ABS cellular-core Pipes Safe?
If the markings on the outside include "NSF-PW" the pipe is rated for potable (drinking) water use. Most "cellular core" pipe won't be so marked, in my experience, since it is only intended for Drain, Waste, Vent (DWV) applications, not drinking water.

I don't know if there's any real hazard to growing plants in them - certainly many plants are grown (or at least started) in plastic pots that are likely not plastic that is rated for eating or drinking from...