through its perennial
running roots. Both of these means are effective in their way. Perfect
i seeds are not often produced until after the plants have become well
established and have spread to some extent by the running roots. In
some localities in Wisconsin and Iowa close observation of the plants
for several years has failed to discover perfect seeds. The plants appear
to be somewhat erratic in this respect, however. In 1894 very few were
found at Washington bearing perfect seeds, while in 1895 nearly all
bore perfect seeds, though such were produced by less than half of the
flowers in each head. In 1896 the plants were again seedless. All
three seasons were alike exceptionally dry for this region. In 1899, an
abnormally wet season in Washington, they were seedless, and no seed-
lings were found in the spring of 1900. There were no apparent fungus
or insect enemies to account for failure to produce seeds. The fact that
the plants are imperfectly dioecious does not fully explain these differ-
ences in seed production.
The seeds are carried from farm to farm by the wind, and along
streams they are carried by the water. The seeds are mature from
midsummer onward through the season, and, as they are easily detached .
from the heads by slight breezes, most of them are disseminated before
the hard winds of late autumn and winter begin to blow. The feathered
pappus is very abundant, and the seeds are comparatively light, so that
they may be carried a mile-rarely farther-in the windstorms that
often precede summer thundershowers, but they are too heavy to float
upon light breezes or in still air. Perfect seeds are easily detached from
the pappus, and the thistle down so often seen floating about is either
that from which the seeds have already fallen or that from fl[overs which.

d(lid not bear perfect seeds. The distribution caused by the wind is
principally from waste land and fence rows to cultivated fields and from
field to field. This distribution is confined to short distances, and is
generally in the direction of prevailing winds.
Every rain falling on a hillside thistle patch washes the seeds down
the slope. Seeds from a patch of Canada thistles growing in a moun-
tain valley are disseminated )by freshets all along the banks of streams
below. Transportation by water becomes a special danger in regions
where irrigation is practiced, as the seeds of thistles growing on the
banks of rivers or irrigation canals will float down the streams and
ditches and be deposited in the fields under the best conditions for
The dissemination of seeds by natural means accounts in part for the
distribution of the Canada thistle over limited areas, but were it not
for the unwitting or careless aid of man its progress would be compara-
tively slow. The seeds were fist brought from Europe to America by
m-an. They have been transported from Europe to this country in
impure seed. The hay or straw used in packing the cheaper kinds of
crockery is a very frequent means of introducing these seeds. Thistles
are brought to the barn in hay or grain. The seeds reach the straw
stack or manure heap and are taken back to the fields. They are car-
ried from field to field by harvesting machinery and from farm to farm
by thrashing machines. In one county in Oregon the Canada thistle
was first noticeil where a thrashing machine from the East was first
used. It would have cost less than 10 cents to have cleaned the ma-
chine before it left the thistle-infested region where it had been used in
tlhe East, but it would now probably cost thousands of dollars to
exterminate the tlhistles that have sprung from that introduction of
se'( ls.
Di..ssminati n, ini field .seeds.-As ian impurity in commercial seeds;
Canada thistle seeds are found most frequently in Canada blue grass,
Kentucky blue grass adlulterated with Canada blue grass, and in the
lovers, especially alsike. If tlhe thistles are cut and thrashed with any
(if the ( cl v(ers (ir grnss s((eeds it is often impossible t completely separate
tlie seeds. Tlie pIppus or down never clings to perfect thistle seeds
after passing through a t crashing :machline. Th(e seeds are so nearly of
tli, sa ,iie 'L (i)lor as tIhose o(f Canada blue grass that they are detected
with difi(fcuulty, even Iby trained eyes; but the yellow thistle spines,
which iarc oore readily 'seen, indicate when present that the seeds may
bei present alsi). ('n;t1'il; thistle seeds are sometimes found in wheat
ii oid ,its, ibut tI (ir presence in these grains indicates very careless clean-
ing, :s tle.y ;are so sitiall tilat they iy Ilv ,e easilyy separated by proper
S(:re'E'1 111 .


Distribution in hay.-During the past fifty years the transportation
of hary has been one of the most potent agencies in the dissemination of
Canada thistle seeds. During the war immense quantities of hay were
shipped from thistle-infested regions to the armies in the field. Since
that time baled hay has been shipped very extensively to lumber camps
and to workmen constructing railways. An evidence of this is left in
the patches of Canada thistles about deserted lumber camps and along
new railway lines. Thistles may frequently be found in the bales of
hay received for city consumption and an abundant growth of thistles
is often seen on farms where manure from city stables is used.
A great many methods for killing the Canada thistle have been de-
vised, and many have been described in various publications, yet there
still seems to be need of a more widely disseminatedl knowledge of those
which have been tried and found successful. Mr. Ambrose Stevens in
an excellent essay on the Canada thistle, published in the Transactions
of the New York Agricultural Society for 1846, states that no entirely
new methods for the destruction of the thistle had been discovered or
developed for at least forty years previous to that time. He gives a
summary of about twenty-five different methods, the details and results
of which had been published in agricultural papers. Each of these
methods had proved successful and each had proved unsuccessful, show-
ing a wide variation in results from the same treatment where no
account was taken of the surrounding conditions. All of the methods
there discussed are still used and are still meeting with success and
failure. Mr. Stevens's conclusions, deduced from a careful study of his
own experiments and from those of others, are. in general, applicable
to present conditions throughout the thistle-infested region, although
experience has proved some of the statements to be too sweeping.
They are as follows:
Whatever will effectually exclude the plant from the light and air will destroy
it. This may be done by plowing, in some soils, and in others by a close grass
sod. Plowing, if repeated frequently in soils where the root does not descend
beyond the reach of the plowing, will, in dry seasons, always destroy the thistle,
and often in moist ones. In soils which are light, deep, rich, friable, and, of
course, permeable to the air, and are in some measure always moist, plowing
will always fail.
Wherever a dense sod can be formed, thie thistle mav lie destroyed hvby seeding.
The grasses, wherever they are adapted to tihe purpose, will be found thie easiest.
means of destruction, although lint so rapid as plowing, hoeing, salting, or burn-
ing, where these latter are available.
In all uplands, where thle soil is of a depth admitting the root to be reached
and affected in its whole extent. by the plow, hoe, fire, or salt, the thistle may
be destroyed by these means, and they will be found the most rapid ofnes.
In all bottom lands where tlte root descends deep and the soil permits access
of air, neither the plow, hoe, fire, nor salt will destroy tlie thistle; ihere tlhe
grasses shotild be applied, and will be found tlie best de.-trov ers.

N ..


Mowing will destroy those parts of the thistle which have thrown up flowering
stalks, and will not in the least affect those which have not. Mowing 'should
take place when the plant Is in bloom.
Whatever limits the thorough application of the means of destruction, will
proportionately diminish success. Hence it will be found difficult in very stony
grounds ever to eradicate the thistle; the plow can not effectually reach its roots,
and such ground is rarely a good grass bearer. Salt and sheep, with the scythe,
will be found best for stony grounds. In grounds filled with stumps, where the
soil is rich and will grow a dense sod, the grasses will be best, and in such the
plow should not be used, as it will not effectually reach all the roots. Fences
that obstruct the application of the plow or hoe should be removed.
If it be desirable to destroy the thistle by thie grasses it will be found best to
make the land rich by manure. This will force the grass and enable it more
readily, by vigorous growth, to kill the plant. And in the application of all
remedies care should be taken to reduce the soil by proper cultivation to a fine
tilth, that all the seeds of the thistle in the ground mnay germinate and not lie
dorinant. The seed is very hardy., and escapes all the ordinary means of
destruction, except fire.
The following specific methods of treatment have been found most
successful in subduing or destroying the Canada thistle:
Mowing twice each year, just after the flowers open, usually in June
and August, will keep the plants in subjection. This will prevent the
production-of seeds, and thus serious injury to crops may be avoided,
hut it will rarely cause the death of the thistle roots except in good
grass land or in wet seasons, and will therefore need to be repeated each
year. It is generally as effective as pulling or grulbbing twice a year.
This plan is recommended for roadsides and waste land, and for
meadows and pastures where tihe methods'for complete eradication seem
to be too expensive for immediate application. Canada thistle plants
are often killed by mowing them just as a heavy rain sets in late in
June or early in July, when they are in bloom and the stalks are hollow.
Tlie rain, keeping the cut surface moist and filling up the hollow stalks,
favors tlhe growth of fungi, inducing decay, which often extends down
to t]e root system.
A more effective method, especially in dry seasons, is to go over the
ground once during every two weeks arter tih mowing in June and cut
off every tlIistle about two indies below tlihe surface with a hoe or spud.
A spud inside of a strong, sharp chisel on the end of a pitchfork handle
will lie found most convenient for this work. Tihe second year the
spudding should begin as soon as the thlistles show in the spring, and
should lie coinitintiued through tle season, although there will be few to
cut after mni(lsuImer, if the work lhas been well (done. The land should
be (lookedl over occasionally eaIch year afterwards to detect and destroy
plants that may spring from dormant seeds.
Sualting thistle plants every week or two during two successive grow-
ing s(IasMIins in pastures ilwhere sleep have access to them, usually :
destroys tlheii. I::

i i


Small patches of the plant have been killed by covering them with
strav% tanbark, or apple pomace; but these methods can not be rec-
ommended. Canada thistle roots will live for three years or longer in
porous soils under straw stacks or piles of tanbark, and they are likely
to creep out and send up shoots. Apple pomace, applied thick enough
to kill the thistle, ruins the land for the growth of any crop for several
years; but this period may be shortened by repeated applications of air-
slaked lime.
The application of chemicals or some substance that, being absorbed
into the tissues of the plant, will kill the roots, is recommended as one
of the best methods for destroying small patches. Trials on a small
scale, proving the ease and effectiveness of this method, may encourage
its extension to larger areas. The following substances given in the
approximate order of their effectiveness, beginning with the poorest,
have been used for this purpose: Salt, brine, quicklime, kerosene,
gasoline, turpentine, lye, sodium arsenite, carbolic acid, muriatic acid,
nitric acid, sulphuric acid. Salt, brine, and quicklime are most
effective when applied liberally to places where the thistles have been
grubbed out. Brine is often applied hot with good effect. Salt and
kerosene are often used together. Kerosene, gasoline, turpentine, and
lye may be applied in the above manner, or they may be poured into
the hollow stems when the plants are cut in flower. This process is
too laborious to be recommended, except in case of small patches.
Strong lye and the other substances mentioned will be found effective
if applied to the top of the plants when they are growing most rapidly
during May and June. None of these substances, except salt, injure
the land to any appreciable extent if applied only in sufficient quan-
tities to kill the thistles. Salt must be applied in such large quantities
that in some cases it may not be washed out of the soil for two or three
years. The stronger acids and alkalies are somewhat difficult and
dangerous to handle, because of their corrosive properties. They have
to be stored and applied in glass bottles. Crude sulphuric acid, which
is much used in eastern Pennsylvania, is applied by means of a glass
bottle with a glass tube or a clay pipestem running through the cork.
Of these strong chemicals, a few drops applied to each plant are suffi-
cient. Carbolic acid and the less corrosive substances may be applied
by means of an ordinary machine oil can, or a watering pot with a
small rose or nozzle. A teaspoonful of strong commercial carbolic
acid applied to each plant is sufficient. This should be applied without
dilution on the buds and tender upper leaves of the growing plant.
Care should be exercised to prevent the poisoning of stock from chem-
icals applied to thistles in pasture fields.
Patches of Canada thistles discovered in grain fields at harvest time,
as they often are, should be left standing until after the crop is
removed, then mowed and burned on the spot as soon as they are

dry enough. This treatment arrests the distribution of seeds and, in
some cases, it has killed the plants.
In shallow, dry soils summer fallowing during a dry season will
destroy the thistles.
The first plowing should be done when the plants are in bloom in
June or early in July. If they can not be turned under cleanly with
chain or jointer, they should be mowed and burned before plowing.
The land should he alternately harrowed and cross-plowed as often as
any green plants appear until it is time to sow winter grain. Thorough
cultivation with hoed crops will produce almost the same effect if the
cultivation is continued through the summer. In this case a hoe must
he used to destroy thistles growing in the hills and others that escape
the cultivator. In wet seasons cultivation generally fails to kill the
roots. After cultivating either in barren fallow or with hoed crops, the
land should be thickly seeded in August or September with crimson
clover, rye, or winter oats where the winter climate will permit the
growth of these crops. These may be pastured (luring the early spring
and then plowed un(ler. Winter wheat and other grain crops that will
permit the thistle to remain undisturbed during spring and early sum-
mner should not be grown. Where the climate is too severe for winter
crops, cultivation should continue late in the fall and begin early in
spring. Thistle-infested areas ought to be plowed and cultivated by
themselves to avoid scattering roots to other parts of the field.
('anade thistles that persist in spite of cultivation on low lands may
soon disappear when the land is seeded and made to produce two good
crops of hay each year. On soils not adapted for permanent grass
lands it is often possible to raise good crops of annual grasses which
will choke out the thistles. Millet, fodder corn, or sorghum are good
crops for this purpose, and good results have been obtained by the cul-
tivation of rape.
l)r. T. J. Burrill, in Bulletin No. 12, Illinois Agricultural Experi-
ment Station, recommends the following method as the "best for
exterminating Canada thistles when in full possession of tillable ground:
k'" 1. ('ut tlhe thistles when in full bloom [July] as close to the ground
as possible.
2. Plow about 3 inches deep and sow millet or Hungarian grass,
seedliIng ieivily; harrow. This may follwv the preceding at once or
ifter some two weeks' delay.
,,. In Septemuber plow under theim crp or save it for hay, as desired.
At :aill events, plolw aMi seu(l liberally3' with rye.
S4. Plow under tlhe rye in Mlay and seed again with millet or Hungarian
grass, or p)lant to sme P hoed crop) [corn] and give the most thorough
cult ivat imin. with c ( lntillul searchiiiig for and destruction of every
reimih ,i thistle.


"5. Continue the clean cultivation and sharp lookout for thistles
another year."

Although the larve of several different kinds of insects live in the
stems of Canada thistles, while others feed more or less upon the roots
and still others eat the foliage, they seem to produce comparatively
little effect upon the vigor or productiveness of the plants. The
American goldfinch or "yellow bird," often called the "thistle bird,"
is sometimes unjustly accused of scattering thistle seeds. It does
scatter the down from which it has detached the seeds. Thistle seeds
form one of its favorite kinds of food, and it is undoubtedly one of the
best natural agencies tending to keep the thistle in check.
Among fungus diseases which attack Canada thistle, the thistle rust,
Puccinia suadolens, is the most destructive. This often prevents the
production of seeds and sometimes kills the plant to the ground. It
is most effective d(luring wet seasons, but even under the most favorable
conditions it rarely spreads so as to destroy all of the plants in a patch.
Experiments made thus far in New York, New Jersey, and Pennsyl-
vania, in introducing the rust into uninfested patches, indicate that
while it may often aid materially in checking the growth of the thistles,
it rarely exterminates them, and its action is too uncertain to warrant
more than a qualified recommendation of its use as a thistle-destroying

Canada thistle is proscribed as a noxious weed by the laws of the
I following twenty-four States:
California. Kansas. Nebraska. Pennsylvania.
Connecticut. Kentucky. New Jerzey. South Dakota.
Delaware. Maryland. New York. Vermont.
Illinois. Michigan. North Dakota. Washington.
Indiana. Minnesota. Ohio. Wisconsin.
Iowa. Missouri. Oregon. Wyoming.
In most of these States penalties are prescribed for permitting the
thistle to produce seeds. Illinois is the only State in which the law
directs that the plants be killed, and this also the only State in which
the law appears to be vigorously enforced. In several counties in
different parts of the State Canada thistles are reported as practically
exterminated through the rigid enforcement of the law. In some of
the large cities of this country where there are thousands of acres of
vacant lots grown up to thistles and other weeds, neither the thistles
nor the thistle laws have received much attention.
The majority of progressive farmers know that Canada thistle can
be exterminated on their farms, but they need the aid of a good law,
well administered, to prevent their well-tilled fields from being seeded


by the thistle patches of careless neighbors. While it is not regarded
as necessary or desirable that the Canada thistle should be treated in
a law distinct from laws relating to other weeds, it is earnestly recom-
mended that it be proscribed by just and comprehensive laws in all of
the States where it is at present abundant, and especially in those
States of the Rocky Mountain region and Great Basin where it is now
becoming established. An effort should be made to enforce the laws
that now exist unheeded in some of the older States, and if they are
found inadequate, they should be repealed and replaced by better ones.
Assistant Botanist.
Secretary of Agriculture.

WASHINGTON, D. C., June 17, 1901.


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