Rhizomes, Root Tubers and Stem Tubers, Say What?

Last week, we removed our cannas from flower beds where we are now planting tulips and have stored them for the winter. As I was doing a little botanizing (read: plant nerd with a curious brain and a lunch hour to kill), I discovered something I had neither heard of nor learned long ago in some dusty university lecture hall. There are such things as stem tubers (such as a potato), and root tubers (such as a dahlia). I had been operating under the assumption that a tuber was a tuber was a tuber: a modified stem used to store energy for the plant. Yes, a stem tuber like a potato is not a root, but actually a stem! Who knew? The potato eye is a node, where one or more leaves meet the stem.

Do not confuse a root tuber with root crops such as radish, carrots and turnips. Those crops have tap roots. They are shaped differently, but serve the same purpose: to store energy. Root tubers, on the other hand, are swollen portions of a root also used to store energy. Think of a dahlia root or some varieties of day lily. The root is thin, swells and gets thin again, forming on a fibrous root system.

Esoteric? Mind numbing? Perhaps, but if you have read this article to this point, you are a hooked plant nerd so how about a bit more? Back to cannas and tubers. Cannas are neither stem nor root tubers. They are rhizomes, or underground stems like stem tubers, with a few key differences in morphology, but easiest to remember is rhizomes grow horizontally underground.

All these root and stem adaptations function to store energy, which is key to my final point- storing these tender perennial species for winter. There is great information out there on how to dig and store cannas and dahlias, but I do want to pass on two thoughts. First, it is recommended to leave cannas in place until frost kills the tops off. I am not sure how critical this is really. If you pull cannas a bit early, fine-but know you might get some leaf regrowth. More important is the curing process and storage conditions later (see above links for details). Second, I would use a spading fork when digging up any plant you hope to transplant, divide or store, regardless of soil type. Using a spading fork rather than a shovel to leverage the plant loose tends to preserve more of the root system intact where a shovel might slice or shear too much.

Josh Steffen
Horticulture Manager

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