Yellow starthistle is a horrifying invasive weed that outcompetes natives, is covered in horrible spines (so strong that they stick in car tires and are distributed on roadsides), and is fatal to horses (once it becomes ~60% of their diet). I have waged war against it for one summer and I have it in my sights for the next 6-10 years until the seed bank is fully depleted.
Update: for a serious infestation, spraying seems like the clear winner (March/April should let them see the rosettes as they walk around with their sprayers) . Some counties will have cost-sharing programs.
After weeks of googling and talking to land management folks at different government agencies as well as local master gardeners, I am confident that this is the best guide for understanding and managing a yellow starthistle infestation:
This is an area where I have a surprising amount of expertise, so really do reach out if you have questions. I don’t have easy solutions (it will take years to get a handle on a problem like this), but I have a very good understanding of the problem at this point and I also have recommendations for strategies, technique, and equipment.
Though I really think I/we should focus mostly on building awareness — we all have a common enemy here and we need to train our friends and neighbors to spot the plants:
The way I recognize young plants is mostly by color — they’re an eerie light blue-green shade in the spring and early summer, which contrasts with the surrounding native grasses. And once the native plants start to brown, the yellow starthistle stands out like a sore thumb.
The first best solution is always hand-pulling—which is easy because they tend to start off in disturbed soils. If you get them as soon as the first few plants show up on the roadside, it’ll be easy to keep ahead of them and prevent them from ever going to seed. Even once the tell-tale spines start showing up, you should have a couple weeks before they get viable seed (though I still generally put them in thick contractor bags and they should never go in the green bin!)
Their seeds aren’t spread by the wind, so unless there are horses around or other animals, over 90% of the seeds that do fall will land within a 2ft radius of the mother plant, so you’ll want to flag the places where you’ve seen those plants and go back to that spot year after year so that the patches don’t slowly expand outward.
The problem is that they are typically ignored until they’ve overtaken entire fields, or entire roadsides, and then you have to resort to more aggressive and time-consuming treatments. They are annuals, but their seeds lay dormant in the ground for many seasons, so it will take a long time to deplete that “seed bank” even if you prevent all plants from going to seed once you figure out you have a problem.
These are some of the very useful diagrams from the PDF I linked above. They help you actually understand the plants well enough to make game-time decisions about what to do.