Answer: The herbicide Clopyralid is a restriced use chemical, not available to homeowners.
Herbicides such as glyphosate are the best choice for killing Canada thistle. Glyphosate is absorbed through foliage and translocated through the entire plant, roots and all. Glyphosate should always be applied to plants that are in a vigorous state of growth. Glyphosate is a nonselective herbicide, which will kill all plants, so care should be taken to avoid damage to desirable vegetation. Herbicides may be applied by spraying or by direct application with a wick or brush to minimize off-target effects. Effects may take two weeks to become noticeable, longer under cool or cloudy conditions.
Goathead weeds, so called because of their spiny seeds, are more correctly called "puncture vine." This is important to know because you will find no herbicides labeled for "goathead", rather you will find them for "puncture vine".
Pre-emergent herbicides labeled to control puncture vine can be applied in the spring before the puncture vine weeds begin to germinate. Such herbicides work by killing the seedling as it germinates. They will not kill existing plants, only those just germinating from seeds. Read and carefully follow the label directions to get the maximum benefit.
Once the puncture vine has germinated and is growing, a "broad-leaf" (post-emergent herbicide such as 2,4-D) may be used. Again, follow the directions. You can supplement this with manual removal (digging or pulling the weeds). It is important to prevent the weeds from forming seeds, so diligence is required. If seeds do form successfully, that is the source of a problem for subsequent years. Even if a few plants do manage to form seeds, if you can limit the number formed you can reduce next year's problem. Since the seed can remain viable in the soil for several years, you must continue vigilant management until no live seeds remain in the soil to create problems.
For more detailed information and recommendations specifically for your area, contact your local Cooperative Extension Service Agent or Master Gardeners. Many nurseries and plant centers can also provide information and help you read and understand the herbicide labels so that you can select one appropriate for your location and conditions.
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