I've sone this with PVC
pipe. Your flowerpots will come out awesome. Be sure to post some pics.
You can't use any silicone that contains color or antifungal chemicals.
GE Silicone-II clear silicone adhesive is aquarium safe. You can get it at Home Depot, Lowes, etc. Make sure it says 100% silicone, and is clear.
All-Glass makes silicone adhesive too. You can get it at any lfs
, but it's much more expensive than the GE product.