Organic Matter !

After harvest, most people just disregard or completely neglect the leavings of the plants !
After you harvest and get your pretty buds, you have the leafs which could be made into hash, and the trunk with all its roots below.
Nothing of this should be thrown away, if you have enough space, you can store the soil used lately for the grow. You leave the roots
there, put the leaves if not used for hash in it, and store it for at least 6 months !
Plant roots will decompose into organic matter in the soil, and will in turn feed your plants, so they should be left in the soil they grew
in just as some of the top growth should be left on the soil to add even more organic matter. You also get the benefits of the soil micro
organisms that were living on the root system of said plants.

Interesting plant photo, It's not Canna. Some sort of bean? Root nodes = Nitrogen fixing points.

You can if you want the work of re-amending your used soil. I simply work it into my Veggie gardens. The wife makes fast use of any I may leave out to long by working it into her flower gardens

OP, roots of any plant are tough to break down. Just leaving them in a pot for 6 months isn't going to have much effect plus decomposition would need continued moisture. Chop them up and throw into a compost pile imo.

I boil mine and add to my fish soups, the lady up the road use her roots as hair in her voodoo dolls

Powdered root
Very good for when someone wants some roots, powdered.