A close-up view of a dahlia tuber clump

Part 1: What Makes a Tuber a Tuber?

How Dahlia Roots Are Different, and Why It Matters


Copyright © 2025 by Steve K. Lloyd
All Rights Reserved

All scientific claims and conclusions presented in this article are thoroughly documented with direct references in Part 7: For Curious Readers & Researchers .

I am a gardener who grows hundreds of dahlias each year. I also sell tubers online, shipping them to customers across the United States every spring.


Until recently, I thought I had a pretty good handle on how dahlia tubers are formed and what growing practices promote or inhibit my tuber harvest.


When I decided to write this article series, I began studying scientific research papers about dahlia tubers. Only then did I realize how truly remarkable they are, and how fragile the process can be that encourages tubers to form and grow.


Most growers pursue their dahlia passion for the beauty, enjoyment and endless variety of blooms these plants produce. But at the end of the season, we hope to be rewarded with a bountiful crop of firm, storable tubers that will carry each prized variety into the next season. When our dahlias fail to make tubers, or when the tubers they create don’t survive winter storage, it can be heartbreaking.


To some gardeners, the dahlia tuber is simply that knobby, potato-like thing you plant in spring and dig up in fall. But to truly understand how to grow great dahlias, and more importantly, how to get great tubers from them, we need to look a little closer.


What exactly is a dahlia tuber, and how is it different from other plant storage organs such as bulbs, rhizomes, or even seeds? How are tubers distinct from other types of roots? The answers hold surprising clues to successful cultivation.


A large dahlia tuber clump from Penhill Watermelon

These dahlia tubers formed in a single season. The original plant grew from one tuber.

More Than Just a Root: The Dahlia's Specialized Storage System


At its simplest, a dahlia tuber is a specialized underground storage unit. Its primary purpose is to allow the dahlia plant to survive periods of dormancy, typically cold winters or dry spells, by storing carbohydrates (sugars), water, and nutrients. This energy reserve then fuels the plant's regrowth when favorable conditions return.


However, calling a dahlia tuber a root doesn't quite do it justice. While they are indeed modified roots, they are distinct from the fibrous roots that dahlias use to absorb water and nutrients. Whether you grow dahlias for garden color, cut flower production, or primarily for tubers, understanding these tiny powerhouses is the first step to unlocking bountiful blooms and a thriving dahlia garden.


Unlike the bulbs of tulips or the corms of gladiolus (which are modified stems), dahlia tubers are formed when a special kind of root receives signals to swell and store nutrients for next year’s growth.

A very large clump of tubers from a seed-grown dahlia

This seed-grown dahlia produced a massive tuber clump after growing for two seasons in the author's garden


Adventitious roots are those that grow from non-root tissue, such as stems, rather than from a pre-existing root system. In dahlias, the crown of the plant, located at the base of the stem, directly produces these special roots.


Until I started reading about the biology of roots in preparation for these articles, I never stopped to wonder about the meaning of this word, which scientists repeatedly use when they write about dahlia experiments.


Like strawberry runners, tomato vines, and many other plants, these roots are triggered in response to certain chemical or environmental clues. For strawberry and tomato plants, coming into contact with moist soil causes that part of the plant to sprout roots that it will use to anchor itself to the ground, eventually forming a self-sustaining plant of its own.


When we take stem cuttings to propagate dahlias, and place the stem in moist rooting medium, that’s an environmental cue. If we first dip the stem in rooting hormone, that provides a chemical cue. The roots that form (if we’re lucky) allow us to transplant that cutting and grow it into a beautiful dahlia just like its parent.

The Transformation: How a Root Becomes a Tuber


The journey from a slender adventitious root to a viable dahlia tuber is an engrossing biological process that involves a remarkable transformation:


Initiation of Adventitious Roots: Early in the dahlia's life cycle, a network of adventitious roots begins to grow from the bottom of the stem, just above where the original tuber might be, assuming it was grown from a tuber. These initial roots function like typical roots, absorbing water and nutrients.


The Signal for Storage: At a certain point in the growing season, under specific environmental cues (which we'll explore in Part 2: What Makes a Dahlia Root Become a Tuber?), some of these roots receive a signal to begin storing energy.


Swelling and Reprogramming: Instead of continuing to grow as fibrous roots, the cells within these selected roots begin to swell dramatically. They accumulate starches, sugars, and water, transforming them from slender tendrils into thickened tubers. This process genetically reprograms the root. Its destiny shifts from seeking nutrients to hoarding them.


A close-up view of the eyes on a dahlia tuber clump

In this close-up view, eyes are visible where the tubers join the stem tissue.

 Formation of Eyes: Viable dahlia tubers must have at least one eye, which is actually a tiny bud located on the crown (the portion of the tuber directly attached to the stem). These eyes are embryonic shoots that contain the genetic material and stored energy necessary to produce a new plant the following season. Without an eye, even the plumpest tuber remains just a root and cannot grow a new dahlia plant.


Understanding that dahlia tubers are specialized roots, formed at the crown and requiring eyes for regrowth, is fundamental. It explains why tubers lacking a piece of the crown won't sprout, and it sets the stage for understanding the delicate balance of conditions needed to encourage their formation in the first place.

Why Does This Matter to You, the Gardener?


When I first started growing dahlias, I had no clue what I was doing. The package said to plant my tubers a few inches deep, so that’s what I did. I was lucky. Up came a gorgeous dahlia that bloomed prolifically and attracted lots of bees. I was hooked.


But I hadn’t yet learned anything about what the plant was up to under the soil. The next spring, a couple of the plants came back on their own, and I got to enjoy them for a second full season. However, the dahlias that spent their first season in plastic pots on my patio didn’t come back.


When I finally dumped out the pots so I could re-use them for tomatoes, I found a few soft, slimy lumps that I threw away. I had learned my first lesson about dahlia tubers, though I didn’t realize it at the time.

Rotten, frozen dahlia tubers that will never grow

Dahlia tubers that have experienced freezing are incapable of producing growth.



In my Zone 8b growing zone in coastal Washington State, dahlia tubers can be left in the ground through all but the harshest winters and will grow again. But in pots, which are far more susceptible to temperature extremes, my tubers had frozen and rotted, never to be seen again.


Since those early efforts, I have grown, divided and replanted thousands of dahlia tubers. I actively seek out reliable, science-backed information to advance my knowledge and skills as a grower and seller.


My goal in writing this article series is to help readers avoid some of the mistakes I made and shorten their learning curve. Knowing the true nature of a dahlia tuber has direct implications for your success in the garden. Here are some basics, before we jump too far into the science:


Planting Depth: Because tubers form at the crown, planting your dahlia too deeply can delay or inhibit tuber formation. The crown needs to be at the right level to initiate new roots and give them the environmental cues they need to form tubers.


Dividing Tubers: When dividing your clumps in the fall or spring, each viable division must have a piece of the crown with at least one eye.


Understanding Blind Tubers: Sometimes you'll dig up a beautiful, fat tuber, but it has no eye. Knowing that the eye is the growth point explains why such a tuber, despite its healthy appearance, won't sprout a new plant.


Optimizing Conditions: With this understanding, you're ready to explore the next parts of the series, where we look at how light, temperature, soil, and certain growth regulators can either help or hinder tuber formation.


The dahlia tuber is much more than a fattened root. It’s a finely tuned survival tool that holds the key to how the plant regrows and multiplies.


A tidy clump of well-formed dahlia tubers after digging

Some dahlias formed tubers in a congested formation that makes them difficult to divide.

In Part 2: What Makes a Dahlia Root Become a Tuber? , we'll take a deep dive into the specific environmental cues and internal changes that tell a dahlia root it's time to become a tuber.

For Further Reading: Scientific Claims and Documentation


Throughout "Part 1: What Makes a Tuber a Tuber?", we've explored the foundational science behind dahlia tubers. Every substantive claim we've made is backed by peer-reviewed research and authoritative texts.


For a complete list of these claims and their supporting scientific documentation, please visit Part 7: For Curious Readers & Researchers  .


  • Dahlia tubers are storage organs that allow the plant to survive adverse conditions and regrow. (See Claim 1.1 )

  • Unlike true bulbs or corms, dahlia tubers are swollen adventitious roots, not modified stems. (See Claim 1.2 )

  • Tuber formation in dahlias specifically involves the reprogramming of certain adventitious roots under specific environmental and hormonal cues. (See Claim 1.3 )

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