Table of Contents
- Introduction
- Selecting the Right Varieties for Maximum Size
- The Best Location for Giant Dahlias
- Soil Preparation and Feeding Strategies
- Step-by-Step Planting Guide
- Pinching for a Sturdier Plant
- The Art of Disbudding for Exhibition-Sized Blooms
- Essential Staking and Support
- Watering Techniques for Lush Growth
- Maintaining Blooms Throughout the Season
- Troubleshooting Smaller Than Expected Flowers
- Conclusion
- FAQ
Introduction
There is a unique kind of magic in watching a dinnerplate dahlia unfurl. One moment you have a green bud the size of a walnut, and a few days later, you are staring at a flower the size of a literal dinner plate. These giants are the undisputed celebrities of the late-summer garden, often stopping neighbors in their tracks and making every bouquet look like a masterpiece. If you have ever wondered how some gardeners manage to produce those massive, head-turning blooms, you will be happy to know that it is much simpler than it looks.
At Longfield Gardens, we believe that every gardener can find success with these spectacular flowers, and our 100% Quality Guarantee helps make that promise easier to trust. While dahlias are generally easy to grow, achieving those exhibition-sized blooms requires a slightly different approach than growing smaller bedding varieties. It is all about giving the plant the right foundation and then using a few simple "tricks of the trade" to focus the plant's energy.
In this guide, we will walk you through the essential steps for growing the largest dahlias possible. We will cover variety selection, soil preparation, and the specific pruning techniques that turn a standard dahlia into a garden giant. Whether you are a beginner or looking to level up your garden, these tips will help you grow the biggest, boldest blooms of your life.
Growing massive dahlias is a rewarding journey that combines the right plant choice with consistent, simple care.
Selecting the Right Varieties for Maximum Size
The most important step in growing large dahlias happens before you ever pick up a shovel. You must choose varieties that have the genetic potential to grow large. In the world of dahlias, these are typically categorized as "Dinnerplate" dahlias. While a standard decorative dahlia might produce flowers 4 to 6 inches across, a dinnerplate variety is bred to produce blooms that reach 8, 10, or even 12 inches in diameter.
When you are browsing our selection, look for specific names known for their reliable size. Some of our favorites for giant blooms include:
- 'Café au Lait': This is perhaps the most famous dinnerplate dahlia, loved for its creamy, blush-pink petals and massive size.
- 'Kelvin Floodlight': A classic choice that produces enormous, glowing yellow blooms that can easily reach 10 inches across.
- 'Spartacus': This variety offers deep, velvety red flowers with recurved petals that add incredible depth and volume.
- 'Emory Paul': If you want the absolute biggest blooms possible, this variety can produce flowers that exceed 12 inches under the right conditions.
- 'Thomas Edison': A reliable deep purple variety that, while slightly smaller than 'Emory Paul', still produces impressively large, formal decorative blooms.
It is helpful to remember that "Dinnerplate" is not an official botanical classification but a descriptive term used for dahlias with large flower heads. These plants also tend to be taller and more robust than other types, often reaching 4 to 5 feet in height. By starting with these high-performing varieties, you ensure that your efforts in the garden will result in the giant flowers you desire.
The Best Location for Giant Dahlias
To reach their full potential, giant dahlias need the right environment. Think of these plants as high-performance athletes; they need plenty of fuel and the right "stadium" to perform. The two most critical factors for your site selection are sun exposure and soil drainage.
Sunlight Requirements
Dahlias are sun-loving plants. To produce the energy required to create massive blooms, they need at least 6 to 8 hours of direct sunlight every day. Morning sun is particularly beneficial because it dries the dew off the leaves, which helps prevent powdery mildew and other fungal issues. If you plant them in too much shade, the stems will become "leggy" and weak as they stretch for the light, and the flowers will be significantly smaller.
Soil and Drainage
Dahlias have a love-hate relationship with water. They need plenty of moisture to grow, but their tubers are very susceptible to rot if they sit in soggy soil. This is why drainage is so important. If your garden has heavy clay soil that stays wet after a rain, consider planting your dahlias in raised beds. Raised beds allow you to control the soil mix and ensure that excess water drains away from the tubers.
If you are planting in the ground, you can improve drainage by mixing in organic matter like compost or well-rotted manure. This not only helps water move through the soil but also provides the nutrients the plants need to thrive.
Key Takeaway: A sunny spot with "fast-draining" soil is the most important factor for plant health. If the soil stays wet like a sponge, the tubers may struggle.
Soil Preparation and Feeding Strategies
Because large dahlias produce so much vegetation and such massive flowers in a single season, they are considered "heavy feeders." This means they require a high level of nutrients in the soil. However, the type of nutrients you provide matters just as much as the amount.
Preparing the Bed
Before planting, we recommend working 2 to 3 inches of compost into the top foot of soil. Compost improves the soil structure and provides a slow-release source of nutrients. Avoid using fresh manure, as it can be too high in nitrogen and may contain salts that can damage the tender tubers.
Understanding Fertilizers
When you look at a bag of fertilizer, you will see three numbers (like 5-10-10). These represent Nitrogen (N), Phosphorus (P), and Potassium (K).
- Nitrogen promotes green, leafy growth.
- Phosphorus encourages strong root development and flower production.
- Potassium supports overall plant health and tuber quality.
For large dahlias, you want to avoid high-nitrogen fertilizers once the plant is established. Too much nitrogen will give you a giant, beautiful green bush with very few (or very small) flowers. Instead, look for a fertilizer where the second and third numbers are higher than the first. A 5-10-10 or 10-20-20 formula is often ideal.
We suggest a light application at planting time and then feeding the plants every 3 to 4 weeks once they reach about 12 inches tall. Stop fertilizing by late August or early September to allow the plant to begin focusing its energy back into the tubers for winter storage. See How to Overwinter Dahlias for more on storing dahlia tubers.
Step-by-Step Planting Guide
Timing is everything when it comes to planting dahlias. How to Plant Dahlias can help if you want a quick video walkthrough.
When to Plant
Wait to plant your dahlia tubers until the soil temperature has reached at least 60°F. A good rule of thumb for most US gardeners is to plant around the same time you would plant your tomatoes or peppers, and the hardiness zone map can help you check your timing. If you plant them in cold, wet soil in early spring, the tubers are likely to rot before they even start to grow.
Planting Depth and Spacing
For large dahlia varieties, proper spacing is essential for air circulation and light penetration.
- Dig a hole: Make it about 6 to 8 inches deep.
- Add support: If you are using a single stake, drive it into the hole now. Doing it later can puncture the tuber.
- Place the tuber: Lay the tuber horizontally at the bottom of the hole. Look for the "eye" (the small bump where the sprout emerges) and point it toward the stake or upward.
- Cover with soil: Fill the hole with 3 to 4 inches of soil. As the plant grows, you can continue to fill in the rest of the hole.
- Space them out: Give dinnerplate varieties at least 18 to 24 inches of space between plants.
The "No-Water" Rule
This is one of the most common mistakes beginners make. Do not water your dahlias immediately after planting unless your soil is extremely dry. The tuber has enough stored moisture to send up its first sprout. Adding water to the soil before the plant has roots to absorb it is the fastest way to cause rot. Wait until you see the first green shoots poking through the soil before you begin a regular watering schedule.
What to do next:
- Check your local frost dates and wait for warm soil.
- Lay tubers horizontally with the eye facing up.
- Resist the urge to water until you see green growth.
Pinching for a Sturdier Plant
It might feel counterintuitive to cut a perfectly healthy plant, but "pinching" is a vital step for growing large, high-quality blooms. See How to Pinch and Stake Dahlias for a simple step-by-step guide.
How to Pinch
When your dahlia plant is between 12 and 16 inches tall and has at least four sets of leaves, use a clean pair of snips to cut off the very top of the center stem. You want to remove about 2 to 3 inches of growth, cutting just above a set of leaves.
Why It Works
This simple act tells the plant to stop putting all its energy into growing upward and instead start growing outward. The plant will respond by sending out multiple side branches from the leaf nodes below your cut. This results in a bushier, stronger plant that can support the weight of heavy dinnerplate blooms. A pinched plant will produce more stems, which means more opportunities for those giant flowers.
The Art of Disbudding for Exhibition-Sized Blooms
If your goal is to grow the largest blooms possible, "disbudding" is the single most effective technique you can use. This is the secret used by professional dahlia growers and exhibitionists to get those massive, 12-inch diameters.
Understanding the Bud Trio
If you look at the tip of a dahlia branch, you will usually see a cluster of three buds. There is one large central bud and two smaller side buds.
How to Disbud
To get a giant flower, you want to carefully pinch off the two side buds, leaving only the central bud to bloom. You should also remove any small "axillary" buds (the ones that grow where the leaf meets the stem) for the next couple of leaf sets down the branch.
Why It Works
By removing the competing buds, you are forcing the plant to direct 100% of its energy and nutrients into that single central flower. This results in a significantly larger bloom and a longer, stronger stem for cutting. While this means you will have fewer flowers overall on the plant at one time, the flowers you do get will be the "giants" you are looking for.
What to do next:
- Identify the cluster of three buds at the end of a stem.
- Carefully pinch off the two smaller side buds.
- Remove side shoots for the top two sets of leaves to focus energy upward.
Essential Staking and Support
Dinnerplate dahlias are heavy. When you combine a 5-foot-tall plant with lush foliage and flowers that can weigh as much as a grapefruit, you have a recipe for disaster during a summer thunderstorm. Without proper support, the hollow stems of a dahlia can easily snap or the entire plant can be blown over.
Staking Options
- Single Stakes: Sturdy wooden, bamboo, or metal stakes are the most common choice. As mentioned earlier, drive the stake into the ground at planting time. Use soft twine or garden ties to secure the main stem to the stake every 12 to 18 inches as it grows.
- Tomato Cages: For home gardeners, a heavy-duty tomato cage can work well for shorter dinnerplate varieties. It provides 360-degree support for the side branches.
- Corral Staking: If you are growing a row of dahlias, you can drive T-posts into the corners of the bed and wrap twine around the entire perimeter at heights of 18 inches and 36 inches. This "corrals" the plants and keeps them upright.
Always tie the plant loosely. You want to provide support without strangling the stem, which will thicken as the season progresses.
Watering Techniques for Lush Growth
Once your dahlias have established a root system and are growing vigorously, they need a consistent supply of water. Large dahlias have a high transpiration rate, meaning they lose a lot of water through their large leaves, especially on hot summer days.
Deep Watering
The goal is to keep the soil consistently moist but not saturated. Instead of a light daily sprinkling, it is better to water deeply 2 to 3 times a week. This encourages the roots to grow deeper into the soil, making the plant more drought-tolerant and stable.
Keep the Foliage Dry
Whenever possible, water at the base of the plant using a soaker hose or drip irrigation. Getting the leaves wet, especially in the evening, can encourage diseases like powdery mildew. If you must use an overhead sprinkler, do it early in the morning so the sun can dry the leaves quickly.
Mulching
Once the weather heats up in mid-summer, applying a 2-inch layer of mulch around the base of the plants can be very helpful. Mulch helps retain soil moisture and keeps the roots cool. Use clean straw, wood chips, or shredded leaves. Just be sure to keep the mulch a few inches away from the actual stem to prevent rot and discourage slugs.
Maintaining Blooms Throughout the Season
To keep your dahlia plant producing large flowers from July until the first frost, you need to stay active in the garden.
Deadheading
If you don't cut the flowers for bouquets, you must "deadhead" them as they fade. When a flower starts to wither, the plant begins to shift its energy into producing seeds. By cutting off the old flowers, you signal the plant to keep producing new buds. When deadheading, don't just snip the flower head; cut the stem back to the next set of leaves or a main branch to encourage longer stems for the next flush of growth.
Harvesting for the Vase
Dahlias are spectacular cut flowers, but they do not open much after they are cut. For the best results and the largest appearance in a vase, wait until the flower is about 75% to 90% open before harvesting.
Cut your dahlias in the cool of the morning or late evening when the plant is fully hydrated. Place the stems immediately into a bucket of clean, room-temperature water. To extend their vase life, you can strip any leaves that would be below the water line and change the water every two days.
Troubleshooting Smaller Than Expected Flowers
If you have planted a dinnerplate variety but the flowers are coming out small, don't worry. This is a common part of the gardening learning curve. Here are the most likely reasons:
- Lack of Sunlight: If the plant is getting less than 6 hours of sun, it simply doesn't have the "fuel" to make a giant bloom.
- Too Much Nitrogen: High-nitrogen fertilizer promotes leaves over flowers. Switch to a bloom-boosting fertilizer with higher phosphorus.
- Missing the "Disbud" Step: If you leave all three buds on the tip, the energy is split three ways. Next time, remove the side buds early.
- Heat Stress: In extremely hot climates, dahlias may produce smaller or slightly distorted blooms during the peak of summer. Often, the size will return to normal once the temperatures cool down in late August.
- Thirsty Plants: If the soil dries out completely, the plant will prioritize survival over flower size. Ensure consistent moisture.
Remember that gardening is a conversation with nature. Every season is different, and sometimes the weather plays a bigger role in bloom size than our best efforts.
Conclusion
Growing large dahlias is one of the most rewarding projects you can take on in your summer garden. By starting with the right varieties, like the stunning 'Café au Lait' or the vibrant 'Kelvin Floodlight', you set yourself up for success from day one. While these giants require a little more attention than your average perennial, the payoff of seeing those massive blooms in your yard is well worth the effort.
To summarize the path to giant dahlias:
- Choose "Dinnerplate" varieties with large genetic potential.
- Wait for warm soil (60°F) before planting to avoid tuber rot.
- Pinch the plants at 12 inches to create a strong, bushy structure.
- Disbud the side flowers to focus all energy into one massive central bloom.
- Provide sturdy staking early to protect heavy stems from wind and rain.
Growing a dahlia is a lesson in patience and a reward in beauty; there is nothing quite like the feeling of holding a flower that you helped grow to its absolute maximum potential.
We invite you to explore our selection of premium dahlia tubers at Longfield Gardens and start planning your most impressive garden yet. See our shipping information page for order details.
FAQ
Why are my dinnerplate dahlias not reaching their full size?
The most common reasons for smaller blooms are a lack of sunlight, too much nitrogen fertilizer, or failing to disbud. To get maximum size, ensure the plant gets 6-8 hours of sun and remove the two side buds from every flower cluster so the plant focuses all its energy on the center bloom.
When is the best time to start feeding dahlias for big flowers?
Start with compost at planting time, but wait until the plants are about 12 inches tall before applying a supplemental fertilizer. Use a low-nitrogen, high-phosphorus formula (like 5-10-10) every 3-4 weeks until the end of August to encourage large, healthy blooms without excessive leaf growth.
Do I really need to stake my dahlias if they aren't very tall yet?
Yes, it is best to stake dahlias at planting time or shortly after they sprout. Dinnerplate varieties grow very quickly and can become top-heavy almost overnight; staking early prevents you from accidentally damaging the tuber later and ensures the plant has support before a summer storm hits.
Can I grow large dahlias in containers?
You can grow large dahlias in containers, but you will need a very large pot—at least 5 to 7 gallons—to accommodate the root system and the height of the plant. Containers dry out much faster than the ground, so you will need to be diligent with watering and fertilizing to ensure the plant has enough resources to produce giant flowers.