Question
Gentlemen, now to my next question:
Is it better to let the water run through the bog during winter dormancy, or
shut if off and let it (partially) dry out? I pump the spring water up to the bog
and can easily turn this off.
3) Finally, I plan on treating the plants with fungicide before I cover them with
mulch (I live in zone 6). Is it better to first cover the bog with a tarp and then
mulch, or vice versa?

Answer
The main issue during dormancy is making sure the soil never dries out.  You know your weather system better than I do.  If it rains or snows a lot where you live, then you probably could shut off the water and let nature take care it.  If your winters are generally dry, then you may need to keep the water on at a trickle to make sure it doesn't dry out.

I'm not a big fan of using tarp over plants for an extended period of time.  Use burlap fabric instead.  You can find it at many garden centers.  After spraying your plants, place the burlap over the ground and then add the mulch.  You may need to spray the burlap as well.  Iv you have lots of wind in your area, you may need a second layer of burlap over the mulch.  The burlap allows for air exchange.

Good growing!
Jacob Farin