How can you spray paint plastic without it peeling off?

make sure sure the the pot is clean and dry, then use a paint that says it is for plastic, and that should solve your problem.

Use the spray paint from Rustoleum or Krylon for plastic.Spray two light coats allowing to dry in-between

I would lightly sand them with a fine grit paper to clean off the old paint and debris. After you are done sanding, I would wash the flower pots first, let them dry, then spray paint them. There are spray paints by Rustoleum and Krylon that are for plastic and have a built in primer in them which will help. You could also paint them with acrylic paint.

You need to sand down the pot and then use extreme bond primer paint. That's what we do for plastic rain barrels.

Multiple coats as well!