Bulbs, Corms, Tubers, and Rhizomes
Making Sense of the Storage Structures of Summer-Flowering Plants

When many people think of bulbs, they think of spring flowering daffodils and tulips planted in the fall. These are hardy bulbs left in the ground over the winter. However, there are also summer flowering plants coming into bloom now and in the coming months that utilize bulb and bulb-like structures.

Bulbs and bulb-like structures can be put into categories including: true bulbs, corms, tubers/tuberous roots, and rhizomes. A plant’s underground storage structure functions to store nutrients for the plant. The plant can then access these nutrients during times when it is not flowering, like during the winter, hot summers, and droughts. Understanding the difference can help you better care for and propagate your favorite plants.

Take note that most summer-flowering bulbs are not hardy, they are planted in the spring after the danger of frost has passed. In the fall when the leaves turn yellow, most may be carefully dug up and stored in a frost-free location, like a garage. Some bulbs can survive in the ground over a mild winter, but use a very heavy layer of mulch for protection. Daylilies and iris are an exception as they are hardy and can handle cold winters in the ground.

Specialized storage organs, often referred to as bulbs.
Illustration and Caption credit: University of Georgia Extension.

True Bulbs:

A true bulb comprises overlapping layers like an onion. It has a growing point at the top and basal plate at the bottom where roots grow out of. Bulbs are planted upright. The outside of a true bulb forms into “scales”. On tunicate bulbs, the scales become dry and papery, serving as a protective layer around the bulb. On non-tunicate bulbs, like a lily, the scales can break off from the basal plate.

Examples:

• Lilly
• Alliums
• Amaryllis
• Grape Hyacinth

Sprouting bulbs.
Photo credit: Cornell Cooperative Extension Rockland County

Corms:

A corm is an underground solid stem enclosed by a dry outer covering. As with true bulbs, this outer covering protects the inner layers. The plant grows from the top of the corm. Corms only grow for one season, with new corms being produced. New corms are called cormels.

Examples:

• Gladiolas
• Ranunculus
• Freesia
• Crocosmia

Gladiola corms.
Photo Credit: Rutgers New Jersey Agricultural Experiment Station

Rhizomes:

A rhizome is a specific kind of stem which grows near the surface of the soil. They can be determinate or indeterminate, depending on whether or not they terminate in a stem. For example, an iris is determinate because the rhizome produces one stem, while Lilly-of-the-Valley is indeterminate because the rhizome produces many shoots. Rhizomes are planted on their sides. You may be familiar with what a ginger root looks like--it’s a rhizome.

 Examples:

• Canna
• Iris
• Daylilies
• Kniphofia
• Agapanthus
• Calla Lilly
• Lilly of the Valley
• Trillium
• Liriope

Canna rhizomes.
Photo credit: UMass Amherst Extension

Iris rhizomes.
Photo Credit: University of Maryland Extension

Tubers/Tuberous Roots:

Tubers are underground, fleshy, modified stem tissue. A potato is a tuber. Tubers have nodes or “eyes” capable of growing into a separate plant.

Tuberous roots form from the swelling of roots near the base of the plant. They do not have eyes and cannot regrow from root pieces. They lack nodes but can produce buds near the stem.

Examples:

• Dahlia
• Begonia
• Caladium
• Elephant Ear
• Peony

Dahlia tuberous roots planted in April by the Rutgers Master Gardeners of Burlington County
at the Burlington County Agricultural Center.

Photo credit: Alyson Ricketts, Rutgers Master Gardener of Burlington County

References and Additional Resources:

University of Illinois Extension: “Bulbs”
https://extension.illinois.edu/flowers/bulbs

Rutgers New Jersey Agricultural Experiment Station: “Spring Flowering Bulbs”
https://njaes.rutgers.edu/fs1220/

Rutgers New Jersey Agricultural Experiment Station: “Cannas”, Fact Sheet 1155.
https://njaes.rutgers.edu/pubs/publication.php?pid=FS1155

Rutgers New Jersey Agricultural Experiment Station: “Gladiolis murielae-Without Doubt a Beautiful Flower”
https://njaes.rutgers.edu/plant-of-the-month/gladiolus.php

Penn State Extension Chester County: “Bulbs, Corms, Rhizomes, and Tubers”
https://extension.psu.edu/programs/master-gardener/counties/chester/how-to-gardening-brochures/bulbs-corms-rhizomes-and-tubers

NC State Extension: “Summer and Fall-Flowering Bulbs for the Landscape”
https://content.ces.ncsu.edu/summer-and-fall-flowering-bulbs-for-the-landscape

NC State Extension: “Canna x generalis”
https://plants.ces.ncsu.edu/plants/canna-x-generalis/

University of Georgia Extension: “Flowering Bulbs for Georgia Gardens”
https://extension.uga.edu/publications/detail.html?number=B918&title=flowering-bulbs-for-georgia-gardens

University of Tennessee Agricultural Extension Service: “Flowering Bulbs for Tennessee Gardens”
https://www.shelbycountytn.gov/DocumentCenter/View/1120/Flowering-Bulbs?bidId=