Introduction
Few flowers bring as much joy to the summer garden as dahlias. From the massive, pillowy blooms of dinnerplate varieties to the intricate, honeycomb petals of pompons, these plants are the undisputed stars of the late-season border. At Longfield Gardens, we know that once you experience your first successful dahlia season, you’ll want to ensure those tubers return even bigger and better next year.
Whether you must dig up your dahlia tubers—often called "bulbs"—depends largely on where you live and the nature of your soil. While dahlias are tropical plants that love the heat, many gardeners find they can save time and effort by leaving them in the ground under the right conditions. This guide will help you determine the best path for your specific garden so you can enjoy these spectacular flowers year after year.
The decision to lift or leave your dahlias comes down to a balance of winter temperatures, soil drainage, and your goals for next year’s garden.
Understanding Dahlia Hardiness Zones
The most important factor in deciding whether to dig up your dahlias is your USDA hardiness zone. Dahlias are native to Mexico and Central America, which means they are not naturally equipped to handle deep freezes. In the gardening world, we categorize plants by their ability to survive winter temperatures, and dahlias are generally considered hardy only in warmer climates.
Zones 8 to 11: The "Safe" Zones
If you live in USDA zones 8, 9, 10, or 11, you are in luck. In these regions, the ground rarely freezes deep enough to reach the dahlia tubers. Most gardeners in these areas can safely leave their dahlias in the ground all year round. The tubers will enter a period of dormancy during the cooler months and begin growing again once the soil warms up in the spring.
Zones 2 to 6: The "Dig" Zones
For those living in zones 2 through 6, the winter frost is usually too deep and sustained for dahlia tubers to survive. If left unprotected in these cold climates, the water inside the tubers will freeze, causing the cells to burst. Once the ground thaws, the tubers will simply turn to mush. To save your favorite varieties in these zones, digging and indoor storage are necessary.
Zone 7: The "Gamble" Zone
Zone 7 is the transition zone where many gardeners experiment. Depending on the specific winter and your local microclimate, dahlias may survive in the ground if they are given extra protection. If you have a mild winter, a heavy layer of mulch might be enough to keep the frost at bay. However, a particularly harsh or wet winter in zone 7 can still result in the loss of your plants.
Key Takeaway: Check your USDA hardiness zone first. If you are in zone 7 or lower, digging is the safest way to ensure your dahlias return. In zone 8 and above, they can usually stay put.
The Role of Soil Drainage
Even if you live in a warm climate like zone 9, temperature isn't the only thing that affects dahlia survival. Drainage—which is how quickly water moves through your soil—is just as critical. In fact, more dahlias are lost to rot caused by wet soil than to cold temperatures alone.
Dahlias have "tuberous roots" that store energy and water. During their winter dormancy, they do not need much moisture. If they sit in cold, soggy soil for months, they are highly susceptible to fungal diseases and rot.
Sandy vs. Clay Soil
If your garden has sandy soil, water moves through it quickly. This is ideal for overwintering dahlias in the ground because the tubers stay relatively dry even during rainy winters. However, if you have heavy clay soil, the ground tends to hold onto moisture like a sponge.
In clay-heavy areas, even a zone 8 winter can be deadly if it is a particularly wet season. If you choose to leave your tubers in heavy soil, you must take extra steps to keep them dry, such as using waterproof covers or planting them in raised beds where drainage is naturally better.
How to Overwinter Dahlias in the Ground
If you’ve determined that your climate and soil are suitable for leaving dahlias in the garden, you can’t simply walk away and forget them. A little bit of "tucking them in" goes a long way toward a successful spring showing.
The Mulching Method
The goal of overwintering in the ground is to keep the tubers dormant, dry, and protected from the occasional light frost. Follow these steps for the best results:
- Wait for the first frost: Allow the first light frost to kill back the foliage. This sends a signal to the plant to move all its remaining energy down into the tubers.
- Cut back the stems: Use a sharp pair of shears to cut the stalks down to about 2 to 4 inches above the soil level.
- Apply a "mulch sandwich": Cover the remaining stems with a thick layer of insulating material. A 4- to 6-inch layer of wood chips, bark, or clean straw works well.
- Optional waterproof cover: If your area gets a lot of winter rain, some gardeners place a piece of heavy plastic or a specialized garden tarp over the mulch. This helps shed water away from the "crown" (the top part of the tuber clump where the stems meet).
Monitoring for Pests
When you leave tubers in the ground, they can become a winter snack for rodents like voles or a hiding spot for slugs. In the early spring, before the first green shoots appear, it is a good idea to check under the mulch. If you see signs of tunneling or slug activity, you can address it early before the new, tender growth is damaged.
Why You Might Choose to Dig Even in Warm Zones
You might be surprised to learn that many professional growers and dahlia enthusiasts dig up their tubers every year, even if they live in a climate where the plants would technically survive the winter. There are several practical reasons to consider lifting your dahlias, even if it isn't strictly required for survival.
1. Dividing for More Plants
Dahlia tubers are incredibly productive. A single tuber planted in the spring will often grow into a large clump of 5 to 15 tubers by the fall. If you leave this clump in the ground year after year, it will eventually become overcrowded.
Overcrowded dahlias compete with themselves for nutrients and water. This often leads to smaller flowers and weaker stems. By digging them up, you can divide the clump into individual tubers. This not only keeps the plants healthy but also gives you "free" plants to expand your garden or share with friends.
2. Preventing Disease and Soil Fatigue
Lifting the tubers allows you to inspect them for any signs of disease or pests that might be hiding in the soil. It also gives you the opportunity to refresh the soil in your flower beds. Adding compost or organic matter to the planting hole in the spring is much easier when the tubers have been removed for the winter.
3. Rearranging Your Garden
Gardening is an evolving art. Digging up your dahlias gives you the freedom to change your color scheme every year. If you found that a tall cactus dahlia was overshadowing a shorter decorative variety, lifting them in the fall allows you to swap their positions when you replant in the spring.
4. Better Bloom Control
When dahlias are left in the ground, they often start growing as soon as the soil reaches a certain temperature. If you have an unseasonably warm spring followed by a late frost, those early shoots can be damaged. By storing tubers indoors, you control exactly when they go back into the ground, ensuring they only start growing when the danger of frost has truly passed.
What to Do Next:
- Identify your USDA zone.
- Assess your soil—is it sandy or clay-heavy?
- Decide if you want to expand your collection by dividing tubers.
- If you're in zone 7 or lower, prepare your storage supplies now.
The Step-by-Step Guide to Digging and Storing
If you’ve decided (or your climate has decided for you) that digging is necessary, timing and technique are key. You want to wait until the plant has finished its job for the year but get the tubers out before the ground actually freezes solid.
Timing the Harvest
The best time to dig is usually a few days after the first "killing" frost. You will know it has happened because the lush green foliage will turn black and limp overnight. This isn't a bad thing; it’s just the plant’s natural cycle. Leaving the tubers in the ground for a few days after this frost allows them to "cure" slightly, toughening the skin for storage.
Lifting the Tubers
Be gentle during this process. Tubers can be surprisingly fragile, and the "neck"—the thin part that connects the tuber to the main stem—is the most vulnerable part. If the neck breaks, the tuber may not be able to sprout.
- Clear the area: Cut the stems down to about 6 inches so you have a handle to work with.
- Dig wide: Use a garden fork rather than a shovel if possible. Start digging about 12 inches away from the stem to avoid accidentally slicing through the tubers.
- Lift slowly: Gently pry the clump upward from several sides until it feels loose, then lift it out by the base of the stems.
- Clean and dry: Shake off the loose soil. Some gardeners prefer to wash their tubers with a hose, while others prefer to let the dirt dry and then brush it off. Both methods work, provided the tubers are allowed to dry completely before going into storage.
The Curing Process
Before you pack them away, dahlias need to "cure." Place them in a frost-free, shaded area with good airflow (like a garage or shed) for 1 to 3 days. This allows any excess moisture on the surface to evaporate. Do not leave them in direct sunlight, as this can cause them to shrivel.
Packing for the Winter
The goal of storage is to keep the tubers dormant: not so wet that they rot, and not so dry that they shrivel up into raisins.
- Storage Medium: Most gardeners use slightly dampened peat moss, vermiculite, or wood shavings.
- Containers: Cardboard boxes, paper bags, or plastic bins with ventilation holes are all excellent choices.
- The "Goldilocks" Temperature: Store your containers in a dark, cool place that stays between 40°F and 50°F. An unheated basement, a root cellar, or an insulated crawl space is usually perfect.
Common Myths About Overwintering Dahlias
There is a lot of conflicting advice online about dahlias, which can make a simple process feel intimidating. Let's clear up a few common misconceptions.
"You must wash them with bleach."
While some people recommend a diluted bleach dip to prevent mold, it is usually unnecessary for the home gardener. As long as your tubers are dry and stored in a clean medium with some airflow, they will likely stay healthy on their own.
"If a tuber feels soft, it's dead."
Not necessarily. While a mushy, rotten tuber should be thrown away, a slightly soft or flexible tuber may just be a bit dehydrated. As long as it isn't rotting, you can often save it by misting your storage medium with a tiny bit of water to plump it back up.
"You have to divide them in the fall."
You can divide dahlia tubers in either the fall or the spring. Many beginners find it easier to divide in the spring because the "eyes" (the small bumps where the new growth begins) are much easier to see once the tuber starts to wake up.
Preparing for Spring
Regardless of whether you left your dahlias in the ground or tucked them away in the basement, spring is the time to get them ready for a new season of growth.
If your dahlias stayed in the garden, wait until the soil temperature consistently reaches 60°F before removing the heavy winter mulch. This prevents the tubers from sprouting too early while the air is still chilly.
If you stored your tubers indoors, check them periodically throughout the winter. If they look shriveled, add a few drops of water to the packing material. If you see any soft spots or rot, remove the affected tuber immediately so it doesn't spread to the rest of the clump.
In late spring, once the danger of frost has passed, you can bring your tubers out and plant them in a sunny spot with well-draining soil. We recommend waiting until the soil is warm to the touch before planting, as dahlias are true sun-lovers and won't do much in cold, damp spring earth.
Realistic Expectations and Success
Gardening is a partnership with nature, and weather can be unpredictable. Even with the best preparation, a freak ice storm or a record-breaking rainy season can affect how well your dahlias overwinter.
Don't be discouraged if you lose a tuber here or there. Professional growers often expect a small percentage of loss each year. The beauty of dahlias is their resilience and their ability to multiply. One successful year often provides you with enough tubers to cover any minor losses from the previous winter.
At Longfield Gardens, we believe that the best way to learn is by doing. If you aren't sure if your dahlias will survive in the ground, try leaving one or two in the garden as a "test" while digging up the rest. This way, you can learn exactly what your local microclimate allows without risking your entire collection.
Safety in the Garden
It is worth noting that dahlia tubers and foliage can be mildly toxic to pets if ingested. While most dogs and cats aren't interested in digging up tubers, it is always a good idea to keep an eye on curious pets during the digging and planting process. If you have a known "digger" in the family, consider storing your tubers in a secure area where they won't be mistaken for a toy.
Additionally, always use caution when working with garden tools like forks and shears. Keeping your tools clean and sharp makes the job easier and safer for both you and the plants.
Conclusion
Deciding whether you have to dig up dahlia bulbs every year depends on your specific environment and how much effort you want to invest. While gardeners in warmer zones can enjoy the ease of leaving them in the ground, those in colder climates find the annual ritual of lifting and storing to be a rewarding way to protect their investment.
- Zones 8-11: Leave them in, mulch well, and ensure good drainage.
- Zones 2-6: Dig them up after the first frost and store in a cool, dry place.
- Zone 7: Mulch heavily and consider a waterproof cover if you want to try overwintering in-ground.
- Any Zone: Consider digging every few years to divide clumps and refresh your garden layout.
We help home gardeners create a more beautiful yard and garden with premium flower bulbs and perennials. With a little bit of winter care, your dahlias will continue to provide spectacular color and stunning bouquets for many years to come.
The next step is easy: check your hardiness zone and enjoy the satisfaction of knowing your garden is ready for whatever winter brings.
FAQ
Can I leave dahlias in the ground in Zone 7?
Yes, it is possible to leave dahlias in Zone 7, but it involves some risk. You should cut the plants back and apply at least 4 to 6 inches of mulch, like straw or wood chips, to protect the tubers from frost. In very wet or unusually cold winters, the tubers may still rot or freeze, so many gardeners in this zone choose to dig up their favorites just to be safe.
Do I have to wait for a frost before digging up dahlias?
It is highly recommended to wait until the first frost has blackened the foliage before digging. This frost signals the plant to stop growing and store its energy in the tuber, which helps it survive the long winter dormancy. If you must dig earlier due to travel or scheduling, try to wait as late into the autumn as possible so the tubers can reach full maturity. For additional guidance, see this guide to bringing in dahlia bulbs.
What is the best material to store dahlia tubers in?
The most successful storage materials are those that can hold a tiny bit of moisture without becoming soggy. Slightly dampened peat moss, vermiculite, or coarse wood shavings are the most popular choices. These materials provide a buffer that prevents the tubers from drying out completely while still allowing for necessary air circulation. For more details, read this dahlia bulb storage guide.
Why did my dahlia tubers rot in the ground even though it didn't freeze?
The most common cause of rot in dahlia tubers is poor soil drainage. If the soil remains saturated with water for long periods, the tubers cannot "breathe" and will succumb to fungal or bacterial rot. This is especially common in heavy clay soils during rainy winters, regardless of how warm the temperature stays.
