Introduction
There is a special kind of excitement that comes with seeing the first green shoots of spring. For many of us, dahlias are the crown jewels of the summer garden, offering a spectacular array of colors and shapes that last until the first frost. While many gardeners wait until the soil is warm to plant their tubers directly into the ground, there is a simple technique that can give you a significant head start. At Longfield Gardens, we love helping you get the most out of every growing season, and "forcing" your tubers is one of the best ways to do just that.
Forcing dahlia tubers, also known as pre-starting or "waking them up," involves potting them up indoors before the outdoor planting season begins. This practice allows the plants to develop strong root systems and early foliage in a controlled environment. By the time the weather is warm enough for them to live outside, you will have established plants ready to take off. This guide is for any gardener who wants to see those first blooms earlier in the summer and protect their young plants during their most vulnerable stage.
Learning how to force dahlia tubers is a straightforward process that rewards a little bit of indoor effort with weeks of extra flowers. By following a few basic steps regarding timing, light, and moisture, you can transform your gardening schedule and enjoy a more productive, vibrant landscape.
Why You Should Consider Forcing Dahlias
Forcing dahlias is not a requirement for success, but it offers several distinct advantages that make the extra effort worthwhile. For many, the primary motivation is the timeline. Dahlias are tropical plants at heart, and they require warm soil to begin growing. In many parts of the United States, the ground doesn't reach the ideal temperature of 60°F until late May or even June. If you wait until then to plant, you might not see flowers until late August.
When you force tubers indoors, you essentially shift the calendar in your favor. By starting them four to six weeks before the last frost, you can often enjoy blooms starting in July. This extended flowering window is particularly beneficial if you live in a region with a short growing season. It gives the plants more time to reach their full height and produce the massive dinnerplate dahlias that so many gardeners adore.
Beyond the timing, forcing tubers provides a safety net for your plants. Young dahlia shoots are a favorite snack for slugs and snails. When tubers are planted directly in the garden, these pests can devour the tiny sprouts before you even know they’ve emerged. By growing them in pots indoors, you allow the stems to become taller and tougher. Once the leaves are more mature and the stems are woody, they are much less appealing to garden pests. This head start ensures your dahlias are sturdy enough to handle the outdoor environment from day one.
The Best Time to Start Forcing Tubers
Timing is everything when it comes to forcing dahlia tubers. If you start too early, the plants may become too large for their indoor space and grow "leggy" or weak. If you start too late, you won't gain much of a head start. The goal is to find the perfect window that allows the plant to grow just enough to be ready for the transition outside.
In most regions, the ideal time to begin the forcing process is late March or early April. Generally, this is about four to six weeks before your local average last frost date. Because we ship our products based on USDA hardiness zones, your order from Longfield Gardens will typically arrive around the right time for you to begin this process or prepare for outdoor planting.
It is important to remember that dahlias are very sensitive to cold. They should not go into the ground outdoors until all danger of frost has passed and the soil has warmed up. Forcing them indoors gives you something beautiful to tend to while you wait for the spring weather to settle. If you aren't sure of your last frost date, our Hardiness Zone Map is a great resource for accurate regional data.
Key Takeaway: Start your dahlia tubers indoors 4 to 6 weeks before the last expected frost to get a head start on the season and protect young growth from pests.
Choosing the Right Tubers for Forcing
Before you begin potting, it is a good idea to inspect your tubers. Whether you are using tubers you stored over the winter or you have just received a fresh shipment, you want to ensure they are healthy and ready to grow. A healthy dahlia tuber should feel firm, similar to a potato. It is normal for them to have a few wrinkles, but they should not feel mushy or completely dried out and hollow.
The most important part of the tuber is the "eye." The eye is the small bump or growth point located on the "neck" of the tuber, where it connects to the old stem. This is where the new sprout will emerge. Sometimes the eyes are very easy to see, especially if they have already begun to sprout a tiny bit of green. Other times, they are "blind," meaning the eye hasn't quite woken up yet.
If you don't see an eye immediately, don't worry. Placing the tubers in a warm, slightly bright spot for a few days can often help them "eye up." Once you see that little bump or a green tip, you know exactly where the growth will come from, which helps you position the tuber correctly in the pot.
What to Look For:
- Firmness: The tuber should be solid and heavy for its size.
- Intact Neck: The thin part connecting the tuber to the crown must be unbroken.
- Visible Eye: Look for a small "pimple" or sprout near the stem end.
- Clean Surface: No signs of soft rot or fuzzy mold.
Gathering Your Materials
One of the best things about forcing dahlias is that you don't need specialized equipment. Most of what you need is likely already in your garden shed or can be easily found at a local garden center.
Containers and Pots
You can use a variety of containers for forcing. Individual plastic pots that are 6 to 8 inches in diameter work well. If you have a lot of tubers, you can also use large, shallow trays or even recycled nursery flats. The most critical requirement is drainage. "Drainage" simply means how fast water leaves the soil. Dahlias do not like to sit in soggy soil, as this can lead to rot. Ensure your pots have plenty of holes in the bottom.
Potting Soil
Always use a high-quality, well-draining potting mix. Avoid using "garden soil" or "topsoil" from the yard, as these are often too heavy and can contain pests or diseases. A potting mix that contains peat moss, perlite, or vermiculite is ideal because it holds just enough moisture while allowing excess water to drain away easily.
Light and Warmth
Once the tubers are potted, they will need a warm spot to stimulate growth. A room temperature between 60°F and 70°F is perfect. Once the green shoots appear, they will need a lot of light. A very bright, south-facing window can work, but many gardeners find that a simple LED grow light helps keep the plants from stretching and becoming thin.
Step-by-Step Guide to Forcing Dahlia Tubers
Now that you have your materials and your tubers are ready, you can begin the potting process. This is a rewarding afternoon project that sets the stage for a summer of color.
Step 1: Prepare the Soil
Start by slightly dampening your potting mix. You want it to feel like a wrung-out sponge—moist but not dripping wet. Fill your pots or trays about halfway with the soil. Don't pack it down too tightly; you want the roots to be able to move through it easily.
Step 2: Position the Tubers
Lay the tuber horizontally on top of the soil. If the tuber has a visible sprout or eye, make sure that part is facing upward or toward the center of the pot. If you are using a large tray to start multiple tubers, leave a little space between them so the roots don't get tangled later.
Step 3: Cover and Label
Cover the tubers with another inch or two of soil. It is perfectly fine if the very top of the neck or the sprout is still slightly visible. This can actually help you monitor their progress. Most importantly, label each pot immediately. Many dahlia tubers look identical, and you will want to know which one is the tall Thomas Edison and which is a shorter border variety when it comes time to plant them in the garden.
Step 4: Initial Watering
Give the pots a very light watering just to settle the soil. At this stage, the tuber has all the energy it needs to start growing, so it doesn't need much water. In fact, overwatering before the plant has leaves is the most common cause of tuber rot. Wait until you see green growth emerging before you start a more regular watering schedule.
What to Do Next:
- Place the pots in a warm spot (65-70°F).
- Check daily for the first signs of green sprouts.
- Keep the soil barely moist to the touch.
- Prepare your light source for when the leaves appear.
Managing Growth Indoors
Once the sprouts emerge from the soil, the plant's needs change. This is the "maintenance phase" of forcing, and it is where you ensure your dahlias grow up to be strong and healthy. For a broader look at season-long care, see All About Dahlias.
Light Requirements
As soon as you see green, the plants need light. Without enough light, dahlias will grow very fast and become "leggy," meaning they have long, weak stems that can't support their own weight. If you are using a windowsill, rotate the pots every few days so the plants don't lean too far in one direction. If you are using grow lights, keep the lights just a few inches above the tops of the plants and move them upward as the dahlias grow. Aim for about 14 to 16 hours of light per day.
Temperature and Airflow
Dahlias enjoy a comfortable room temperature. Try to keep them away from cold drafts or direct heat from a furnace vent. Good airflow is also important to prevent issues like powdery mildew, which can occasionally occur in humid indoor environments. A small fan set on low can keep the air moving and help strengthen the stems.
Watering and Feeding
Once the plant has a few sets of leaves, you can begin watering more regularly. The "finger test" is the best way to know when to water: stick your finger an inch into the soil. If it feels dry, it’s time to water. If it still feels damp, wait another day. You generally don't need to fertilize forced dahlias while they are in their starter pots, as most potting mixes have enough nutrients to last for a few weeks. Save the heavy feeding for once they are settled in the garden.
Pinching for a Bushier Plant
When your forced dahlia is about 8 to 12 inches tall and has several sets of leaves, you might notice it growing straight up like a single pole. To get more flowers and a sturdier plant, we recommend "pinching" it.
Pinching sounds scary to new gardeners, but it is actually very helpful for the plant. Simply use your fingers or a clean pair of snips to remove the very top of the main stem, just above the highest set of leaves. This tells the plant to stop growing upward and start growing outward. The plant will respond by sending out two new stems from the leaf joints below the pinch. This results in a bushier, more stable plant with many more blooming sites.
Key Takeaway: Pinching the center stem when the plant is about a foot tall encourages branching, which leads to a stronger plant and more flowers throughout the season.
Transitioning to the Garden
The process of moving plants from a cozy indoor environment to the great outdoors is called hardening off. This is a crucial step. Your forced dahlias have been living in a stable, protected space, and the direct sun, wind, and temperature swings of the outdoors can be a shock if they aren't introduced gradually.
The Hardening Off Process
About a week or two before you plan to plant, start by taking your pots outside for an hour or two. Choose a sheltered, shady spot. Each day, leave them out for a little longer and gradually move them into more sunlight. Bring them back inside every night. By the end of the week, they should be ready to stay out all night, provided there is no frost in the forecast.
Planting in the Ground
When it is time to transplant, treat your forced dahlias with care. Dig a hole that is slightly larger than the pot. Gently remove the plant from the pot, keeping the root ball intact. Place it in the hole so the top of the root ball is level with the surrounding soil. For more detail on the process, see how to plant dahlia bulbs.
If you are planting tall varieties, such as dinnerplate dahlias, this is the best time to put your stakes in the ground. Driving a stake into the ground later can damage the tubers you worked so hard to grow. Tie the main stem loosely to the stake to provide support as it continues to grow.
Common Questions About Forcing Dahlias
Many gardeners wonder if they can use the forcing process to take cuttings. Yes, this is a fantastic way to multiply your dahlia collection! Once the shoots on your forced tuber are about 3 inches long, you can carefully snip them off at the base and place them in a small pot of moist starting mix. With warmth and humidity, these cuttings will grow their own roots and become brand-new plants that are identical to the parent.
Another common concern is what to do if a tuber doesn't sprout as quickly as the others. Just like people, some dahlias are "early risers" and others like to sleep in. Some varieties can take three or four weeks to show any signs of life. As long as the tuber remains firm and isn't rotting, give it time. Warmth is usually the key to waking up a stubborn tuber.
Summary of the Forcing Process
Forcing dahlia tubers is one of the easiest "pro tips" for a more successful garden. It bridges the gap between the end of winter and the start of summer, giving you a head start that results in earlier blooms and stronger plants.
Next Steps for Success:
- Inspect your tubers for health and look for the growth "eye."
- Pot them in well-draining mix in late March or early April.
- Provide plenty of light and keep the soil lightly moist.
- Harden off the plants carefully before moving them to their permanent garden home.
Forcing dahlias turns the anticipation of spring into an active, rewarding project. By giving your tubers a little extra attention indoors, you ensure they hit the ground running once the summer sun arrives.
Conclusion
At Longfield Gardens, our goal is to make gardening a joyful and achievable part of your life. Forcing dahlia tubers is a perfect example of how a small amount of preparation can lead to a massive payoff in the garden. By starting your plants indoors, you bypass the slow-starting phase of the early season and jump straight into the excitement of vigorous growth. Whether you are growing tiny pompon dahlias or massive decorative types, this head start helps every variety reach its full potential.
Once your dahlias are established in the garden, they will continue to reward you with handfuls of fresh-cut flowers all the way until the first frost of autumn. The extra month of color you gain by forcing is truly a gift to yourself. We hope you find the process as rewarding as we do. If you are ready to start your journey, we invite you to explore our new dahlia collections and find the perfect varieties for your yard.
FAQ
How long does it take for forced dahlia tubers to sprout?
Most dahlia tubers will begin to show green sprouts within two to three weeks if they are kept in a warm environment (65-70°F). However, some varieties are naturally slower and may take up to four or five weeks to "eye up." As long as the tuber feels firm and isn't mushy, it is likely just taking its time to wake up.
Do I need to fertilize my dahlias while forcing them indoors?
Generally, no fertilization is needed during the forcing stage. Most high-quality potting mixes contain a small amount of nutrients that are sufficient for the first few weeks of growth. It is better to wait until the plants are transplanted into the garden and have begun active growth before starting a regular fertilizer routine.
What should I do if my forced dahlias are getting too tall and thin?
Leggy growth is almost always caused by a lack of sufficient light. Move your plants to a brighter location, such as a south-facing window, or place them under a grow light for 14-16 hours a day. You can also pinch the top of the main stem once it is about 12 inches tall to encourage the plant to grow bushier rather than taller.
Can I leave my forced dahlias outside overnight once they are potted?
You should only leave your dahlias outside overnight once you have finished the "hardening off" process and you are certain there is no risk of frost. Dahlias are very sensitive to cold temperatures; even a light frost can kill the tender new foliage. Always check your local weather forecast before leaving them out for the night.
