We often call many of our spring perennial flowers bulbs. However, botanically speaking, not everything we call a bulb is a true bulb. For example, is an iris a bulb? A corm? Or something else, such as a rhizome or tuber? This was a question that came up recently for Cory, the Greenhouse Project manager/farmer, and me. All of these are underground storage organs for plants. Each can be propagated by division. The general term geophyte includes all of the above.
True bulbs are layered on the inside much like an artichoke and are made up of fleshy modified leaves called scales. They might be onion-like bulbs with dry outer skins called tunics, as in tulips, narcissi, daffodils, hyacinths, garlic or onions. Or, they might be non-tunicate bulbs without outer skins, such as lilies. When cut in half vertically, you would find all the structures you find in a bud — the flowers and leaves. The shoots emerge from the top pointy bit of the bulb and the roots from the bottom, or basal plate.
Corms, while similar to bulbs, do not have fleshy scales. Instead, they are solid and round. While they have a basal plate as a bulb does, they are flatter than bulbs. Gladiolas, anemones, freesias and crocuses are corms.
Although rhizomes are stems, they grow underground horizontally, just below the surface of the soil. The “eyes” or buds grow upward from the underground portion to produce new aboveground stems and leaves at intervals along the rhizome. More plants can be propagated by cutting the rhizome into sections with at least one eye and plant per section. It turns out an iris is a fleshy rhizome as are peonies and lilies of the valley.
Tubers make me think of potatoes. The tuber is the uppermost portion of a thick underground stem that is usually fat and often round. They do not grow horizontally as rhizomes do. They do not have basal plates as bulbs do. They, too, can be cut into one eye per section to create a new plant. The term tuber often covers anything that isn’t one of the above. Examples of tubers (besides the potato) include caladiums, dahlias and tuberous begonias. Oddly enough, daylilies grow from tubers that are long and slender rather than fat and round.
I love the plant world and another horticultural question is resolved. Call them all bulbs or sound like a true botanist and call them “geophytes!”
JoAnne Skelly is associate professor and Extension educator, Emerita at University of Nevada Cooperative Extension. She can be reached at firstname.lastname@example.org.