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

Most gardeners do the same thing: We all tend to lump together plants that grow from bulbs, corms, rhizomes and tubers and call them all bulb plants. Plants like tulip, iris, crocosmia, lily, daffodil, and more have these swollen parts that are the underground storage rooms for the plant. Calling them all a bulb is not exactly accurate, but do you know what the difference really is?

Bulbs, corms, rhizomes and tubers are all considered geophytes. Geophyte is the collective term for the type of plant structure that stores water and nutrients in an enlarged underground part of the plant. Most gardeners - myself included - tend to refer to most geophytes as simply bulbs because, well. . .bulb is easier to say and remember than geophyte.

When it comes to recognizing and understanding the difference between bulbs, corms, rhizomes and tubers, it isn't as difficult as you would think.

Recognizing a Bulb

If you look at a bulb, it resembles an onion. . .because an onion is actually a true bulb. Bulbs have layers (remember, think onion). Bulb plants include tulips, narcissus, alium, amaryllis, blackberry lily, caladium, canna and lilies.

Recognizing a Corm

A corm is a geophyte that is an enlarged, solid base. If you cut open a corm it is solid, unlike true bulbs that are marked by layers. Corms can be propagated by cutting the corm into sections and planting. Each corm section will then root and grow a plant. Crocosmia, crocus, anemone, gladiolus are all examples of corm plants.

Recognizing a Rhizome

Have you ever dug up and transplanted your iris plants? Bingo! You've seen rhizomes then. Rhizomes are those swollen stems on the iris that grow horizontally underground. They grow shallow and will put out leaves and shoots. Besides the iris, other rhizome plants include ginger, ferns, Venus flytrap, bamboo, Chinese lantern. Like corms, rhizomes can be broken or cut into sections for propagation. When planted, each section will grow and develop leaves and shoots.

Recognizing a Tuber

Now we come to the tubers. Tubers are pretty much those plants that are geophytes but don't really fit in as a bulb, corm or rhizome. Tubers are generally separated into two categories - stem tubers and root tubers. Stem tubers are formed when underground parts of the stem swell (like potatoes and begonias) while root tubers are formed when root sections swell (like dahlias and cassavas). Common tubers would be potatoes, gloxinia, dahlias, begonia, cassavas, cyclamen.

The next time you are faced with a geophyte, be sure you know the proper terminology and don't just call it a bulb.

Sources:
http://cemadera.ucdavis.edu/newsletterfiles/Master_Gardeners_Articles4072.pdf.
http://en.wikipedia.org/wiki/Tuber
http://ag.arizona.edu/pubs/garden/mg/botany/plantparts.html