Q.I’ve Sprayed W/ Roundup,leaves Turning Black,do I Trim Each Back & Respray? Need Procedural Advice.
subject is Canadian Thistle

Certified GKH Gardening Expert
Make sure to follow all advice and recommended use for the product that will be found on the product label.
Additionally, certain areas have different procedures on when and how you can apply. Here is an article to help you find the local extension service nearest to you, as they can help you with local regulation:
https://www.gardeningknowhow.com/extension-search
Here is an article that will explain more on correct use of the product, and link you to fact sheets and more:
https://www.gardeningknowhow.com/plant-problems/weeds/glyphosate-information.htm