Colder temps definatelly slow the growth, but is still possible to finish. Consistent really cold temps can kill a plant. Here in the desert my flower room is an outdoor shed. I keep the heater on at night, and it gets down to 50 just before lights-on. Slow's 'em, but doesn't stop 'em. (well, unless yor heater takes a dump, which happened this winter)

Couple of questions...
How cold does the garage get? No heater?
Did you switch to a bloom fert when switching to 12/12 light schedule? Which fert and ammendments do you use? How often?
Do you use tap water? What's your water ph? ...run-off ph?
Any pix of the grow?
If the seeds you made are germinating and growing to the bloom stage, likely they are fine.

Some cannabis stalks are pithy, (The soft, spongelike, central cylinder of the stems of most flowering plants, composed mainly of parenchyma.) Some are hollow. Depends on strain, and is acceptable either way. Keeping a fan on 'em strengthens the stalks, and keeps air from getting stagnant.