Introduction
There is nothing quite like the excitement of seeing the first massive bud of a giant dahlia begin to unfurl in late summer. These spectacular flowers, often called dinnerplate dahlias, produce blossoms that can easily reach 8 to 10 inches across. They are the undisputed stars of the garden, offering a level of drama and color that few other plants can match. At Longfield Gardens, we love helping home gardeners experience the joy of growing these impressive "flower machines" that bloom from midsummer right up until the first frost.
This guide is designed for any gardener who wants to master the art of growing these oversized beauties. We will cover everything from choosing the right spot to the simple techniques that ensure your plants stay strong and upright under the weight of their heavy blooms. By focusing on a few essential steps, you can transform a handful of tubers into a breathtaking display of summer color.
With the right timing and a little bit of preparation, giant dahlias are incredibly rewarding and accessible for gardeners of all skill levels. If you are unsure which zone you garden in, our Hardiness Zone Map can help.
Understanding Giant Dahlia Bulbs
While many people refer to them as "bulbs," dahlias actually grow from tubers. These look a bit like a bunch of carrots or a cluster of potatoes joined at a central point called the crown. When you are looking for "giant" dahlias, you are typically looking for varieties categorized as "dinnerplate" dahlias. This isn't an official botanical classification, but rather a descriptive term for any dahlia that produces flowers with a diameter of 8 inches or more.
The size of the tuber you receive does not always dictate the size of the final flower. Some giant varieties naturally produce smaller, slender tubers, while others produce large, chunky ones. What matters most is that the tuber has a healthy "eye." This eye is a small growth bud located on the crown, similar to the eye on a potato. As long as that eye is present and the tuber is firm, you have everything you need for a successful plant.
Common giant varieties that we often recommend include the creamy, blush-toned ‘Café au Lait’, the vibrant purple ‘Thomas Edison’, or the sunny yellow ‘Kelvin Floodlight’. Each of these produces the massive, multi-petaled blooms that make dinnerplate dahlias so famous. Because these plants have to work hard to produce such large flowers, they require a bit more support and nutrition than their smaller cousins.
Choosing the Best Location
Success with giant dahlias starts with finding the right home for them in your yard. Because these plants grow quite large—often reaching 4 to 5 feet in height—they need plenty of room to breathe. For a broader walkthrough, see When and Where to Plant Dahlias.
Sunlight Requirements
Dahlias are sun-loving plants. To produce those legendary giant blooms, they need at least 6 to 8 hours of direct sunlight every day. Morning sun is particularly beneficial as it helps dry the dew off the leaves, which keeps the foliage healthy. If you plant them in too much shade, the stems will become "leggy" or weak as they stretch toward the light, and you will see fewer flowers.
Soil and Drainage
The term "drainage" simply refers to how fast water leaves the soil. Dahlias have a very specific preference: they love moisture, but they hate sitting in soggy ground. If the soil stays too wet for too long, the tubers can rot before they even have a chance to sprout.
If you have heavy clay soil that stays wet after a rain, consider planting your dahlias in raised beds or creating small mounds of soil for them. This elevated position allows excess water to move away from the tubers. For those with sandy soil, adding a bit of compost can help the ground hold onto just enough moisture to keep the plants happy.
Key Takeaway: Choose a site with full sun and soil that doesn't stay soggy. If your yard has heavy soil, a raised bed is an easy win for dahlia health.
Timing Your Planting
When it comes to dahlias, patience is a virtue that leads to much better results. These are tropical plants at heart, native to the high plains of Mexico. They have no tolerance for frost and do not like cold soil.
The best time to plant is when the danger of frost has completely passed and the soil has warmed up to about 60°F. A good rule of thumb for most gardeners is to plant your dahlia tubers around the same time you would plant your tomatoes or peppers. If the ground is still cold and clammy, the tuber will just sit there and may potentially struggle.
If you live in a region with a short growing season, you can give your giant dahlias a head start by "starting" them indoors. About 4 to 6 weeks before the last frost, place the tubers in pots with slightly damp potting soil. Keep them in a warm, sunny spot. By the time the weather is warm enough to move them outside, you will already have a small plant ready to go, which can lead to earlier blooms in the summer. For planting timing by zone, the Hardiness Zone Map is a useful reference.
Preparing the Soil for Success
Before you put your tubers in the ground, take a moment to prepare the soil. Giant dahlias are "heavy feeders," meaning they use a lot of nutrients to build those thick stems and massive flowers.
Start by loosening the soil to a depth of about 12 inches. This makes it easier for the dahlia's root system to expand and anchor the plant. We recommend mixing in some well-rotted compost or a balanced, all-purpose organic fertilizer into the planting area. For more on getting the planting hole right, How Deep to Plant Dahlias covers the basics.
When choosing a fertilizer, look at the three numbers on the package (NPK). For dahlias, you want to avoid fertilizers that are very high in nitrogen (the first number). Too much nitrogen encourages the plant to grow lots of green leaves but very few flowers. A fertilizer that is higher in phosphorus and potassium (the middle and last numbers) will support strong roots and bigger blooms.
Step-by-Step Planting Process
Once your soil is warm and prepared, it is time to get your giant dahlia bulbs into the ground. Following these steps ensures your tubers are at the right depth and have enough room to thrive.
- Dig the Hole: For giant varieties, dig a hole that is 6 to 8 inches deep.
- Add a Little Boost: Toss a small handful of bone meal or compost into the bottom of the hole and mix it with the soil.
- Position the Tuber: Lay the tuber horizontally (flat) on the bottom of the hole. Ensure the "eye" or the sprout is pointing up. If you have a cluster of tubers, the point where they all meet (the crown) should be the highest part.
- Spacing: Giant dahlias need space. Plant them 18 to 24 inches apart. This allows for good air circulation, which helps prevent powdery mildew and other common leaf issues.
- Cover with Soil: Fill the hole back in with soil. You only need to cover the tuber with about 2 to 3 inches of soil at first. As the green sprout begins to grow taller, you can gradually fill in the rest of the hole until it is level with the surrounding ground.
What to Do Next:
- Check your local frost dates to ensure the soil is warm enough.
- Lay out your tubers to plan the spacing before you start digging.
- Have your stakes or support structures ready before you put the tubers in the ground.
The Secret to Success: Support and Staking
Giant dahlias are essentially victims of their own success. Their flowers are so large and heavy that a summer rainstorm or a stiff breeze can easily snap the stems. Because dinnerplate varieties can grow 4 or 5 feet tall, providing a "backbone" for the plant is mandatory.
The best time to stake your dahlia is at the moment of planting. If you wait until the plant is large, you risk driving a stake through the tuber underground. The step-by-step advice in How to Pinch and Stake Dahlias is especially helpful here.
Staking Methods
- Single Stake: Drive a sturdy wooden or metal stake (at least 5 or 6 feet tall) into the ground about 2 inches away from the tuber. As the plant grows, use soft garden twine to loosely tie the main stem to the stake every 12 inches.
- Tomato Cages: For many home gardeners, a heavy-duty tomato cage is the easiest solution. Place the cage over the planting spot immediately after burying the tuber. The plant will grow up through the rings, and the side branches will be supported by the wire.
- Corner Staking: if you are planting a row of dahlias, you can place a stake at each corner and wrap twine around the entire perimeter of the group to keep them contained.
Watering Wisely
One of the most common mistakes with dahlias happens right after planting. It is very tempting to give your new tubers a big drink of water, but we advise against this.
Until you see the green sprout emerge from the soil, the tuber doesn't have a root system to drink up water. If you water too early, you are just making the soil cold and wet, which can lead to rot. In most cases, there is enough natural moisture in the spring soil to get the plant started. For shipping and timing details, Longfield Gardens’ Shipping Information page is also useful when planning ahead.
Once the plant is about 6 inches tall and has several sets of leaves, you can begin a regular watering schedule. Dahlias prefer deep watering a few times a week rather than a light sprinkle every day. This encourages the roots to grow deeper into the soil, making the plant more resilient during hot weather. Try to water at the base of the plant to keep the leaves dry.
The Magic of Pinching
If you want a giant dahlia plant that is full and bushy rather than tall and spindly, you must practice "pinching." This might feel a bit scary the first time you do it, but it is one of the best things you can do for your plant.
When your dahlia is about 12 to 16 inches tall and has four sets of leaves, take a pair of clean shears and snip off the very top of the center stem. You want to remove about 3 or 4 inches of the main stalk, cutting just above a set of leaves.
This simple act tells the plant to stop putting all its energy into one single tall stem. Instead, it will start growing side branches from every leaf node below the cut. This results in a much stronger, bushier plant with more stems, which ultimately means more giant flowers for you to enjoy. If you want another quick reference on the technique, see How to Pinch and Stake Dahlias.
Feeding for Bigger Flowers
While we mixed fertilizer into the soil at planting, giant dahlias benefit from a little extra boost during the growing season. Think of it as "fuel" for their mid-to-late summer performance.
Once the plants are about a foot tall, you can begin applying a water-soluble fertilizer every 3 to 4 weeks. Again, stick to a low-nitrogen formula. Many dahlia enthusiasts use a fertilizer designed for tomatoes or roses, as these usually have the right balance of nutrients to promote heavy blooming. Always follow the instructions on the label to avoid over-fertilizing, which can stress the plant.
Cutting and Deadheading
The more you cut your dahlias, the more they will bloom. This is especially true for giant varieties. For more bouquet-friendly tips, How to Cut Dahlias for More Blooms offers a detailed walkthrough.
Cutting for Vases
If you want to bring these massive blooms inside, the best time to cut them is in the cool of the morning. Unlike roses or lilies, dahlias will not open much further once they are cut. Wait until the flower is almost fully open but the petals at the back are still firm and fresh. Use a sharp, clean knife or shears and place the stems immediately into a bucket of room-temperature water.
Deadheading
If you decide to leave the flowers on the plant, you must "deadhead" them once they begin to fade. Deadheading is simply the process of removing spent blooms. If you leave the old flowers on the plant, it will start to produce seeds, which signals the plant to stop blooming. By removing the old flowers, you trick the plant into thinking its job isn't done yet, and it will continue to pump out new buds until the frost arrives.
When deadheading, don't just snip off the flower head. Cut the stem back to the next set of leaves or a side bud. This keeps the plant looking tidy and encourages new growth.
Key Takeaway: Consistent cutting and deadheading are the keys to a long blooming season. Don't be afraid to harvest your flowers!
Realistic Expectations for Giant Dahlias
While we aim for perfection, it is helpful to remember that gardening is a partnership with nature. Your giant dahlias will respond to the specific conditions of your yard.
- Weather Variation: If you have a very hot, dry summer, your dahlias may take a "break" from blooming during the peak heat and then start up again when temperatures cool down in late August.
- The First Year: Sometimes a tuber takes a little longer to get established. If your plant is a bit slower than expected, give it time. Most giant dahlias hit their stride in late August and September.
- Bloom Size: The very first bloom of the season is often the largest. Subsequent blooms might be slightly smaller, but they will still be impressively big compared to standard garden flowers.
At Longfield Gardens, we believe that every garden has its own personality. Your soil, your local microclimate, and the amount of rain you get will all play a role in exactly when and how big your dahlias grow.
Conclusion
Planting giant dahlia bulbs is one of the most rewarding projects you can undertake in your summer garden. These magnificent plants offer a scale of beauty that is truly unmatched, turning any sunny corner into a tropical-looking paradise. By waiting for warm soil, providing sturdy support, and practicing a little bit of "pinching," you can grow flowers that will be the talk of the neighborhood.
Remember these simple steps for success:
- Wait for the soil to reach 60°F before planting.
- Provide a strong stake or cage at planting time.
- Avoid watering until the first green shoots appear.
- Pinch the center stem to encourage a bushier, more productive plant.
We invite you to explore the wide variety of dinnerplate dahlias we offer at Longfield Gardens and start planning your most spectacular summer yet. There is nothing quite like the pride of harvesting a bouquet of flowers the size of a dinner plate from your own backyard.
Growing giant dahlias is an accessible way to add high-impact color and drama to your landscape. With a few basic steps, anyone can successfully grow these incredible summer stars.
FAQ
When is the best time to plant giant dahlia bulbs?
You should plant dahlia tubers in the spring once the danger of frost has passed and the soil has warmed to about 60°F. In most regions, this is roughly the same time you would plant tomatoes or other warm-weather vegetables. If you need a refresher on timing, When and Where to Plant Dahlias is a good place to start.
Do giant dahlias really need stakes?
Yes, giant dahlias absolutely require support. The flower heads are so large and heavy that the stems can easily snap during wind or rain. It is best to put your stakes or tomato cages in place at the time of planting to avoid damaging the tubers later.
Why are my giant dahlias not blooming?
The most common reasons for a lack of blooms are too much shade or too much nitrogen in the fertilizer. Ensure your plants get at least 6 to 8 hours of sun and use a fertilizer that is higher in phosphorus and potassium than nitrogen.
Can I grow giant dahlias in containers?
While giant dahlias are quite large, you can grow them in containers if the pot is big enough. Use a heavy pot that holds at least 5 to 7 gallons of soil to prevent it from tipping over, and ensure you have a way to secure a stake or support within the container. For planting depth details, How Deep to Plant Dahlias can help you set them up correctly.
