Table of Contents
- Introduction
- Bulbs, Tubers, and Corms: What Is the Difference?
- The Anatomy of a Dahlia Tuber
- Why We Grow Dahlias from Tubers
- Choosing the Right Spot for Your Dahlias
- When to Plant Your Dahlia Tubers
- How to Plant Dahlias Step-by-Step
- Essential Care for Growing Dahlias
- Staking and Supporting Your Plants
- Extending the Bloom Season
- End-of-Season Care: Lifting and Storing
- Multiplying Your Garden
- Growing Dahlias in Containers
- Common Myths About Growing Dahlias
- Conclusion
- FAQ
Introduction
There is nothing quite like the excitement of watching a dahlia sprout from the soil and transform into a spectacular display of color and texture. These plants are the undisputed stars of the late-summer garden, offering everything from petite pompons to dinnerplate blooms as large as a volleyball. If you have ever wondered whether those funny-looking, potato-like roots are actually bulbs, you are not alone. It is one of the most common questions we receive from home gardeners who are eager to add these beauties to their landscape.
While many people use the term "dahlia bulb" as a catch-all phrase, these plants actually grow from structures called tubers. Understanding the unique way these plants grow is the first step toward a successful and rewarding gardening experience. At Longfield Gardens, we want to help you feel confident as you plan your flower beds and cutting gardens.
In this guide, we will clear up the confusion between bulbs and tubers, explain how dahlias grow, and provide the practical steps you need to take these plants from dormant roots to breathtaking blooms. By focusing on a few simple rules, you can enjoy a garden filled with these impressive flowers all season long.
Bulbs, Tubers, and Corms: What Is the Difference?
In the gardening world, we often use "bulb" to describe any plant that has an underground storage organ. However, there are botanical differences that affect how you plant and care for them. True bulbs, such as tulips and daffodils, are actually modified leaves that store food in layers, much like an onion. If you cut a tulip bulb in half, you can see these layers and even a tiny, undeveloped flower at the center.
Dahlias belong to a different category called tuberous roots. A dahlia tuber is a thickened, fleshy root that stores energy and nutrients for the plant. Unlike a true bulb, which is a self-contained unit, dahlia tubers grow in clumps connected to a central stem. They look more like a cluster of sweet potatoes than a round tulip bulb.
Another common structure is a corm, which is a swollen stem base, like what you find with gladiolus. While it might seem like a lot of jargon, the main thing to remember is that dahlias are tubers. This matters because the way they sprout and the way you store them is different from the fall-planted bulbs you might be familiar with. Knowing this helps you handle them with the right care from the moment they arrive at your door.
The Anatomy of a Dahlia Tuber
To grow a healthy dahlia plant, you need a viable tuber. Not every tuber in a clump will necessarily grow into a plant. For a dahlia tuber to be "viable," it must have three specific parts: the body, the neck, and the crown.
The Body
The body is the fleshy part of the tuber where the energy is stored. These come in many shapes and sizes. Some varieties produce long, skinny tubers, while others produce short, round ones. The size of the body does not dictate the size of the final plant. A small tuber often has just as much energy as a large one to get the plant started.
The Neck
The neck is the narrow portion that connects the body to the crown. This is the most fragile part of the tuber. If the neck is broken or severely creased, the energy stored in the body cannot reach the sprout. We always recommend handling your tubers gently to keep these necks intact.
The Crown and the "Eye"
The crown is the area where the tuber meets the old stem. This is where the "eye" is located. An eye is a small, slightly raised bump that will eventually become the sprout. This is exactly like the eyes you see on a potato in your kitchen pantry.
Key Takeaway: A dahlia tuber must have an intact neck and at least one visible eye on the crown to grow. If you cannot see the eye yet, don't worry. They often become more visible once the tuber is exposed to warmth and moisture.
Why We Grow Dahlias from Tubers
You might wonder why we focus on tubers when dahlias can also be grown from seeds. Most of the famous varieties you see in magazines, such as the creamy Cafe au Lait or the vibrant Thomas Edison, are grown from tubers to ensure they are "true to type."
When you grow a dahlia from a tuber, you are essentially growing a clone of the parent plant. You know exactly what color, shape, and height the flower will be. If you plant a seed from a 'Cafe au Lait' dahlia, the resulting flower will be a complete surprise and will likely not look like the parent at all. This genetic variation makes seeds fun for plant breeders, but tubers are the best choice for gardeners who want specific colors and forms in their landscape.
Another benefit of growing from tubers is the speed of growth. Because the tuber already has a significant store of energy, the plant can grow very quickly once the weather warms up. This allows dahlias to produce an incredible number of flowers in a single growing season.
Choosing the Right Spot for Your Dahlias
Dahlias are not particularly fussy, but they do have a few non-negotiable needs. Matching the plant to the right location is the easiest way to ensure a beautiful result.
Sunlight is Essential
Dahlias love the sun. To get the best blooms and strong stems, they need at least six to eight hours of direct sunlight every day. In very hot climates, they appreciate a little bit of shade in the late afternoon to prevent the blooms from fading, but full sun is generally the goal. If they are planted in too much shade, the plants will become "leggy" as they stretch for the light, and they will produce fewer flowers.
Drainage Matters
Dahlias need soil that drains well. "Drainage" simply refers to how fast water moves through the soil. If water sits in a spot for hours after a rain, the tubers may rot. If you have heavy clay soil, you can improve the drainage by adding organic matter like compost. Alternatively, dahlias grow exceptionally well in raised beds or large containers where you have more control over the soil quality.
Give Them Space
It can be tempting to crowd plants together, but dahlias need room for air to circulate around their leaves. Most varieties should be spaced about 12 to 18 inches apart. Larger "dinnerplate" varieties often need up to 2 feet of space. Good airflow helps keep the foliage healthy and prevents issues like powdery mildew during humid summer months.
When to Plant Your Dahlia Tubers
Timing is everything when it comes to dahlias. Since they are tropical plants native to Mexico and Central America, they have no tolerance for frost. Planting too early in cold, wet soil is one of the few ways to truly struggle with these plants.
Wait for the Warmth
The best rule of thumb is to plant your dahlias at the same time you would plant tomatoes or peppers. This is usually about two weeks after the last expected frost in your area. The soil temperature should be around 60°F. If the soil is too cold and damp, the tuber will sit dormant and may eventually rot before it has a chance to sprout.
Checking Your Zone
Your USDA hardiness zone helps determine your shipping and planting window. We time our shipments to arrive about two weeks before the ideal planting time for your specific zone. If your tubers arrive and it is still too cold outside, you can keep them in a cool, dark, dry place like a basement until the ground is ready.
What to Do Next: Preparing for Planting
- Check your local frost dates to determine your planting window.
- Find a spot in your garden that receives at least 6 hours of full sun.
- Clear the area of weeds and stir in some compost to loosen the soil.
- Ensure you have stakes or supports ready for taller varieties.
How to Plant Dahlias Step-by-Step
Once the danger of frost has passed and the sun has warmed the soil, it is time to get your tubers in the ground. The process is straightforward and very rewarding.
1. Dig the Hole
Dig a hole about 4 to 6 inches deep. If you are planting a single tuber, the hole only needs to be as wide as the tuber itself. If you are planting a larger clump, make the hole a bit wider.
2. Place the Tuber
Lay the tuber horizontally in the hole. If you can see the eye or a small sprout, point it toward the sky. If you aren't sure which side is the top, laying it on its side is perfectly fine; the plant will figure out which way is up.
3. Add Support Early
If you are growing medium or tall varieties, it is best to put your stakes in the ground at the time of planting. If you wait until the plant is large, you might accidentally drive the stake through the tuber underground. Driving the stake in now ensures the plant is supported from day one.
4. Cover with Soil
Fill the hole back up with soil. You do not need to pack it down tightly; a gentle firming with your hands is enough.
5. The Golden Rule of Watering
This is the most important tip for planting dahlias: Do not water the tubers immediately after planting. There is usually enough natural moisture in the soil to get the plant started. Watering a dormant tuber before it has leaves can cause it to rot. Wait until you see the green sprouts poking through the soil before you begin a regular watering schedule.
Essential Care for Growing Dahlias
Once your dahlias have sprouted and are about a foot tall, they are very easy to care for. Following a few simple steps will lead to a more beautiful and productive garden.
Watering Correctly
Once the plants are established and have several sets of leaves, they need consistent moisture. The goal is to water deeply a few times a week rather than giving them a light sprinkle every day. This encourages the roots to grow deep into the soil. In the heat of summer, you may need to water more frequently, especially if you are growing in containers.
The Secret of "Pinching"
To get the most flowers, you should pinch your dahlia plants when they are about 12 to 16 inches tall. This simply means snipping off the very top of the center stem, just above a set of leaves. This feels a little bit scary the first time you do it, but it signals the plant to stop growing one tall stalk and start branching out. More branches mean more stems, and more stems mean more flowers for you to enjoy.
Feeding Your Plants
Dahlias are "heavy feeders," meaning they use a lot of nutrients to produce those big, beautiful blooms. We recommend using a fertilizer that is lower in nitrogen and higher in phosphorus and potassium (look for a 5-10-10 or 10-20-20 formula). High nitrogen can lead to lots of green leaves but very few flowers. Start fertilizing about 30 days after planting and continue once a month through the end of the summer.
Key Takeaway: Consistent deep watering and a mid-summer "pinch" are the two best things you can do to ensure a spectacular dahlia display.
Staking and Supporting Your Plants
Many dahlias can reach heights of 4 or 5 feet. When these tall plants are loaded with heavy flowers, a summer rainstorm or a gust of wind can easily knock them over. Providing support is a simple way to protect your investment.
For individual plants, a sturdy wooden or metal stake works perfectly. You can use soft twine or even strips of old t-shirts to gently tie the main stem to the stake as it grows. If you are growing a long row of dahlias, you can use the "corralling" method. Drive a heavy stake at each corner of the bed and every few feet along the sides. Then, wrap twine around the stakes at different heights to create a supportive cage for the plants to grow through.
Extending the Bloom Season
One of the best things about dahlias is that the more you cut them, the more they bloom. This makes them perfect for anyone who loves having fresh flowers in the house.
To keep the plant producing new buds, you must "deadhead" the spent flowers. Deadheading is simply the process of cutting off flowers that have started to fade. If you leave the old flowers on the plant, it will stop producing new buds and put its energy into making seeds instead. By removing the old blooms, you tell the plant to keep the flower show going.
When cutting flowers for a vase, look for blooms that are almost fully open. Unlike roses or lilies, dahlias will not open much further once they are cut. Place them in warm water immediately after cutting to help them stay fresh for several days.
End-of-Season Care: Lifting and Storing
Dahlias are perennials in warm climates (USDA zones 8 and above), meaning they can stay in the ground all winter. However, in most of the United States, the tubers will freeze and die if left outdoors. To save your favorite varieties for next year, you can "lift" and store them.
When to Dig
Wait until the first frost has blackened the foliage. This sends a signal to the plant to go into dormancy and pack all its energy into the tubers. Cut the stalks down to about 4 inches and let them sit in the ground for a few days before carefully digging them up with a garden fork.
Cleaning and Drying
Gently shake off the excess soil and rinse the tubers with a hose. Let them air dry in a protected, shady spot for about 24 hours. You don't want them to become bone-dry or shriveled, just dry enough that they aren't dripping wet.
Storage Tips
Pack the tubers in a box filled with slightly damp peat moss, vermiculite, or wood shavings. Store the box in a cool, dark, frost-free place, like a basement or a crawl space. The ideal temperature is between 40°F and 50°F. Check on them once a month during the winter. If they look shriveled, give the packing material a light mist of water. If any tubers look mushy or moldy, remove them so the rot doesn't spread.
Multiplying Your Garden
The amazing thing about dahlias is that one tuber planted in the spring will grow into a large clump of several tubers by the fall. This means you can actually increase your flower collection every year for free!
In the spring, you can divide these clumps into individual tubers. Remember the "anatomy" rule: each division must have a piece of the crown and at least one eye. Dividing tubers takes a little practice and a sharp, clean knife, but it is a rewarding way to share your favorite varieties with friends or expand your own garden beds.
What to Do Next: Post-Harvest
- Label your tubers with a permanent marker or tag so you know the variety name next spring.
- Find a storage spot that stays cool but never drops below freezing.
- Set a calendar reminder to check your stored tubers once a month.
Growing Dahlias in Containers
If you have limited space or poor soil, you can still enjoy dahlias by growing them in pots. This is an excellent option for patios, decks, or balconies.
Select a container that is at least 12 to 16 inches in diameter and has drainage holes at the bottom. Use a high-quality potting mix rather than garden soil, as potting mix is designed to drain well and stay fluffy in a container. Choose shorter "border" or gallery dahlia varieties that naturally stay compact. These smaller types usually don't require staking, making them very low-maintenance.
Keep in mind that containers dry out much faster than the ground. In the peak of summer, you may need to water your potted dahlias every day. Because you are watering frequently, nutrients will wash out of the soil more quickly, so regular fertilizing is key for container success.
Common Myths About Growing Dahlias
There are a lot of "hacks" on the internet that can make dahlia growing sound complicated. At our trial garden at Longfield Gardens, we focus on what actually works.
One common myth is that you must use bone meal or specific "miracle" products to get blooms. In reality, healthy soil and a basic balanced fertilizer are all you need. Another myth is that bigger tubers produce bigger plants. As we discussed earlier, the size of the tuber doesn't matter nearly as much as the presence of a healthy eye and an intact neck.
Finally, some people think dahlias are "too much work" because of the winter storage. While lifting tubers is an extra step, it is entirely optional. If you don't want to store them, you can treat your dahlias as annuals and simply plant fresh tubers every spring. The choice is yours, and either way, the reward of those summer blooms is well worth it.
Conclusion
Dahlias are one of the most rewarding flowers you can grow. While they are technically tubers and not bulbs, they are just as easy to plant and care for once you understand their basic needs. By waiting for the soil to warm, providing plenty of sunlight, and remembering not to water until you see sprouts, you are setting yourself up for a season of spectacular color.
We love dahlias because they offer something for everyone, from the beginner gardener to the seasoned pro. Whether you are growing a single pot on your porch or a massive cutting garden, these plants will never fail to impress.
- Dahlias grow from tubers, which are fleshy roots that store energy.
- Wait until the soil is 60°F and the danger of frost is gone before planting.
- Avoid watering until you see green sprouts above the ground.
- Pinch the center stem when the plant is a foot tall to encourage more blooms.
- Cut the flowers often to keep the plant producing new buds all summer.
"The joy of gardening comes from the simple success of watching a dormant root turn into a masterpiece of nature."
Ready to start your own dahlia journey? Explore our collection of premium tubers at Longfield Gardens and find the perfect varieties to bring your garden to life this season.
FAQ
Are dahlias considered annuals or perennials?
Dahlias are technically tender perennials. In warm climates (Zones 8–11), they can stay in the ground year-round. In colder northern climates, the tubers will not survive the freezing winter temperatures, so gardeners either dig them up to store indoors or plant new ones each spring like annuals.
Do I need to soak dahlia tubers before planting?
No, you do not need to soak dahlia tubers. In fact, soaking them or watering the soil too much at planting time can lead to rot. The tuber contains all the moisture and energy it needs to send up its first sprout. Once you see green growth above the soil, you can begin a regular watering routine.
Why aren't my dahlia tubers sprouting?
The most common reason for slow sprouting is cold soil. Dahlias are tropical plants and will sit dormant until the soil reaches about 60°F. If your soil is warm and you still see nothing after three or four weeks, check that you didn't overwater, which can cause the tuber to rot before it starts growing.
Can I grow the large dinnerplate dahlias in pots?
While it is possible, it is quite challenging. Dinnerplate dahlias grow very large and heavy, requiring deep soil for their root systems and very sturdy staking. For the best results in containers, we recommend choosing smaller "border" or "miniature" varieties that stay under 2 feet tall.
