Not all bulbs are bulbs: Understanding the different types: True Bulbs, Corms, Tubers, and Rhizomes

Not all of what we home gardeners call bulbs are—technically speaking—bulbs. Horticulturists identify different structures for different flowers, and these terms highlight the great variety we find amongst our favorite flowers. 

A true bulb has a plate from which roots will shoot, and is made up of several scales or layers like garlic bulbs that contain nutrients for its growth. True bulbs include daffodils, alliums, hyacinths, and tulips. Corms, like crocuses and gladioli, have a similar plate of roots, but no scales; they are a solid, compressed food storage unit. Tubers, like dahlias and begonias, bud from an enlarged rootlike structure which multiplies underground like potatoes, while rhizomes, like canna lilies and lily-of-the-valley, are horizontal structures that look like swollen stems.

The horticultural name for bulbs—for all true bulbs, corms, rhizomes, tubers—is geophytes. It’s fitting, as “geo” means “earth” and “phyte” means plant. These plants of the earth all have underground storage organs from which they will grow.

Hardy versus Tender

Much like Minnesota retirees, the most important distinction amongst geophytes is where they will spend the winter. Do they like the cold or will they need warmer weather?

Hardy bulbs are tough enough to survive outside in the wintertime. In fact, that cold snap is vital! Because of this, hardy bulbs are planted in autumn and will bloom for the first time the following spring. Hardy bulbs are often the first flowers to appear in spring: snowdrops, daffodils, crocuses, alliums, and more. They must be buried to the appropriate depth, and in an area with good drainage. Although hardy bulbs can stay in the ground indefinitely, they do require some maintenance. Every 3-4 years, hardy bulbs may need to be dug up and divided to prevent overcrowding. 

Tender bulbs are too delicate for winter and must be removed from our gardens before a frost sets in. Stored indoors during winter, they can be planted again in late spring for late summer or autumn blossoms. Canna lilies, begonias, freesia, gladiolus, and dahlias are amongst the most popular tender bulbs. Learn how to care for bulbs and rhizomes over the winter to prevent problems and rot.  Take care when digging up bulbs, clean the soil from them gently, and in some cases, let them cure for a week in the sunlight before storing them in paper bags out of the light during winter months. For more information, read this article from the U of M Extension.

If you want to plant hardy bulbs, you will have to wait for the fall. But you can still plant tender bulbs this spring for enjoyment during the summer.

Resources:

U of MN Extension: Planting Bulbs, Tubers, and Rhizomes.  https://extension.umn.edu/how/planting-bulbs-tubers-and-rhizomes#soil-preparation-1410662

University of FL: Geophytes. https://propg.ifas.ufl.edu/07-geophytes/01-bulbs/02-geophytes-bulbs.html

Photo Credits: U of M Extension (1,2), www.flickr.com (3)