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

For a lot of newly sprouting gardeners, if you are planting something underground that is larger, rounder, and more swollen than a seed, it must be a bulb. But, as many of us know, sometimes it’s not. Sometimes we grow from corms, tubers, or rhizomes—but what’s the difference?

What are Bulbs?

True bulbs, like tulips, daffodils, and hyacinths, are enlarged stems that actually contain the embryos of the new plants. So, if you cut it open and took a peek inside just before the growing season, you’d find a little baby flower and stem surrounded by onion-like layers that keep that flower baby fed until it grows up nice and strong.

Some bulbs are like onions and have a papery outer layer called a tunic that helps to protect the bulb from drying out before it gets into the ground. Other bulbs, like lilies, do not and can have a tendency to dry out and bruise easily. However, tunic or not, most bulbs are typically round in shape, and most have a pointy end on top which is where the plant sprouts. They also have a flat bottom, called a basal plate, where the roots come from. This is also where new bulbs— “offsets” or “daughter bulbs”—will form, too.

Probably the most distinctive feature of all for bulbs, though, is the layers of scales. Much in the same way that tree trunks contain rings that can tell you how long they’ve been standing, bulbs also form new layers of scales, which you can see by cutting it open (although, we don’t recommend slicing up a bulb if you’re hoping to grow a flower from it).

What are Rhizomes?

Rhizomes, like irises, cannas, and lily-of-the-valley, are longer, thickened stems that grow sideways along the surface or just below it. Rather than springing forth from a pointy end on top, plants grown from rhizomes will sprout from eyes that form along the top and sides. And when they are ready to multiply, they don’t just duplicate themselves. They simply branch out, forming roots and shoots of their own, which can be cut apart and divided so long as each section holds at least one eye and one plant.

What are Tubers?

Tubers, like potatoes, caladiums, and anemones, are also thickened stems that are fat and round like rhizomes, but they don’t grow horizontally. Also like rhizomes, they grow new shoots from eyes along their surface, and they don’t form any offsets. Instead, tubers tend just to get bigger and bigger every year, forming new eyes as they go.

What are Corms?

Corms, like crocosmias, gladiolas, and crocuses, are a lot more like bulbs but without the layers of fleshy scales and pointy tip. Instead, they have a slightly rounded or flattened top that can often have more than one growing point and are made up of solid stem tissue that won’t show any rings when cut open. Also unlike bulbs, instead of reblooming year after year and forming more daughters as they age, corms simply die off after they bloom and won’t produce another flower. However, before they go, they will leave us with daughter corms—cormels—that form along the tops and sides that will bloom in years to come.

Though they each have fine-tuned differences that set them apart from each other, one thing we can say about all of these thickened stems is that they all give us some pretty amazing plants, no matter which you are growing from. Although, wowing your gardening friends with your knowledge of the subtleties and differences among each type certainly never hurts!