Ask Mr. Smarty Plants

Juncus effusus (common rush) is native to nearly all the states and some Canadian provinces in North America, including Texas and Pennsylvania.  We are a little confused; was your question should you leave it outside and leave it in the pond? When you say "let" we think more of letting the dog in, so should you let the plant in? 

Juncus effusus (common rush)  is hardy from USDA Zones 4 to 10. It appears that Mountville PA, Lancaster County is in Zones 6a to 6b, so the plant should do fine there outside. Juncus effusus 'Spiralis' is a selection for horticultural use, low growing with strongly spiraling stems. According to this Floridata article on Juncus effusus, it is really better to allow the plant to freeze back, remaining outdoors during the winter, because the stems (there are no leaves) will come back much stronger and better-looking. In milder climates (such as indoors) the stems will get brown and less attractive-looking, and will need to be trimmed. However, this page from Blue Stem Nursery on Lesser Corkscrew Rush refers to it as a plant that can be kept in a terra cotta pot and double as a winter houseplant. We would imagine the soil in the pot would need to be kept pretty moist, but we did see a number of illustrations of the plant in pots and indoors. 

As to whether to leave it in the pond outdoors in winter, that probably depends on how hard your pond freezes. The plant needs to be kept moist, frozen or not, so you will have to use your best judgment on whether to pot it up and bring it where you  can water it outside if it's not getting enough moisture, or bring it inside and give it moisture. 

Pictures of Juncus effusus 'Spiralis'