@armando: The foil is filled with very very wet moss to begin with -- so that a squeeze in the middle of the packet oozes water out each end. The twine is tight but not excessively so. The drying wouldn't result from leakage, I don't think, as much as from the roots using the water to do their thing. I haven't ever added water, but if the root development took more than six weeks or so I would probably just use a needle and syringe or poke a turkey baster under the upper-end twine.
@James: Yeah, squirrels have been a big problem around here... until one of my neighbors put out a bait station with very weak anti-coagulant in it. The squirrels have to feed on the stuff day after day before they go off and self-bury. We have a lot of coyotes and wild cats around here so people tend not to use any poison that might kill one of the natural predators. A couple years back, however, the whole neighborhood was swarming with them. I tend to sprinkle cayenne pepper (powder) here and there to discourage squirrels and rabbits as well.
@HarveyC: I have several fake owls -- the ones with the heads that rotate with the shifting winds. I suppose one way to discourage bird-pecking at the foil bundle would be to "whip" the bundle in the twine -- not so hard as to really squeeze it, but enough to frustrate the birds.
Thanks, everybody, for the welcome and kind comments.
Ken.