Bulbs, Corms, Rhizomes and Tubers: What’s the Difference?

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Have you ever wondered what’s the difference between bulbs, corms, rhizomes and tubers? They are all terms for plant adaptations that allow life to be stored in a dormant state during less-than-ideal growing conditions. They are like little packages; some packages store food to set the plant into growth when conditions are right, while other packages contain a baby version of the plant itself. Here is a quick description of how the different adaptation units work.

Bulbs: These are just like miniature plants encased in a protective home. Inside the bulb are the baby leaves, the flower stalk and the flower head, all reduced to the basics and usually packaged in a papery container. Think tulips and daffodils.

Corms: Unlike bulbs, these are food storage “warehouses” filled with all the nutrition the tiny bud of a plant inside needs to burst into growth. The surrounding warehouse will feed the plant until roots grow, supplementing nourishment. Think gladioli, like the yellow flower below.

Rhizomes: These are swollen stems that store meals for hungry plants coming out of dormancy. Think bearded irises, like the pink flower above.

Tubers: Just as rhizomes are storage stems, tubers form rather lumpy shapes that feed sprouts or “eyes” until the plant matures. Think potatoes.

Nature’s adaptations not only help plants survive less-than-ideal seasons, they also give you some guidance about planting. The rule of thumb for bulbs is to set them at a depth that is at least two to three times their size. Set the narrow growth ends pointed upward toward the sky. Corms are planted in a similar way. Both want to be safely nestled underground but they don’t want to have to push new top sprouts through too much soil before reaching light. The bigger the corm or bulb, the deeper it can go.

Since they’re adapted from stems, rhizomes are happiest when planted closer to the surface. Some will even tolerate showing their upper sides to the daylight. Check for little growth buds to guide you to which way is up. Tubers, in general, can be planted a bit deeper. Since they don’t have an obvious top, look for small “eye” spots where sprouts will begin to protrude, or a slight indentation to place toward the top. The “up” side is less important than with bulbs and corms.

Of course, there are exceptions for every rule. For example, the big amaryllis bulb should be potted half out of the soil, and the tuber of the tuberous begonia needs to be set close to the soil surface. Read directions on bulb packages to be sure you are planting at the right depth.

Bulbs, corms, rhizomes and tubers are easy to plant since they are so neat, clean and portable when dormant. Plant them in groups for the biggest show of color. Since they are easily dug up and stored after the foliage dies down, you can even use tender varieties for gardens in cold-winter climates.

Photo credits: Jane Gates

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