Introduction
Imagine walking through your garden and being greeted by flowers that stand nearly as tall as you do. There is a unique thrill in watching a small, potato-like tuber transform into a towering botanical masterpiece in just a few months. Tall dahlias are the heavy hitters of the summer garden. They provide architectural height, vibrant color, and an endless supply of blooms for your kitchen table.
At Longfield Gardens, we believe that every gardener can find success with these magnificent plants. While their size might seem impressive, the process of growing them is straightforward and deeply rewarding. Whether you are looking to create a living privacy screen or want to grow "dinnerplate" blooms that spark conversation, the steps are the same.
In this guide, we will walk you through everything you need to know about growing tall dahlias. We will cover variety selection, the best ways to support heavy stems, and simple maintenance tips to keep your plants thriving until the first frost. By focusing on a few gardening basics, you can enjoy a season full of dramatic, sky-high color.
Choosing the Best Varieties for Height
When you want to grow tall dahlias, success starts with choosing the right tuber. Dahlias come in a vast range of sizes, from tiny border varieties to giants that reach five feet or more. If your goal is height, you should look for specific categories that are naturally inclined to grow tall.
Dinnerplate Dahlias
Dinnerplate dahlias are the most famous of the tall varieties. These plants are known for their massive blooms, which can reach 8 to 10 inches in diameter. Because the flowers are so large, the plants naturally grow robust, tall stems to support the weight. Popular varieties like 'Café au Lait' or 'Thomas Edison' are excellent choices for adding vertical interest to the back of a flower bed.
Cactus and Semi-Cactus Dahlias
Cactus dahlias feature narrow, pointed petals that give them a spiky, starburst appearance. These varieties often have a very upright growth habit. Many cactus dahlias easily reach four feet in height. Their unique texture adds a different visual element compared to the broad, flat petals of decorative types.
Decorative Dahlias
This is the broadest category and includes many of the tallest dahlias available. Formal decorative dahlias have evenly spaced petals, while informal types have a more relaxed, wavy look. Varieties like 'Kelvin Floodlight' are legendary for both their height and their sunshine-yellow blooms.
Key Takeaway: Always check the "mature height" on the plant label or product description. To achieve a tall, layered look in your garden, place these varieties at the back of your borders so they don't shade out shorter plants.
Finding the Right Place to Plant
The secret to a tall, strong dahlia is a location that provides plenty of energy. These plants are sun-worshippers, and their height is directly tied to how much light they receive.
Prioritize Full Sun
Dahlias need at least six to eight hours of direct sunlight every day. If they are planted in too much shade, they will stretch toward the light. This results in "leggy" growth, where the stems are thin, weak, and prone to breaking. A sunny spot ensures the stems grow thick and sturdy enough to support those towering heights.
Check Your Drainage
"Drainage" simply refers to how fast water leaves the soil. Dahlias have fleshy tubers that can rot if they sit in soggy ground. Before planting, observe your garden after a rainstorm. If puddles remain for hours, that spot might be too wet. You can improve drainage by adding compost or planting in raised beds, which we highly recommend for dahlia success.
Consider Wind Protection
Because tall dahlias act like sails in the wind, a particularly gusty afternoon can topple them. If possible, plant them near a fence, a wall, or a line of shrubs. These structures act as a windbreak, protecting the heavy blooms and tall stalks from snapping.
What to do next:
- Identify the sunniest spot in your yard.
- Dig a small hole and fill it with water to see how fast it drains.
- Look for a location that offers some protection from strong north or west winds.
Preparing the Soil for Giants
To grow a plant that reaches four or five feet in height in a single season, you need to provide a lot of "fuel." This fuel comes from the soil.
Dahlias prefer fertile soil that is rich in organic matter. Before you plant your tubers, take some time to prepare the area. Mixing in a few inches of compost or well-rotted manure will give the plants a slow-release source of nutrients. This also helps the soil hold exactly the right amount of moisture—not too much, and not too little.
If you have heavy clay soil, adding organic matter is even more important. It helps break up the clay so the roots can grow deep and wide. A deep root system is the anchor for a tall plant. Without it, the plant may lean or pull out of the ground when the flowers become heavy with rain.
Timing and Planting Depth
Timing is perhaps the most important rule for dahlia success. It can be tempting to plant as soon as the first warm day of spring arrives, but patience pays off.
The 60-Degree Rule
Wait until the soil temperature reaches about 60°F before putting your tubers in the ground. Dahlias are tropical plants at heart. If they are placed in cold, damp soil, they will simply sit there and may eventually rot. In most regions, this means planting around the same time you would plant tomatoes or peppers. For additional guidance, read 8 Tips for Growing Better Dahlias.
Planting Depth and Spacing
When you are ready to plant, dig a hole about four to six inches deep. Lay the tuber on its side with the "eye"—the small bump where the sprout emerges—pointing upward. Learn more about dahlia tubers before planting.
For tall varieties, spacing is critical. Give each plant at least 18 to 24 inches of room. While they look small now, they will eventually become large, bushy shrubs. Good spacing allows for air to circulate between the plants, which keeps the leaves dry and healthy.
Key Takeaway: Do not water your tubers immediately after planting unless the soil is bone-dry. The tuber has enough stored energy and moisture to start growing. Wait until you see green sprouts poking through the soil before you begin a regular watering schedule.
Essential Support: Staking Tall Dahlias
If you are growing dahlias that reach over three feet, staking is not optional. The weight of the large blooms, especially after a rain, is enough to bend or break even the strongest stems. The best time to provide support is at the moment of planting. See additional dahlia staking guidance for support ideas.
Why Stake Early?
If you wait until the plant is tall to drive a stake into the ground, you risk piercing the tuber growing underground. By placing the stake first, you ensure the plant's foundation remains safe.
Single Staking
This is the most common method for home gardens. Use a sturdy stake made of wood, bamboo, or coated metal. Drive the stake at least 12 inches into the ground right next to where you plant the tuber. As the plant grows, use soft twine or garden ties to loosely attach the main stem to the stake every 12 inches.
The Tomato Cage Method
For dahlias that are bushy as well as tall, a heavy-duty tomato cage works wonders. Place the cage over the planting spot immediately after burying the tuber. As the plant grows, the horizontal rings will support the side branches, keeping the entire plant upright without the need for constant tying.
The Corral Method
If you are planting a long row of dahlias, you can use the "corral" or "Florida weave" technique. Drive heavy T-posts at the four corners of your bed and every five to eight feet along the sides. Run sturdy twine between the posts at heights of 12, 24, and 36 inches. This creates a "cage" that the dahlias grow up through, holding them in place.
What to do next:
- Purchase your stakes or cages before planting day.
- Ensure stakes are at least 5 to 6 feet tall, as a significant portion will be underground.
- Have soft garden twine or strips of old fabric ready for tying.
The Power of Pinching
It sounds counterintuitive, but to get a better, more structurally sound tall plant, you need to cut off the top of it early in the season. This process is called "pinching."
When your dahlia plant is about 12 to 16 inches tall and has four sets of leaves, snip off the very top of the center terminal shoot. You only need to remove an inch or two. This tells the plant to stop putting all its energy into one single, lanky stem. For a broader overview, see All About Dahlias.
Instead, the plant will begin to grow multiple side branches from the leaf nodes below the cut. This results in a bushier plant with a stronger base. A "pinched" plant will produce far more flowers and will be much less likely to snap in the wind because the weight is distributed across several stems rather than one.
Watering and Feeding for Growth
Tall dahlias are thirsty and hungry. Because they grow so fast, they need a consistent supply of water and nutrients to maintain their height.
Deep Watering
Rather than giving your plants a light sprinkle every day, it is better to water them deeply once or twice a week. This encourages the roots to grow deeper into the soil to find moisture. Aim to keep the soil consistently moist, like a wrung-out sponge. During the heat of mid-summer, you may need to increase the frequency, especially if you have sandy soil.
Fertilizing Basics
We like to think of dahlias like tomato plants when it comes to feeding. At the start of the season, a balanced, all-purpose fertilizer is great for establishing the plant.
However, as the summer progresses, avoid fertilizers that are very high in nitrogen. Nitrogen promotes lots of green leaves, but too much of it can make the stems "soft" and weak. It can also result in fewer flowers. Switch to a fertilizer with higher phosphorus and potassium (the second and third numbers on the bag) once the plants are a few feet tall to encourage strong stems and vibrant blooms.
Managing Pests and Maintaining Health
A healthy plant is a tall plant. Keeping an eye out for common garden visitors will ensure your dahlias reach their full potential.
Slugs and Snails
In the early spring, slugs and snails love to munch on tender dahlia sprouts. If the main growing tip is eaten, it can stunt the plant's growth. Use a pet-safe slug bait or copper tape around the base of the plant to protect those early shoots.
Earwigs and Beetles
As the flowers begin to bloom, you might notice small holes in the petals. Earwigs and Japanese beetles are the usual suspects. In most cases, these are just a cosmetic issue. You can often manage them by hand-picking or by keeping the area around the base of the plants clear of fallen leaves and debris, which gives these pests fewer places to hide.
Airflow and Mildew
Toward the end of the summer, when the weather gets humid, dahlias can sometimes develop powdery mildew—a white, dusty film on the leaves. The best way to prevent this is to ensure good airflow. This is why the spacing we mentioned earlier is so important. You can also trim off the bottom 12 inches of leaves once the plant is tall to help air move freely around the base.
Harvesting and Deadheading
The more you cut your dahlias, the more they will bloom. This is one of the best parts of growing them!
Cutting for Bouquets
When you harvest a flower for a vase, don't just snip the flower head. Cut a long stem, going back to where it meets a larger branch. This encourages the plant to send out new, long stems for future blooms. For the longest vase life, cut your dahlias in the cool of the morning and choose flowers that are almost fully open. Unlike roses or lilies, dahlias will not open much further once they are cut.
Deadheading
If you aren't cutting the flowers for bouquets, you must "deadhead" them. This simply means removing the flowers as they fade. If you leave the old flowers on the plant, it will start to put its energy into making seeds. By removing them, you signal the plant to keep producing new buds.
Key Takeaway: When deadheading, look closely at the plant. Spent blooms are often pointed or "cone-shaped," while new buds are round and firm. Make sure you are snipping the right one!
Seasonal Care and Expectations
Growing tall dahlias is a journey that lasts from spring through the first hard frost. It is important to remember that dahlias take time to reach their peak. They often spend the first half of the summer building their root system and foliage. You might not see your first bloom until July or August, but once they start, they won't stop until the weather turns cold.
In most parts of the country, dahlia tubers cannot survive the winter in the ground. Once the foliage has been blackened by the first frost, you have a choice. You can treat them as annuals and plant fresh tubers next spring, or you can dig them up and store them in a cool, dry place like a basement or garage for the winter. For more seasonal information, browse dahlia growing resources.
At Longfield Gardens, we see many gardeners enjoy the process of "lifting" their tubers to grow their favorite varieties year after year. It is a wonderful way to see how much the tubers have multiplied over a single season!
Conclusion
Growing tall dahlias is one of the most rewarding projects a gardener can undertake. By choosing the right varieties, providing a sunny spot with plenty of support, and following a few simple care steps, you can transform your backyard into a floral paradise. These plants are resilient, generous, and endlessly beautiful.
- Select naturally tall varieties like Dinnerplate or Cactus dahlias.
- Plant in full sun with excellent drainage.
- Stake your plants at the time of planting to protect tall stems.
- Pinch the center shoot when the plant is 12–16 inches tall for a stronger structure.
- Water deeply and feed regularly for the best results.
We are here to help you every step of the way. With high-quality tubers and a little bit of patience, your garden will soon be reaching new heights. Visit our dahlia collection to find the perfect varieties for your next growing season and start planning your most beautiful garden yet.
"There is no such thing as a 'difficult' dahlia; there are only plants waiting for the right spot and a sturdy stake."
FAQ
How tall do dahlias actually get?
The height of a dahlia depends entirely on the variety. While border dahlias may only reach 12 to 18 inches, many decorative and dinnerplate varieties regularly grow to 4 or 5 feet. In ideal conditions with plenty of sun and food, some varieties have even been known to reach 6 feet or more.
Why are my tall dahlias falling over?
If your dahlias are leaning or snapping, they likely need more support or more sunlight. Tall dahlias are top-heavy, especially after rain. Ensure you have a sturdy stake or cage in place and that the plant is tied to it every 12 inches. If the plant is "reaching" for the sun in a shady spot, it will grow weak stems that cannot support its own weight.
Do I really need to pinch my dahlias if I want them to be tall?
Yes! It sounds like you are making the plant shorter, but pinching actually helps it grow "better" tall. By removing the main tip, you encourage the plant to grow a wider, stronger base. This prevents the plant from becoming a single, fragile stalk that is easily broken by the wind.
Can I grow tall dahlias in containers?
You can grow tall dahlias in pots, but you will need a very large, heavy container to prevent it from tipping over. A 5-gallon pot is the minimum size for a tall variety. You will also need to secure the stake to the pot or a nearby structure, as the wind can easily catch a tall plant and blow the entire container over.
