ridgeballs can only be bred from other ridgeballs.

try mutating a multiflora, you have a 50/50 chance it will mutate into a ridgeball (or a weeper); or if you get lucky and see one in the store, buy the ridgeball from the store.

If you have VST, it gives lots of breeding and mutation advice on the chart page.

Click HERE to download the latest version of Virtual Seed Tray for Plant Tycoon (warning: This link takes you to a note that has spoilers in it. To download the program without reading the spoilers, read the non-spoiler information at the top, and keep reading until you reach a point that warns you of spoilers. then scroll down quickly to the bottom of the note while looking at your shoes. At the bottom of that page you will find the link to download the program.