First some personal observations about some of the universal truths of weed control.
1. "Control" usually doesn't mean eradication. Total removal of a localized weed infestation is possible in many situations, but usually not. However, thinking "eradication" during weed control work is good for the soul.
2. Any method, or methods, of weed control of invasive, difficult to control weeds must be applied repeatedly over a number of years. No single treatment will totally eliminate any of these bad guys. If the correct methods are being used, the job will get smaller each year.
3. To be effective, weed control must eliminate seed production EVERY year.
4. Relying totally on one control method is usually much less effective.
5. For many of the widespread weeds, biological control is the only real hope for region-wide control.
6. Not all weeds are widely spread by the wind or wildlife. This means that it isn't always your neighbors fault.7. Roundup (glyphosate) is almost never the best choice of herbicide to use for chemical weed control. It also kills the plants needed to compete with weeds.
The photo is a young knapweed seedling at a very fragile stage of life.