Canadian Thistle
Aiton's Encyclopedia (1910)
a well known and troublesome weed. It is an emigrant from Europe, where it is known as the field thistle. It is the most slender of all our thistles; the flowers are rose-purple; the heads are small and numerous. The root-stocks creep and interlace, and form extensive mats that defy the plow. The Canada thistle is a plague in old fields, pastures, and waysides. Nothing else can grow where a patch has taken possession, and it is the most difficult of all weeds to eradicate. Quack grass has more vitality and is harder to kill, but it lacks the sharp, prickly leaves. Quack grass affords valuable pasturage, but the thistle is an unmitigated nuisance, not to say, a torment, for stock avoid it like a plague. There seem to be three common sense ways of exterminating the pest,--cultivating, smothering, and salting. The stalks may be mowed, raked, and dried to clear the way for the team. The thistle sod may then be turned over. If disked early and late and between times through a dry season, literally chopped to pieces and so frequently that no green blade peeps up, the thistles can be killed out. The chances are, however, that a rainy spell or a little neglect during a busy time, as haying or harvest, will enable the thistle to reset worse than ever. The whole secret lies in the principle that no ordinary plant, weed, or grain can live many seasons if prevented from forming leaves. The second method is dependent on a similar principle. No ordinary plant can live if kept in the dark. If a straw stack or manure pile be left on a thistle patch for a sufficient length of time, the thistle will rot in the ground. A covering of tarred paper, weighted down so that the cracks will not blow open, will answer the purpose. Smothering is a practical plan for small patches. The third method is dependent on the principle that ordinary plants cannot live in alkaline soil. If enough salt or ashes be spread on the thistle mats to soak the soil with brine or lye the thistle will die, and that right speedily. Subsequent rains will leach the salt or lye out of the soil and distribute it so widely that the field will be none the worse, but rather the better for farming.
What Has Changed in the Last Century
Aiton's 1910 remedies were cultivation, smothering and salting; the salt cure in particular is now rejected, since sodium sterilizes soil far longer than the writer's confidence that "subsequent rains" would leach it away harmlessly. The decisive change came after 1945 with selective herbicides — 2,4-D, and later clopyralid and glyphosate — which finally gave farmers a tool against the creeping root-stocks that "defy the plow," though repeated treatment is still needed because the plant regrows from root fragments. The species, now known as Cirsium arvense and still misnamed after Canada despite its Eurasian origin, is listed as a noxious weed in most US states and Canadian provinces and ranks among the world's worst agricultural weeds. Modern control combines herbicides with the old principle Aiton's grasped correctly — starve the roots by never letting leaves persist — plus introduced insect enemies and competitive pasture.
Written for this edition, 2026.