Ask Mr. Smarty Plants

Once and for all is probably beyond us, but eventually, with work, we can try for.

You truly have our sympathy, but it may be beyond difficult to eradicate a weed that has held sway for 50 years, and probably long before that. We can't be sure about the identification as Scotch thistle, either, but can tell you what we found out about it. Onopordum acanthium, Scotch Thistle, also known as cotton thistle or wooly thistle, is a biennial that can grow up to 12 feet tall. According to this Texasinvasives.org Invasives Database Scotch Thistle, it is often confused with several other thistles:

Cirsium arvense, Canada thistle-native to Europe, has a deep and wide-spreading root system, flourishes in cultivated and disturbed ground

Cirsium vulgare, Bull thistle- native to Europe, western Asia and North Africa

Carduus nutans. Musk thistle - native to western Europe

Obviously, all of these plants are non-native and therefore out of the range of our expertise. The Lady Bird Johnson Wildflower Center is dedicated to the growth, propagation and protection of plants native not only to North America but to the area in which the plant is being grown. However, we are always interested in invasive plants, whether they are native or non-native.

From what we read on the websites we have referred you to, it is more likely that the Cirsium arvense, Canada thistle (misnamed, it came from Europe) is the culprit in your case. The others are all annuals and biennials, spreading by seed. Theoretically, keeping those thistles mowed as low as possible and preventing going to seed would eventually eliminate them. However, seeds can be viable in the soil for up to 20 years, so seeds dropped long ago would probably keep germinating. However, a plant that spreads vegetatively, from roots, can continue to put up new shoots almost indefinitely.

What you have to do in a case like this is to starve and/or kill those roots. Spraying the upper plant, as you already observed, is to no purpose and can likely kill other, desirable plants. Be sure and read all the websites we gave you for their ideas, but here is what we would suggest; whether you want to actually tackle it is up to you.

First step, mow thistles as low as possible and OFTEN. It's not enough just to prevent seeding; those leaves that are above ground are producing food and keeping the roots alive. So, down with the weed, every last leaf, frequently. Attack the roots directly by first cutting each one below the surface of the soil. Immediately, within 5 minutes, paint the stub with a broad-spectrum herbicide and a disposable sponge brush. Try to avoid getting it on anything else, and do it within 5 minutes of cutting, to keep the stalk from healing over and preventing the herbicide from getting to the roots. It won't happen right away, and you probably will always have the plant to deal with, but eventually they will be slowed down and become more manageable. Whatever you mow or cut down needs to be bagged and disposed of in a landfill to prevent any further seed spread.