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. While they are usually planted similarly, there can be important differences. For instance, although most bulbs should be planted two to three times as deep as their circumference, bearded iris, which grows from rhizomes, would rot if buried that deeply.
Many of the flowering bulbs we plant in the fall are true bulbs. A true bulb is an underground stem with fleshy, scale-like layers surrounding a center bud. The scales are food-storing leaf bases and they are attached to what is called a basal plate (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 basal plate. You can separate these offsets and plant them, to create more plants.
There are two types of true bulbs:
- Tunicate bulbs have a papery outer skin that protects the scales, which are the bulb's 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, and tulip.
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, though it might take a few years to build up enough reserves to bloom again.
Examples of corms include crocosmia, crocus, freesia, and gladiolus.
Rhizomes are also underground stems, but they grow horizontally (and often quickly). Many plants that we think of as aggressive or invasive, such as bamboo, grow by rhizomes. But that doesn't make all rhizomatous plants a problem. The bearded iris, shown here, spreads slowly and is easy to keep in check. The best option for growing rhizomatous plants is to keep them in a container or raised bed.
Examples of rhizomes include bamboo, calla lily, canna, cast iron plants, grass, ground ivy, bearded iris, lily of the valley, and waterlily.
Tubers are yet another type of swollen stem. There is no basal 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 need to do is lift the plant and cut off healthy pieces of tuber, each with about three eyes on it.
Examples of tubers include anemone, cyclamen, caladium, dahlia, daylily, peony, sweet potato, and potato. Tuberous roots, which are similar to plant tubers, are also swollen stems.
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, but they do not propagate the plant.