What's the Difference Between Bulbs, Corms, Rhizomes, and Tubers?

  • 01 of 04

    What is a Bulb?

    Lucky Strike variety of tulip bulb ready for planting, Wales, UK
    Chris Howes / Getty Images

    Both gardeners and plant companies have a tendency to call any roundish, knobby plant root a bulb, but there are distinctions between true bulbs, corms, and rhizomes. For the most part they are all planted similarly, however there are times when you need to be aware of what it is you are planting. For instance, although the rule of thumb for planting bulbs is to plant them 2 - 3 times as deep as their circumference, bearded iris, which are rhizomes, would rot is buried that deeply.

    So, just what do these 4 terms mean?

    What is a Bulb?

    Many of the flowering bulbs we plant in the fall are true bulbs. So are onions. A true bulb is an underground stem with fleshy, scale-like layers surrounding a center bud. Think of the layers when you peel and onion. The scales are food storing leaf bases and they are attached to what is called a basil plate. It's the bottom of the bulb where the roots come out. The center bud is the future flower.

    Bulbs reproduce by forming offset, smaller versions of themselves attached to the basil plate. You can separate these offsets and plant them, to create more plants.

    There are 2 Types of True Bulbs:

    Tunicate bulbs have a papery outer skin that protects the scales, which are the bulbs food source. Onions and tulips are both tunicate bulbs.

    Imbricate or non-tunicate bulbs don't have a papery covering. They remain plump and moist. Lily bulbs are a good example of imbricate bulbs.

    Examples of True Bulbs Include: alliums, amaryllis, daffodil, lily, onions, tulip

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  • 02 of 04

    What is a Plant Corm?

    Anemone Corms
    Corms often look like small stones. You can just make out the hair-like roots at the botton of these anemone corms. Marie Iannotti

    They may look like a pile of stones, but corms are actually very much like true bulbs. Just like bulbs, they are swollen underground stems that store food for the plant during dormancy. Unlike bulbs, corms are solid and do not have scales or fleshy leaves. Since they are solid, the bud, or growing tip, is on the top of the corm, instead of in the center of the bulb's scales.

    As the plant grows and uses up the stored food, the corm shrivels and all but disappears. Luckily a new corm forms, however it might take a few years to build up enough reserves to bloom again.

    Examples of Corms Include: crocosmia, crocus, freesia, gladiolus

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  • 03 of 04

    What is a Rhizome?

    Bearded Iris Rhizomes
    To divide bearded iris rhizomes, just select a portion of the rhizome with eyes or a fan of leaves. Marie iannotti

    Rhizomes are also underground stems, but they grow horizontally - and often quickly. Many plants that we think of as aggressive or invasive, for instance bamboo, grow by rhizomes. But that doesn't make all rhizomatous plants a problem. The bearded iris, shown here, spread slowly and are easy to keep in check. If there's a plant that spreads by rhizomes that you love, you can always grow it safely in a container.

    Examples of Rhizomes Include: bamboo, calla lily, canna, grass, ground ivy, bearded iris, lily of the valley, waterlily

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  • 04 of 04

    What are Plant Tubers?

    Dahlia Tuber
    You can see the eyes on this dahlia tuber. These are the new flower stems forming. Marie Iannotti

    Tubers are yet another type of swollen stem. There is no basil plate and the outside tends to be leathery. Tubers have eyes, or growth nodes, from which the new plants grow. To propagate plants all you would need to do is lift the plant and cut off healthy pieces of tuber, each with about 3 eyes on it.

    Examples of tubers include:  anemone, cyclamen, caladium, dahlia, daylily, peony, potato

    To complicate matters further, there are also tuberous roots, like tuberous begonias. Just as with swollen stems, these swollen roots store extra food for the plant.