I'm starting canna bulbs for the first time. Rather than filling up my 16" styrofoam pot with "soil", i need to know approxiately how long the roots will get so that i can put light weight materials in the bottom, like packing peanuts & sections of plastic milk jugs.

I'm planting 5 of Burpee's dwarf mix & in the catalog they show a picture of this mix in a pot that looks like it is maybe 9" deep, but i would rather not guess.

Thanks in advance for your help! I so appreciate being to come here for gardening advice!