Now I know I need to get some rest!* :shock:* So sorry about that.*
My Juncus would get sun inside, because, as I said, I would place it in an unheated SUNPORCH!... Not a "garage".
I think the sunporch would work and yes, you should keep the soil moist, even keeping water in the saucer as long as it doesn't freeze.* What you don't want is for the soil to freeze, thaw and freeze again. That's when you risk killing your plant. * In winter the plants will go dormant in frozen ground.* On the sunporch the sun will heat up the air during the day, possibly bringing on a flush of growth.* Then the temps might drop below freezing during the night and cause frostbite to any new growth. I've seen this happen in unheated greenhouses.* As long as the temps don't go below 28*F and then above 45*F to 55*F, your plants should be ok.* I still think sinking the pot into the ground and applying mulch around the rim would be best.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.