Introduction
There is nothing quite like the first time a giant "dinnerplate" dahlia opens in your garden. These magnificent flowers, which can reach the size of a literal dinner plate, bring a sense of wonder and scale that few other plants can match. At Longfield Gardens, we believe every gardener should experience the thrill of harvesting a bloom so large it takes two hands to hold.
While their size may look like the result of professional secrets, growing giant dahlias is remarkably straightforward. Success comes down to choosing the right varieties and following a few simple steps to support their heavy stems. This guide is for any home gardener who wants to turn their backyard into a late-summer showcase of massive, colorful blossoms.
By matching your local conditions with the right care routine, you can produce prize-worthy flowers from midsummer until the first frost.
Understanding Giant Dahlias
When people talk about giant dahlias, they are usually referring to "dinnerplate" varieties. It is important to know that "dinnerplate" isn't a formal botanical classification. Instead, it is a descriptive term used for any dahlia variety that consistently produces flowers measuring 8 to 12 inches in diameter.
These plants are the heavyweights of the summer garden. Because they put so much energy into creating enormous petals, the plants themselves tend to be quite large, often reaching 4 to 5 feet in height. They act almost like flowering shrubs, filling gaps in the landscape with lush green foliage before the grand finale of their blooming season.
Because these varieties are bred for size, they have slightly different needs than the smaller "pompon" or "border" dahlias. They require more support and a bit more focus on nutrient timing. However, the reward is a focal point that will have neighbors stopping at your garden gate to ask for your "secret."
Selecting the Best Varieties
The first step in growing giant dahlias is starting with the right genetics. Not all dahlias are capable of producing 10-inch blooms, no matter how much you fertilize them. You must look for varieties specifically labeled as dinnerplate dahlias.
We have spent years trialing different types in our test gardens to ensure they perform well for home gardeners. Some of the most reliable and popular giant varieties include:
- Café au Lait: A world-renowned favorite featuring creamy, peachy-pink petals. It is the gold standard for weddings and elegant arrangements.
- Thomas Edison: A classic variety with deep, royal purple blooms that stand out beautifully against green foliage.
- Fleurel: For a clean, crisp look, this variety produces massive, snowy white flowers that look stunning in the moonlight.
- Emory Paul: This is one of the true giants, often producing pinkish-purple flowers that exceed 12 inches in diameter.
- Kelvin Floodlight: A bright, sunny yellow dahlia with perfectly formed petals that consistently win awards for size and shape.
- Penhill Dark Monarch: This variety offers a more "shaggy" or informal look with smoky pink and purple tones.
When you receive your tubers, remember that the size of the tuber does not dictate the size of the flower. A small tuber with a healthy "eye" (the small bump where the sprout emerges) contains all the energy needed to grow a massive plant.
Choosing the Perfect Spot
To grow large flowers, your dahlias need plenty of fuel, and that fuel starts with the sun. Right plant, right place is the most important rule in gardening.
Sun Exposure
Giant dahlias are sun-seekers. They require at least six to eight hours of direct sunlight every day. In cooler northern climates, they can handle full sun all day long. In very hot southern regions, they may benefit from some light afternoon shade to keep the delicate petals from scorching, but they still need that strong morning sun to develop their large heads.
Soil and Drainage
"Drainage" is a term gardeners use to describe how fast water leaves the soil. Dahlias have fleshy tubers that can rot if they sit in puddles. You want soil that holds some moisture but allows the excess to drain away freely.
If you have heavy clay soil that stays wet, consider planting your dahlias in raised beds. This keeps the tubers above the "waterline" of the surrounding yard. If you are planting in the ground, mixing in some compost or well-rotted manure before planting can help improve the soil structure. For more guidance, see this dahlia planting and growing guide.
Key Takeaway: Choose a spot with 6–8 hours of sun and soil that doesn't stay soggy after a rainstorm.
Preparing the Soil
A giant dahlia is a "heavy feeder," meaning it uses a lot of nutrients to build its tall stems and huge flowers. Preparing the soil before you plant gives the tuber a head start.
Start by loosening the soil to a depth of about 12 inches. This makes it easier for the new roots to spread out and find water. Mix in a few inches of compost or aged leaf mold. Avoid using high-nitrogen fertilizers at this stage, as too much nitrogen can encourage lots of green leaves but very few flowers. A balanced, all-purpose fertilizer mixed into the planting hole is usually all you need to get started.
Timing and Planting Depth
Timing is the most common hurdle for dahlia growers. Because dahlias are tropical plants, they are very sensitive to cold.
The Soil Temperature Rule
Do not be tempted to plant your dahlias as soon as the first warm day of spring arrives. The soil needs to be warm—around 60°F—before the tubers go into the ground. A good rule of thumb is to plant your dahlias at the same time you would plant tomatoes or peppers. If the ground is too cold and wet, the tuber will simply sit there and may rot before it ever has a chance to grow.
How to Plant
- Dig a hole: Make it about 6 to 8 inches deep.
- Place the tuber: Lay the tuber horizontally (flat) at the bottom of the hole. If you can see the "eye" or a small sprout, point it upward.
- Space them out: Giant dahlias need room to breathe. Space your plants at least 18 to 24 inches apart. This ensures good airflow, which helps prevent leaf diseases later in the summer.
- Cover: Fill the hole back in with soil. You do not need to mound the soil; just keep it level with the surrounding ground.
For additional planting guidance, see how to plant dahlia flowers.
What to do next:
- Wait for the last frost date in your area.
- Check the soil temperature with a simple kitchen or garden thermometer.
- Dig your 6-inch holes and space them at least 18 inches apart.
- Lay the tubers flat and cover them completely.
The Golden Rule of Watering
This is the most important "pro tip" for growing dahlias: Do not water your tubers immediately after planting.
Until the dahlia sprout emerges from the soil and has its first set of leaves, it does not have a way to process a lot of water. The tuber has enough stored moisture to get the plant started. Watering too early is the number one cause of tuber rot.
Once you see the green shoots poking through the soil, you can begin a regular watering schedule. As the plants grow larger, they will need more water. During the heat of the summer, aim for about an inch of water per week.
When you water, try to soak the ground deeply rather than giving it a light sprinkle every day. Deep watering encourages the roots to grow further down into the soil, which makes the plant more stable and drought-resistant. Always try to water at the base of the plant to keep the leaves dry, as wet leaves can attract mildew.
Supporting the Weight: Staking Systems
When you are growing giant dahlias, staking is not optional. A single "dinnerplate" bloom can be heavy, especially after a rain shower when the petals hold onto water. Without support, a summer breeze can easily snap the brittle stems.
Stake Early
The best time to put a stake in the ground is at the moment of planting. If you wait until the plant is large, you risk driving the stake right through the tuber underground.
Staking Options
- Single Sturdy Stakes: Use a 5-foot or 6-foot tall stake (wood, bamboo, or metal) driven at least 12 inches into the ground right next to the planting hole. As the plant grows, use soft twine or garden ties to secure the main stem to the stake every 12 inches.
- The "Tomato Cage" Method: For home gardeners, a heavy-duty tomato cage can work well. It provides support on all sides as the plant grows into a bushy shape.
- The Corral Method: If you are planting a row of dahlias, you can drive sturdy posts at the corners and wrap twine around the entire group at different heights. This creates a "corral" that keeps the plants from flopping over.
See the complete guide to planting and caring for dahlias for additional staking advice.
Key Takeaway: Support is essential for giant blooms. Install your stakes at planting time to protect the tubers and ensure your flowers stay upright all season.
Fertilizing for Massive Blooms
While we mentioned avoiding high nitrogen at the start, giant dahlias do appreciate regular feeding once they are established.
Wait until the plants are about 12 inches tall before you start a regular fertilizing routine. Look for a fertilizer that is lower in Nitrogen (the first number on the bag) and higher in Phosphorus and Potassium (the second and third numbers). These nutrients help with root development and flower production.
A water-soluble fertilizer applied every two to three weeks is a simple way to keep the plants happy. If you prefer organic options, liquid seaweed or fish emulsion are excellent choices. Stop fertilizing in late summer (usually around early September) to allow the plant to start focusing on tuber health for the winter.
The Secret to Giant Blooms: Disbudding
If you want your dahlias to reach their maximum potential size, you need to use a technique called "disbudding."
Dahlias naturally grow in clusters of three buds at the end of every stem. There is one central "terminal" bud and two side buds. If you leave all three, the plant divides its energy among them, resulting in three medium-sized flowers.
To get one giant flower, use your fingers or small snips to gently remove the two smaller side buds, leaving only the central bud. By doing this, the plant directs 100% of its energy, water, and nutrients into that single blossom. This is the "secret" used by competitive growers to get those massive, 12-inch wide prize winners.
For more detail, read about dahlia pinching, deadheading, and disbudding.
To Pinch or Not to Pinch?
"Pinching" is a common gardening practice where you snip off the top of the main stem when the plant is about 12 to 18 inches tall. This encourages the plant to branch out, resulting in a bushier plant with more flowers.
For giant dahlias, there is a trade-off. Pinching will give you more flowers, but they might be slightly smaller. If your goal is to have the absolute largest single bloom possible, you might skip pinching. However, for most home gardeners, pinching is recommended because it creates a sturdier, more attractive plant that produces many more beautiful "giant" flowers over the course of the season.
Routine Care and Maintenance
Gardening is a rewarding journey, and a little bit of weekly maintenance will keep your dahlia patch looking its best.
Deadheading
"Deadheading" means removing flowers as they begin to fade. If you leave old flowers on the plant, it thinks its job is done and will stop producing new buds. By cutting off the old blooms, you signal the plant to keep making more. When you deadhead, don't just snip the flower head; cut the stem back to where it meets a main branch. This keeps the plant tidy and encourages longer stems for future flowers.
Pest Management
Dahlias are generally hardy, but they can attract a few common visitors. Slugs and snails love the tender young shoots in the spring. You can manage them by hand-picking or using simple copper barriers or organic slug bait.
Later in the season, you might see aphids or mites. A strong blast of water from the hose is often enough to knock them off. If you notice holes in the leaves, it may be earwigs or grasshoppers. At our trial gardens, we find that maintaining a clean garden area with few hiding spots is the easiest win for pest control.
Mulching
Once your dahlias are about a foot tall, you can apply a layer of mulch (like straw or shredded bark) around the base. This helps keep the soil cool and moist during the heat of July and August. Just be sure to keep the mulch a few inches away from the actual stem to prevent any moisture-related issues.
Harvesting for Bouquets
One of the best things about giant dahlias is bringing them inside. A single dinnerplate dahlia in a vase is a complete centerpiece.
The best time to harvest is in the cool of the early morning or late evening. Unlike roses or lilies, dahlias will not open much after they are cut. You should wait to harvest until the flower is about 75% to 90% open. Check the back of the flower; if the petals are starting to turn brown or feel soft, the bloom is past its prime.
Use sharp, clean bypass pruners or garden shears. Cut the stem at an angle and place it immediately into a bucket of room-temperature water. To help the bloom last longer, remove any leaves that would be submerged in the vase water. Change the water every day or two to keep it fresh.
For further guidance, see this dahlia planting and care resource.
What to do next:
- Wait for the flower to be nearly fully open before cutting.
- Cut early in the morning when the plant is most hydrated.
- Use a clean vase and change the water daily.
Planning for Next Year
Dahlias are perennials in warm climates (Zones 8-11), but in most of the United States, the tubers need to be protected from freezing.
When the first frost of autumn hits, the foliage will turn black. This is your signal that the growing season is over. You can cut the stalks down to about 4 inches. If you live in a cold climate, you will need to carefully dig up the tubers and store them in a cool, dry, frost-free place (like a basement or garage) for the winter.
Many gardeners find this part of the process exciting because dahlia tubers multiply underground. The single tuber you planted in May will often become a clump of three to five tubers by October, meaning you’ll have even more to plant next year!
Read more about seasonal dahlia care and overwintering.
Realistic Expectations
Growing giant dahlias is a joyful experience, but it is helpful to remember that nature has its own timeline.
The exact size of your flowers and the timing of your first bloom will depend on your local weather, soil quality, and how much sun the garden receives. Some years might bring a "heat dome" that slows down production, while other years might provide the perfect balance of rain and sun for record-breaking sizes.
If your first flower isn't quite 12 inches across, don't worry. The plants often produce their largest and most vibrant blooms as the nights begin to cool down in late August and September. Patience is part of the reward.
"The secret to gardening is getting the basics right: sun, drainage, and support. Once you provide those, these giant dahlias do the rest of the work themselves."
Conclusion
Giant dahlias are the true "showstoppers" of the summer garden, offering an incredible return on the little bit of effort they require. By selecting the right dinnerplate varieties from Longfield Gardens and giving them a sunny, well-drained home with plenty of support, you can enjoy a season filled with breathtaking color and scale. Whether you are growing them for competition, for gorgeous bouquets, or simply to enjoy the view from your porch, these flowers are sure to bring a smile to your face.
- Start with high-quality dinnerplate tubers.
- Wait for warm soil before planting.
- Provide strong stakes early in the season.
- Disbud the side growth for the largest possible flowers.
We are here to help you succeed every step of the way. If you have questions about which variety is right for your zone or need help with your order, our team is ready to support your gardening journey. Grab a few tubers this season and get ready to grow something truly magnificent!
FAQ
Why are my dahlia flowers smaller than expected?
The most common reasons for smaller flowers are too much shade, lack of fertilizer, or not "disbudding" the side growth. If the plant has to feed dozens of buds at once, the flowers will naturally be smaller. To get those giant sizes, make sure the plant has full sun and remove the side buds so the energy focuses on one main bloom per stem.
Can I grow giant dahlias in containers?
While possible, it is challenging because giant dahlias need a lot of space and very heavy support. You would need a very large, heavy pot (at least 5-10 gallons) to prevent the tall plant from tipping over. In most cases, these large varieties perform much better when planted directly in the ground or in large raised beds. See the guide to growing dahlias in containers for more information.
Are dahlia tubers toxic to pets?
Yes, dahlia tubers and foliage can be toxic to dogs and cats if eaten, potentially causing skin irritation or stomach upset. It is always a good idea to keep your tubers out of reach while planting and monitor your pets if they like to dig in the garden. Consult your veterinarian if you suspect your pet has ingested any part of the plant.
When will my dinnerplate dahlias start blooming?
Dinnerplate dahlias generally take a bit longer to reach maturity than smaller varieties because they have so much growing to do. Most will start blooming in mid-to-late July and will continue until the first frost. If you live in a short-season climate, you can "start" your tubers indoors in pots about 4-6 weeks before the last frost to get an earlier start on the blooming season.
