Richard: Casoron is quite effective on horsetail but will seriously damage/kill grass, herbaceous  broadleaf plants, and bulbs.  It is safe around most established woody plants.  But read the label for the few woody plant exceptions so you can avoid treating their root systems.

It is best applied to the soil surface  November through late February. With the current weather pattern, we are past being able to use it that way.

As a long shot, you might be able to get some activity if you apply it and immediately cover it with two or more inches of mulch. Then water the mulch to release the chemical and hope that it gets into the soil before it evaporates.  

You can't plant grass, herbaceous plants, bulbs, etc. in the treated areas for at least a year after the treatment.

If you have it in areas that can't be treated (lawns, flower beds, etc.) you will need to remove each shoot every day right after they show up. Some catch-up may be needed. This takes incredible effort but is possible. By taking away the green stems, you are slowly starving the plant of carbohydrates, which slows stem and root growth. But this effort can work if the space is somewhat small and the focus and intensity to do this for two to three years is there. You get time off in the winter! 

If I am not understanding your situation, please reply to this answer or you can contact me directly at <personal data hidden>

Chip Bubl OSU Extension Agent/Columbia County (retired)