RE: Box elder problem
I happen to be in the brush control business so hopefully my advice is based on real world understanding.
You have a couple alternatives to explore. I assume you do not have a restricted use permit.
1) Cut trees and treat stump. Your local coop will have or can get a product called Tordon RTU. It is not resticted and is formulated for cut stump treatments.
2) Basal Bark: You can buy a product called Crossbow. Read the label and do EXACTLY what it says in the Crossbow+Diesel Basal bark section. Basal bark involves spraying the lower 15 inches of the trunk all sides and any exposed root surfaces. This can be done any time of the yaear except when you have snow cover and will kill the tree.
Round up will not work effectively or consistantly. Arsenal would be the LAST thing Id ever use, its made for control of woody underbrush in pine plantations or in special situations ahd has many drawbacks in the hands of an amature. 2,4-D wont work.