My suggestion would be to ask a local contractor where he thinks the frost "line is". You might be suprised how far down you'll have to go? Don't let this years mild winter fool you. My guess would be a little over two feet?

I am trying to do the same thing. In Zone 5, the frost line is 4 ft.

The only reasonable solution is to use something that will not crack when frozzen. I talked to a couple of people. One just used a good quality garden hose, the other 'green line' polyethylene tubing.

Last year I tried a good ruber hose. Left it above ground, full of water, all winter. No visable damage. This is a thick hose and even in full sun it does not seem to expand like the cheap stuff. Polyethylene is even stronger, but less flexible.

I am not sure what to use either but will try something this fall.