Cirsium Arvense: Canada Thistle, Creeping Thistle, Field Thistle, Common Thistle, California Thistle, Corn Thistle, etc...
The Canada Thistle is known by many different and colorful names in many different regions, and is gererally considered a nuisance weed and an invasive species to those enterprising in agricultural and pastoral pursuits. Not to be confused with many similar plants within the Cynareae tribe, such as the nasty Bull Thistle, the spikey flowered pride of the Scots. The Canada Thistle is of European and Mediterranean origin and has propagated itself throughout much of the temperate climates of the world, accidentally, as a contaminant in cereal crop seeds. The Missouri Department of Conservation describes this weed in detail:
"Canada thistle is a 2- to 5-foot (0.6 to 1.5 meters) tall forb with deep, wide-spreading, horizontal roots. The grooved, slender stems branch only at the top and are slightly hairy when young, becoming covered with hair as the plant grows. The oblong, tapering, sessile leaves are deeply divided, with prickly margins. Leaves are green on both sides with a smooth or slightly downy lower surface. Numerous small, compact (3/4 inch or 1.9 cm diameter), rose-purple or white flowers appear on upper stems from June to September. Seeds are small (three-sixteenths of an inch or 0.5 cm long), light brown, smooth and slightly tapered, with a tuft of tan hair loosely attached to the tip." [ 1 ]
What really makes the Canada Thistle a serious pain-in-the-ass is it's ability to propagate itself through it's deep taproots. A single plant, if allowed to grow undisturbed from a wind-born seed will grow a slender taproot to a depth exceeding a foot which will then continue to grow in a more horizontal manner. This rambling horizontal rootstock will then grow many buds along it's length which shall result in an abundance of new shoots in the following years. On the soil's surface, it will appear as infestation of multiple thistle plants where these shoots may be originating from the same root stock.
These second-year shoots will flourish in the Springtime, generating the carbohydrates to feed and grow the rootstock. A gardener may pull the plant, and may succeed in pulling up a good portion of the sub-soil shoot, but the taproot will be undisturbed and the shoot will re-emerge in a short time. What is even more devilish, damage to the shoots can cause the rootstock to develop new buds and many more shoots will then emerge from the soil. To the average gardeners, the careless lax of weeding in the summer can lead to beds full of prickly shoots which disturb the delicate rows of vegetable seedlings.
A large, neglected field may be choked out with Canada thistle in this manner, often with other undesirable vegetation with their own methods of domination. Such a field can be reclaimed either chemically, mechanically, or with a beneficial competitor cover crop. Applications of Glyphosate during the hot summer months, timed shortly before the emergence of the plants' purple flower heads, will be absorbed throughout the rootstock and will eradicate an infestation within two seasons. Mechanical destruction of new shoot growth during the spring time, either through repetitive mowing or tillage will eventually exhaust the stored carbohydrates in the rootstock. This can take several seasons before the rootstock is destroyed. A cover crop with rapid vertical growth during the same period that Canada Thistle shoots emerge will out-compete the Thistle. Spring Alfalfa is such a crop. The rootstock will be forced to grow tall, spindly shoots which will be unable to replenish the carbohydrates spent in the growth phase. In this manner, the rootstock will starve and die in a few seasons.
The average gardener who is unwilling to forgo multiple fallow seasons would be best off not to neglect the first year growth of Canada Thistle in the first place. If, however, multiple year growth takes a-hold in the garden, there is no substitute for regular removal of new shoot growth in the Springtime. Shoots directly in the beds can either be removed by hand or with hand tools. New multiple shoots will likely emerge. Plan the spacing between rows so that a tool such a stirrup hoe can easily work between rows without damaging the shallow roots of the vegetable seedlings. Eventually, the taproots will exhaust themselves and will expire even though this may take more than a single season.
There is no substitute for regular weeding. You deserved it.
Reference: [ 1 ] http://mdc.mo.gov/your-property/problem-plants-and-animals/invasive-plants/canada-thistle-control