J- /^ ^ ? Circular Xo. 27.— Revised. United States Department of Agricult DIVISION OF BOTANY, CANADA THISTLE. {Carduus arvensis (L.) Robs. In the history of weeds in America none n^lM s / '»n«f> bV^' 5i^Sii;oRY plained of than the Canada thistle (fig. 1). regarded b}' the farmers of New P^ngland as the greatest pest of their fields. When the sons of these farmers moved West the Canada thistle went with them. It grows vigorously, sometimes spreads rapidlj^ and is always difficult to kill by ordinary cultivation. It forms dense patches, sometimes to the complete exclusion of other plants, and its abundant sharp spines make it disagree- able to handle. To these char- acters are due its traditional reputation in tiie Northeastern United States as the worst of all weeds. With little d«ul)t, however, it causes in the aggre- gate less real injury to farm products than does bull thistle, ragweed, or pigeon grass, and its distribution if* confined to a smaller area than that of any of these weeds. XAMKS. A century ago it was 1. Canada thistle, showjiiy horizontal r«>()ts, youii shoots, anri mature plant in flower. The phmt received the name p,, Canada thistle in this country soon after the war of the Revolution, as it was suprosed to have been introduced from Canada.- This name is now used generally in both Canada and the United States. In England the plait is known by the nanies corn thistle, green thistle, and creeping thistle. In Australi.i 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 j^ears this plant has ))een described under three different technical names, Carduus arvettsis, CirsiHm arvense, and Cuicus arveiisis. Recent studies upon the synonj-nn- confirm Card t( us arveiisis as the correct name. DESCRIPTION. Canada thistle is usually first introduced into new localities by the seed. The seed germinates and a rosette of leaves lying almost fiat on the ground is first formed. These leaves are prickly on the margins, somewhat woolly on the under surface, but green and nearlj' smooth on the upper. The following year a flowering stalk branching at the top grows up to a height from one to three feet (20 to 100cm.), rarel}' higher. The stalk is more slender than that of most other thistles and bears very few spines (fig. 2, n) . The earlier lower leaves are four to eight inches (10 to 20 cm.) long and about one-fifth as wide. The later leaves on the upper parts of the stalks and branches are smaller. The Fiwer SUrfaCCS ot the earlier rf. seed, eniartrixi. with pappus di'taiiu'd. leavcs are at lirst somcwhat woolly, l)ut/ the upper siufaces of all the leaves are bright green and smooth or slightly hairy. They are uneven or rufHed and the margins are irn^guhirly toothed and very spiny, the longer spines being one-lifth to thre(;-(;ighths of an inch (5 to S mm.) long. The ilovvers are rose- I)urple, rarely white, in licads one-half to seven-eighths of an inch (1.5 to 2 cm.) in diameter, clustered at the ends of the branches. The green bracts surrounding the Mower heads (fig. 2, h) are entirely devoid of hard, stiff prickles or spines. I'sually a conii)aratively small number of the flowers produce seeds (strictly, akenes) and in many localities no perfect seeds have been found. The abundant white, feathered I)api)us or down (fig. 2, c) is usually fornKid, however, even though the seeds are not developed. The seeds are smooth, brown, about an I eighth of an inch (3 mm.) in length, nearly cjdindrical, 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. OTHER THISTLES MISTAKEN FOR CANADA THISTLES. 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 ver^- 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 rapidlj'. 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, Cardiui.s spinosissi- mus, 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 bj' 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. mk On the Pacific coast milk thistle, Silybum mariannm, 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 maj^ be recognized by its coarse, thistle-like habit, large leaves, mottled with white, and milky juice. The curled thistle, Cardutis cri spu s , introduced at a few- points from Vermont to Pennsylvania, and in ballast about sea- ports, has been mis- taken for the Canada thistle, and in this case the resemblance is very close. It has the same slender habit, small heads, and light-colored foli- age. The best char- acters for distinguish- ing between them are the prickly winged stems, the spine- pointed scales, and the plumeless or non- feathered pappus bris- tles of the curled this- tle, as compared with the feathered pappus of the Canada thistle. In case of doubt as t(^ any plant su])i)()sed to be Canada thistle, tlic )>est method is to destroy it so as to pre- vent seed production — alter mailing to the Department of Agriculture for identification some of the leaves and (lower heads. Specimens of Canada thisth', or of plants supposed to be Canada thistle, are especially (h'sired from localities when' its presence is not indicated by marks on the accompanying map. IIISTOKV. Th(; Canada thistle was a troublesonu! weed in the lields of southern Europe as early at least as the beginning of tlie sixteenth century. By the middle <»f the eighteenth century it had spread throughout the Via. :{. Hull thistle, n, piece of main stem, with leaf; />, llowei hea,'elant appeared in 1S94. Canada thistle seeds were undoubtedly carried to many parts of the South in hay during the war. and during recent- years the increased trafhc between the Northern and Southern States has given aljundant opportunity for tb(? introduction of thistle seeds. These facts indicate that it is not likely ever to l^ecome such a pest in the South as it is now in the North. It may be expected, liow- ever, in the rich valleys in the Piedmont regions, where the clifuatic conditions are similar to those of the mountain^ regions of rcnnsyl- vani;i. DANGER OF ITS IXTRODUCTIOX IX XORTHERX PRAIRIE STATES AND ROCKY 3I0UXTAIX REGIOX. 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 manj^ 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. 3IETH0DS OF DISTRIBUTIOX. 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 vcay. Perfect 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 nearh^ 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. DISSEMINATION OF SEEDS BY NATURAL MEANS— WIND AND WATER. 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 thej' 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 flowers which 8 did not bear perfect seeds. The distribution caused b\' 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 vallej' are disseminated b^^ 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 propagation. DISSEMINATION OV SEEDS BY ARTIFICIAL MEANS. 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- tivel}' slow. The seeds were first brought from P]urope to America by man. 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 veiy frequent means of introducing these seeds. Thistles are brought to the barn in ha}' or grain. The seeds reach the straw stack f)r manure heap and are taken back to the fields. They are car- ried from field to field by harvesting machiner}' and from farm to farm ))y thrashing machines. In one county in Oregon the Canada thistle was first noticed where a thrashing machine from the East was first usc(l. It would have cost less than 10 cents to have cleaned the ma- <-hine before it left the thistle-infested region where it had been used in the East, but it would now probabh' cost thousands of dollai-s to exterminate the thistles that have sprung from that introduction of seeds. Disseniindfion in field seeds. — As an impurity in commercial seeds, Canada thistle seeds are found most frequently in Canada blue grass, Kentucky })lue grass adulterated with Canada blue grass, and in the clovers, especially alsikc. If the thistles are cut and thrashed with any of the clovers or grass seeds it is often im])ossible to completely sei)arate the seeds. The i)appus or down nevei- (flings to i^erfect thistle s(»eds after passing through a thrashing niacliinc. The seeds are so nearh' of tlie same color as those of Canada blue grass that tli(y are detected with difliculty, even by trained eyes; but the yell<>\v thistle spines, which are more readily s<'en, indicate when picsent that the seeds may be i)i-esenl also. Canada thistle seeds are sometimes found in wheat an«l oats, but theii- presence in these grains indicates very careless clean- ing, as tliev aic so small that they ma\- l»e easily separated l)y pr(»per SCI'eeniliL'. Distribution in hay. — During the past fifty years the transportation of hay has been one of the most potent agencies in the dissemination of Canada thistle seeds. During the war immense quantities of haj^ 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 railwaj^s. 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. METHODS OF ERADICATION. A great mam* methods for killing the Canada thistle have been de- vised, and many have been described in various publications, 3'et there still seems to be need of a more widely disseminated 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 studj- 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. ]*lowing, if repeated frequently in soils where the root does not descend beyond the reacli 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, Ihe thistle may be destroyed by seeding. The grasses, wherever they are adapted to tiie purpose, will be found the easiest means of destruction, although not so rapid as ])lowing, hoeing, salting, or burn- ing, wliere these latter are available. In all ui)lands, where the soil is of a (h'ptli achnitting the root to be reached and affected in its whole extent by the ]iIow, hoe, fire, or salt, the thistle may be destroyed by these means, and they will be found the most rapid ones. In all bottom lands where the root descends deep and the soil permits access of air, neither the plow, hoe, (ire, nor salt will destroy the thistl(>: licrc the grasses should l)e a]i])1ied, and will be found the best destntyers. 10 Mowing will destroy those parts of the thistle wiiich have thrown up Howering 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 ditlicult 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 tilled 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 the 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 may germinate and not lie dormant. 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, }>ut it will rareh' 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 grubbing twice a year. This plan is recommended for roadsides and waste land, and for meadows and pastures where the methods 'for complete eradication seem to be too expensive for innnediate application. C'anada thistle plants are often killed b\' 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. The rain, keeping the cut surface moist and filling up the hollow stalks, favors the growth of fungi, inducing decay, which often extends down to the root system. A more effective method, especiallx' in djy seasons, is to go over the ground once during every two weeks al'tci' the mowing in .hinc and cut off every thistle about two inchc^s Ix'low the surface with a lioi' or spud. A spud made of a strong, sliarp chisel on the end of a pitchfork handle will be foimd most convenient h)r this work. The second year the spudding should begin as soon as the thistles show in tlu* spiing, and should be continuecl througli tlui season, although theic will l>e few to cut after niidsinnmer. if the work has been well done. 'I'lie land should \)(t looked over occasionallN' each yeai" aflerwaids to detect and desti'oy plants that may spring from dormant se<'ds. Salting thistle plants every week oi- \\\n during two successive grow- ing seasons in pjistures where sheep have access to them, usually destrovs them. 11 Small patches of the plant have been killed by covering them with straw, 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 the}' are likeh- 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 3^ears ; 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 destro3dng 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 thej^ are growing most rapidh- during May and June. None of these substances, except salt, injure the land to any appreciable extent if applied only in sufBcient 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. Thej' 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 b}^ 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 ])e 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 cro]) is removed, then mowcnl and l)urned on the spot as soon as they are 12 dry enough. This treatment arrests the distribution of seeds and, in some cases, it has killed the plants. In shallow, dr}- 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 earlj' in July. If they can not be turned under cleanlj- with chain or jointer, they should be mowed and burned before plowing. The land should be 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 be used to destroy- thistles growing in the hills and others that escape the cultivator. In wet seasons cultivation generalh' 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, rj'e, or winter oats where the winter climate will permit the growth of these crops. These may be pastured during the early spring and then plowed under. Winter wheat and other grain crops that will permit the thistle to remain undisturbed during spring and earh' sum- mer should not be grown. Where the urrill, 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: "1. Cut the thistles wlicii in full bloom [July] as close to the ground as possible. "2. Plow about 'i inches deep and sow millet or Hungarian grass, seeding heavily; harrow. 'I'liis may follow (he i)receding at once or after some two w(M;ks' delay. " .S. In Sei)teml)er plow under the ciop or save it for hay. as desired. At all events, plow and seed libei'ally with rye. " 4. Plow under the rye in May and seed again with millet or Hungarian grass, or i)lant to some hoed crop [cornl and give the most thorough cultivation, with continual seaicliing for and destruction of every remaining thistle. 13 "5. Continue the clean cultivation and sharp lookout for thistles another j^ear." NATURAL ENEMIES. Although the larvae 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 suavfolens, is the most destructive. This often prevents the production of seeds and sometimes kills the plant to the ground. It is most effective during 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 rareh^ exterminates them, and its action is too uncertain to warrant more than a qualified recommendation of its use as a thistle-destroj^ng agent. STATE LAWS RELATING TO CANADA THISTLE. Canada thistle is proscribed as a noxious weed by the laws of the following twentj-four States : California. Kansas. Nebraska. Pennsylvania. Connecticut. Kentucky. New Jersey. 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 majorit}' 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 14 b}' 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 earnestl}- recom- mended that it be proscribed b}' just and comprehensive laws in all of the States where it is at present abundant, and especialh' 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 bj' better ones. Lyster H. Dewf.y, Assistant Botanist. Approved : James Wilson, Secretary of Agriculture. Washington, D. C, June 17, 1901. O n UNIVERSITY OF FLORIDA 3 1262 09221 4864