Introduction
There is nothing quite like the sight of a dahlia garden in full bloom during the late summer months. Watching those first green shoots emerge from the soil and transform into towering plants topped with dinnerplate-sized blossoms is one of the most rewarding experiences a gardener can have. Whether you are looking to create a dramatic backdrop for your flower beds or want extra-long stems for spectacular indoor bouquets, achieving impressive height is a common goal for many enthusiasts.
At Longfield Gardens, we want to help you reach those gardening milestones by providing high-quality tubers from our Dahlia Collections and the practical knowledge to make them thrive. Growing tall, sturdy dahlias is not a matter of luck; it is about understanding how these plants respond to their environment. By focusing on a few key factors like variety selection, light, and proper support, you can encourage your plants to reach their full potential.
This guide will walk you through the essential steps to maximize the height and vigor of your dahlias. For a broader refresher, see 8 Tips for Growing Better Dahlias. We will cover everything from choosing the right tubers to the specific care routines that push plants upward. With a bit of planning and the right techniques, you can enjoy a garden filled with magnificent, sky-high blooms all season long.
Choose the Right Varieties for Height
The single most important factor in determining how tall your dahlias will grow is the specific variety you choose to plant. Dahlias are incredibly diverse, with thousands of cultivars ranging from tiny border plants to giants that can tower over a person. For a broader overview, see All About Dahlias. If your goal is height, you must start with the right genetics.
Dinnerplate and Decorative Dahlias
Most gardeners looking for height turn to the Dinnerplate dahlias or large "Decorative" categories. These varieties are naturally bred to produce thick, strong stems and massive plant structures. Types like the famous Café au Lait are known for their ability to reach 4 to 5 feet in height under the right conditions.
The classic Thomas Edison is another vigorous dinnerplate dahlia with a large leaf structure that supports upward movement.
Cactus and Semi-Cactus Types
Cactus-flowered dahlias are another excellent choice for vertical interest. These cactus dahlias often have a more open, airy growth habit compared to the dense Decorative types, which allows them to stretch upward gracefully. Many Cactus dahlias easily reach 4 feet or more, and their spiky, starburst blooms look stunning when swaying at eye level.
Avoiding Border Varieties
If you want tall plants, be sure to avoid Border dahlias. These are bred specifically to stay low to the ground, usually topping out at 12 to 24 inches. While they are wonderful for the front of a garden bed or for small containers, no amount of fertilizer or care will turn them into tall plants. Always check the expected mature height on the packaging or our website before you buy.
Key Takeaway: Start with varieties genetically programmed for height. Dinnerplate and Decorative dahlias are the most reliable choices for reaching 4 to 5 feet.
Maximize Sunlight for Upward Growth
Dahlias are sun-loving plants, and the amount of light they receive directly influences their height and stem strength. Sunlight is the energy source that fuels the rapid growth required to produce tall stalks in a single season.
The Power of Full Sun
For the best results, plant your dahlias in a spot that receives at least 6 to 8 hours of direct sunlight every day. In regions with cooler summers, more sun is almost always better. When a plant receives ample light, it has the energy to build a thick, woody stem that can support its own weight as it climbs.
Avoiding the "Leggy" Look
While it is true that plants in lower light may "stretch" toward the sun and technically become taller, this is not the kind of height you want. This growth is known as being "leggy." Leggy stems are thin, weak, and prone to snapping under the weight of the flowers or a light breeze. A dahlia grown in full sun will be tall but also robust and sturdy.
Afternoon Shade in Hot Climates
In parts of the country where summer temperatures regularly soar above 90°F, your dahlias may benefit from a bit of dappled afternoon shade. Extreme heat can cause the plants to wilt or slow their growth. Providing protection from the harshest sun during the hottest part of the day can actually help the plant maintain its vigor and continue growing taller throughout the summer.
What to do next:
- Observe your garden for a full day to identify the sunniest spots.
- Ensure there are no overhanging branches or tall structures blocking the southern exposure.
- If you live in a very hot climate, prioritize a spot with morning sun and afternoon relief.
Prepare the Soil for Vertical Success
A tall plant needs a deep, nutrient-rich foundation. Dahlia tubers are "heavy feeders," meaning they require a significant amount of nutrients and water to sustain their rapid growth. Preparing your soil before the tubers go in the ground is a simple win that pays off in height later.
Organic Matter and Drainage
Dahlias grow best in loose, well-draining soil that is rich in organic matter. Before planting, we recommend digging in a generous amount of finished compost or well-rotted manure. This improves the soil structure, allowing roots to penetrate deep into the earth. Deep roots provide the stability a tall plant needs to stay upright during summer storms.
Nutrient Balance for Height
When it comes to fertilizer, the balance of nutrients matters. Nitrogen is the element responsible for leafy, green growth and stem elongation. Early in the season, a balanced fertilizer (such as a 10-10-10) can help the plant establish its height. However, it is important not to overdo the nitrogen as the season progresses. Too much nitrogen late in the summer can result in lush leaves but very few flowers.
Soil pH and Accessibility
Dahlias prefer a slightly acidic to neutral soil pH, ideally between 6.0 and 7.0. If your soil is too alkaline or too acidic, the plant may struggle to take up the nutrients it needs to grow tall. A simple soil test from a local extension service can provide you with the exact measurements and recommendations for your specific area. If you need help confirming your zone, see the Hardiness Zone Map.
Key Takeaway: Healthy, compost-enriched soil encourages deep root systems. Deep roots are the secret to supporting the physical weight of a 5-foot-tall plant.
Watering for Strength and Stature
Water is the primary component of a plant's structure. For dahlias to reach their maximum height, they need a consistent and reliable supply of moisture. This keeps the stems "turgid," or firm, allowing them to stand tall and continue pushing upward.
Deep Watering vs. Frequent Sprinkling
The best way to water dahlias is deeply and less frequently. Instead of a light daily sprinkle, give the plants a thorough soaking once or twice a week. This encourages the roots to grow deeper into the soil in search of moisture. Shallow watering leads to shallow roots, which makes tall plants much more likely to fall over.
The Critical Growth Phase
Once your dahlia shoots are about 12 inches tall, they enter a phase of rapid expansion. During this time, they should never be allowed to dry out completely. If a plant experiences drought stress during its main growing phase, it may "harden off" or stop growing vertically, resulting in a shorter, bushier plant.
Mulching to Retain Moisture
Applying a 2- to 3-inch layer of organic mulch, such as shredded bark or straw, around the base of the plants is a great way to keep the soil moist and cool. Mulch prevents water from evaporating and helps maintain the consistent conditions that dahlias love. Just be sure to keep the mulch a few inches away from the main stem to prevent rot.
What to do next:
- Install a soaker hose or drip irrigation for easy, deep watering.
- Check the soil moisture by sticking your finger an inch or two into the ground.
- Apply mulch only after the soil has warmed up and the plants are established.
Support Your Dahlias Early
As dahlias grow taller, they become increasingly top-heavy. This is especially true for Dinnerplate varieties, where a single bloom can weigh nearly a pound. Without proper support, even the tallest dahlia will eventually bend or break. Supporting them early ensures they maintain their vertical stature.
Individual Staking
For the tallest varieties in a home garden, individual staking is often the best method. Use a sturdy wooden stake, a steel T-post, or a piece of heavy-duty rebar at least 5 to 6 feet tall. It is best to drive the stake into the ground at planting time so you do not accidentally damage the tuber later. As the plant grows, use soft garden twine to loosely tie the main stem to the stake every 12 inches.
Tomato Cages for Mid-Sized Tall Varieties
Heavy-duty tomato cages can work well for dahlias that reach about 3 to 4 feet. The cage provides a 360-degree support system that keeps the side branches from splaying out. For the best results, use the square, folding metal cages, as they are often sturdier than the round wire versions.
The Corral Method
If you are growing a row of dahlias, the corral method is highly efficient. Drive heavy posts into the four corners of your dahlia bed. Run several levels of strong twine around the perimeter of the posts, essentially "coralling" the plants inside. As they grow, they will lean against the twine for support. This is a common practice for cut-flower growers who need to manage dozens of tall plants at once.
Key Takeaway: Staking doesn't make a plant grow taller, but it ensures the height you've achieved isn't lost to wind or the weight of the blooms. Always stake at planting time to protect the tubers.
Strategic Pinching for Stronger Height
It may seem counterintuitive, but cutting off the top of your dahlia can actually lead to a taller, more impressive plant overall. This process is known as "pinching" or "topping." For a step-by-step refresher, see How to Pinch and Stake Dahlias.
Why Pinching Works
When a dahlia is about 12 inches tall and has four sets of leaves, you can snip off the very top of the main center shoot. This signals the plant to stop putting all its energy into a single, thin stalk and instead develop two or more strong main branches from the base. While this might slightly delay the very first bloom, it creates a much broader, more stable foundation.
Achieving "Better" Height
A plant that hasn't been pinched often produces one tall, spindly stem that is very difficult to support. A pinched plant will produce multiple tall stems that are thicker and more resilient. For the gardener who wants a massive presence in the yard, these multi-stemmed plants look much "taller" and more substantial than a single-stemmed plant.
How to Pinch
Simply locate the central growing tip once the plant reaches 12 inches. Use a clean pair of snips or your fingernails to remove the top inch of growth. Ensure you are leaving at least three to four pairs of leaves on the plant. Within a week, you will see new shoots emerging from the leaf axils, ready to climb skyward.
What to do next:
- Wait until the plant has at least four pairs of true leaves.
- Use clean tools to prevent the spread of garden diseases.
- Monitor the new side shoots and continue tying them to your support system as they grow.
Manage Pests and Disease to Protect Height
A healthy plant is a fast-growing plant. If your dahlias are struggling with pests or fungal issues, they will divert energy away from vertical growth and toward survival. Keeping your plants clean and healthy is a key part of reaching those height goals.
Air Circulation is Essential
As dahlias get taller and bushier, air can become trapped in the center of the plant. This creates a humid environment where powdery mildew and other fungal diseases thrive. To prevent this, you can prune away the bottom 12 inches of leaves once the plant is established. This improves airflow and keeps the plant focused on upward growth and flower production.
Defending Against Sap-Suckers
Aphids, thrips, and mites are small pests that suck the sap from new, tender growth at the tips of the stems. If these pests are left unchecked, they can stunt the plant’s growth or cause the stems to become distorted. Regularly inspect the growing tips of your tall dahlias and treat any infestations early with a gentle stream of water or an appropriate garden soap.
Slug Protection for Young Shoots
Slugs and snails love the tender shoots of young dahlias. If the main growing point is eaten early in the season, the plant may struggle to regain its momentum. Protecting your plants when they are small ensures they get the "head start" they need to reach their maximum potential height by mid-summer.
Key Takeaway: Healthy foliage produces the energy needed for growth. Protect the growing tips from pests to ensure the plant never stops its upward climb.
Fertilizing for Late-Season Stature
To keep your dahlias growing tall throughout the entire season, they need a "boost" of nutrients as they begin to bloom. While we recommend starting with a balanced fertilizer, the needs of the plant change as it matures.
Switching to Low-Nitrogen Food
Once your dahlias reach their desired height and begin setting buds, switch to a fertilizer that is lower in nitrogen and higher in phosphorus and potassium (the middle and last numbers on the bag). This encourages the plant to maintain its height while focusing its energy on producing large, vibrant flowers and strengthening the stems.
Liquid vs. Granular
Granular fertilizers are excellent for long-term feeding because they break down slowly over time. However, if you notice your plants' growth slowing down in mid-July, a water-soluble liquid fertilizer can provide an immediate pick-me-up. Always follow the instructions on the label to avoid over-fertilizing, which can stress the plant.
Stop Feeding in Early Fall
If you live in a climate where you plan to dig up and store your tubers for the winter, it is a good idea to stop fertilizing by early September. This encourages the plant to stop its rapid vertical growth and start sending energy back down into the tubers for winter storage. This ensures your tubers will be healthy and ready to grow tall again next year.
What to do next:
- Set a calendar reminder to feed your dahlias every 3 to 4 weeks.
- Always water your plants well before and after applying fertilizer to prevent root burn.
- Switch to a bloom-boosting formula once the first buds appear.
Understanding the Growth Timeline
Dahlias are not instant-gratification plants; they require patience. Understanding their natural rhythm helps you manage your expectations and provide the right care at the right time.
The Early Slow Phase
For the first few weeks after planting, it may seem like nothing is happening. During this time, the tuber is busy establishing a root system. Do not be tempted to overwater or over-fertilize during this stage. Once the roots are set, the "exponential" growth phase begins.
Mid-Summer Growth Spurt
In most parts of the US, dahlias do the bulk of their vertical growing in July and August. During these months, a healthy Dinnerplate dahlia can grow several inches in a single week. This is the time to stay on top of your staking and watering routine.
Fall Maturity
By September, most dahlias have reached their maximum height for the season. At this point, the plant's job is to produce as many flowers as possible before the first frost. Regular deadheading—removing spent blooms—will keep the plant looking neat and encourage it to keep producing those high-elevation flowers until the very end of the season.
Key Takeaway: Patience pays off. The most dramatic height gains happen during the heat of mid-summer, provided the plant has consistent water and support.
Conclusion
Making dahlias grow taller is a rewarding journey that combines variety selection with attentive care. By starting with naturally tall types like Dinnerplates or Decoratives and providing them with ample sun, deep water, and steady nutrients, you set the stage for a spectacular vertical display. Remember that height is only half the battle; ensuring your plants are well-supported and healthy is what allows those tall stems to shine.
At Longfield Gardens, we take pride in helping you achieve your gardening dreams. Whether you are a beginner or a seasoned pro, the joy of watching a dahlia reach for the sky is a highlight of the summer season. We hope these tips help you grow your most impressive garden yet.
- Select tall-growing varieties like Dinnerplate or Decorative types.
- Provide at least 6–8 hours of direct sunlight for strong, thick stems.
- Stake or support your plants early to prevent bending or breaking.
- Water deeply and consistently to fuel rapid mid-summer growth.
"The secret to tall dahlias is a combination of the right genetics and a strong foundation of healthy soil and steady support."
FAQ
Why are my dahlias so short this year?
The most common reason for short dahlias is variety selection; border dahlias will never grow tall regardless of care. If you chose a tall variety but it remains short, it may be due to a lack of sunlight, insufficient water during the peak growth phase, or poor soil nutrition. Ensure your plants get 6–8 hours of sun and deep, weekly watering to encourage vertical growth.
Do all dahlias need to be staked?
While low-growing border dahlias are self-supporting, almost all varieties that grow over 3 feet tall require some form of support. Tall dahlias have heavy blossoms and hollow stems that can easily snap in high winds or heavy rain. Using a wooden stake or a tomato cage helps maintain their height and keeps the garden looking tidy throughout the season.
Will more fertilizer make my dahlias grow taller?
Fertilizer helps, but more is not always better. High-nitrogen fertilizer early in the season encourages leafy, upward growth, but too much can lead to weak stems and fewer flowers. It is best to use a balanced fertilizer at planting and switch to a phosphorus-rich "bloom booster" once the plant has reached its desired height to strengthen the stems and support the flowers.
Does pinching dahlias make them shorter?
Pinching (removing the growing tip) temporarily pauses vertical growth to encourage the plant to branch out. While this might make the plant look slightly shorter for a week or two, it ultimately creates a much sturdier and more substantial plant. A pinched dahlia will produce multiple tall, strong stems rather than one single, fragile stalk, leading to a better overall display.
