Introduction
Few things match the sheer joy of a dahlia garden in mid-August. These plants are the heavy lifters of the summer border, producing an endless supply of vibrant, intricate blooms that last until the first frost. At Longfield Gardens, we believe dahlias are one of the most rewarding flowers any gardener can grow, offering everything from tiny pompons to massive dinnerplate varieties in every color of the rainbow.
As the growing season winds down and the first crisp nights of autumn arrive, every dahlia lover faces the same important question: Do I have to dig up dahlia tubers, or can I leave them in the ground? The answer isn't a simple "yes" or "no." It depends on where you live, the nature of your soil, and how much risk you are willing to take with your favorite varieties.
This guide is designed to help you make the best decision for your garden. We will cover the specific temperature thresholds for dahlia survival, the pros and cons of lifting your tubers, and the step-by-step methods for both over-wintering them in the ground and storing them indoors. If you are eyeing dinnerplate varieties, these tips still apply. Whether you are a busy beginner or a seasoned collector, understanding these basics ensures your dahlias come back stronger and more beautiful next year.
The decision to dig or stay depends on balancing your local climate with the specific needs of these spectacular Mexican natives.
Understanding Your USDA Hardiness Zone
The most significant factor in deciding whether to dig up your dahlias is your USDA Hardiness Zone Map. Dahlias are native to the high-altitude regions of Mexico and Central America. In their natural habitat, they experience a distinct dry season and never face the deep, penetrating freezes common in much of the United States.
For gardening purposes, dahlias are generally considered hardy in Zones 8 through 11. In these warmer regions, the ground rarely freezes deep enough to reach the tubers. If you live in one of these zones, you can often leave your dahlias in the ground year-round with minimal extra care.
If you live in Zone 7, you are in a "transition" area. Depending on the specific winter weather and your local microclimates, your dahlias might survive in the ground, but it is not a guarantee. For gardeners in Zones 6 and colder, the ground freezes deep enough to turn dahlia tubers into mush. In these climates, digging and storing them indoors is essential if you want to keep the same plants for next year.
Key Takeaway: Check your USDA Hardiness Zone before winter. If you are in Zone 7 or colder, digging up your tubers is the only way to ensure they survive a typical winter.
What is a Microclimate?
Sometimes, a plant survives in a zone where it shouldn't. This is often due to a microclimate—a small area where the temperature is slightly different from the surrounding region. For example, a dahlia planted right against the foundation of a heated house may stay just warm enough to survive a Zone 7 winter because the house radiates heat. Similarly, a garden bed at the bottom of a hill may be colder than one at the top because cold air settles in low spots.
The Role of Soil Drainage
Even if you live in a warm climate like Zone 9, temperature isn't the only thing that kills dahlias in winter. Drainage is equally important. "Drainage" simply refers to how fast water leaves the soil after a rainstorm or watering session.
Dahlias go dormant in the winter, meaning they stop growing and rest. During this time, they do not "drink" water. If they are sitting in heavy, wet, clay-heavy soil during a rainy winter, the tubers will likely rot. They need to stay relatively dry while they sleep.
If your soil is sandy and water moves through it quickly, your tubers have a much higher chance of surviving in the ground. If your soil stays soggy or has puddles for hours after a rain, the tubers will almost certainly rot, regardless of the temperature.
Reasons to Dig Up Your Dahlias
While it takes some effort, there are several excellent reasons why many gardeners choose to dig up their tubers every autumn, even if they live in a slightly warmer climate.
1. Guaranteed Survival
The biggest benefit of digging is peace of mind. Weather is unpredictable. An unusually cold snap or a record-breaking rainy season can kill dahlias that have survived in the ground for years. By lifting them and storing them in a controlled environment, you remove the guesswork.
2. Dividing and Multiplying
Dahlias are incredibly productive. A single tuber planted in May will grow into a large dahlia tubers by October. If you dig up that clump, you can divide it into several individual tubers. Each of these new tubers will grow into a full-sized plant the following year. This is the most cost-effective way to expand your garden or share favorite varieties with friends.
3. Preventing Overcrowding
If you leave dahlias in the ground for three or four years, the clumps can become massive. When a clump gets too large, it sends up too many stems. This leads to overcrowding, which reduces air circulation and makes the plants more prone to diseases like powdery mildew. It also results in smaller flowers because the tubers are competing with each other for nutrients.
4. Soil Health and Bed Rotation
Digging up your tubers allows you to amend the soil in the fall or early spring. You can add compost or organic matter to the bed without worrying about hitting the dormant tubers with a shovel. It also gives you the flexibility to move your dahlias to a new location next year, which helps prevent soil-borne pests from building up in one spot.
Reasons to Leave Dahlias in the Ground
If you live in Zone 8 or higher, or if you are a gardener who prefers a "low-maintenance" approach, leaving dahlias in the ground has its own set of advantages.
1. Less Physical Work
Lifting, cleaning, and storing dozens of dahlia clumps is a significant chore. If you have a large collection, leaving them in the ground saves hours of labor in the fall and spring.
2. Earlier Blooms
Dahlias that stay in the ground often start growing as soon as the soil warms up in the spring. Because they have an established root system and don't experience the "shock" of being replanted, they often bloom two to three weeks earlier than tubers that were stored indoors.
3. Larger Clumps
In the first year or two, a dahlia that is left in the ground can grow into a very robust plant. The large, undisturbed clump provides plenty of energy for the plant to produce a massive display of foliage and flowers.
Key Takeaway: If you choose to leave your dahlias in the ground, ensure your soil has excellent drainage and apply a thick layer of mulch to protect the crowns from light frosts.
How to Overwinter Dahlias in the Ground (Zones 8-11)
If you have decided that your climate and soil are suitable for leaving dahlias in the ground, you can't just walk away and forget them. A little bit of winter preparation goes a long way.
Step 1: Wait for Dormancy
Wait until the foliage has been hit by a light frost or has naturally turned yellow and brown. This signals that the plant has finished storing energy in the tubers for the winter.
Step 2: Cut Back the Stems
Cut the stems down to about 2 to 4 inches above the soil line. Some gardeners prefer to leave a little more stem so they can find the plant in the spring, but a shorter stem is easier to cover with mulch.
Step 3: Manage the Hollow Stems
Dahlia stems are hollow, like straws. If water gets into the hollow stem and sits at the base of the tuber (the "crown"), it can cause the plant to rot from the top down. You can prevent this by placing a small piece of aluminum foil over the cut stem or by piling your mulch in a way that sheds water away from the center of the plant.
Step 4: Mulch Heavily
This is the most important step. Apply a 4- to 6-inch layer of mulch over the top of the plant. You can use wood chips, shredded leaves, straw, or bark mulch. This layer acts as insulation, keeping the soil temperature stable and protecting the tubers from occasional cold snaps.
Step 5: Mark the Spot
It is very easy to forget where a dahlia was planted once the foliage is gone. Use a sturdy garden stake or a labeled marker so you don't accidentally dig into the tubers when you are planting spring bulbs or weeding in March.
How to Dig and Store Dahlias (Zones 3-7)
For most gardeners in the US, digging and storing is the standard practice. At Longfield Gardens, we recommend this method for anyone who wants to ensure their favorite varieties return year after year.
Timing is Everything
The best time to dig up dahlias is about two weeks after the first "killing frost." A killing frost is a freeze that turns the dahlia foliage black or dark brown. This transition period is important because it triggers the tubers to go into deep dormancy. For more detail on timing, see When to Dig Up Dahlia Bulbs: Timing & Storage Tips. However, do not wait so long that the ground itself begins to freeze hard.
Step-by-Step Digging Process
- Cut the stems: Use sharp loppers or pruners to cut the stems down to about 6 inches.
- Loosen the soil: Use a garden fork or a shovel. Start digging about 12 inches away from the main stem. Dahlias grow horizontally, and you want to avoid slicing into the tubers.
- Lift gently: Once you have loosened the soil all the way around the plant, gently pry the clump upward. Use your hands to lift it the rest of the way.
- Remove excess dirt: Shake the clump gently or use a small stick to poke away large chunks of soil.
Cleaning and Curing
Once the tubers are out of the ground, they need to be cleaned. Many gardeners use a garden hose to wash away the remaining soil. This makes it easier to see the "eyes" (growth points) and check for any signs of damage or rot. For tuber basics, see Dahlia Tubers: What You Need to Know.
After washing, the tubers need to "cure." This means letting them dry out slightly so the outer skin toughens up. Place the clumps upside down (stems pointing down) in a cool, dry place for 1 to 3 days. This allows any water trapped in the hollow stems to drain out. Do not leave them in direct sunlight or in a place where they might freeze.
Key Takeaway: Curing is a vital step. It prevents excess moisture from being trapped in the storage container, which is the leading cause of mold and rot.
Storing for the Winter
At our facility at Longfield Gardens, we know that the storage environment is the secret to success. You are looking for a "Goldilocks" environment: not too cold, not too warm, not too wet, and not too dry.
- Temperature: Aim for 40°F to 50°F. An unheated basement, a crawl space, or a cool closet in a garage is usually perfect. If the temperature drops below 35°F, the tubers may freeze and die. If it stays above 55°F, they may start to sprout too early or shrivel up.
- Packing Material: Place the tubers in a container (like a cardboard box, plastic bin with air holes, or a paper bag). Surround them with a slightly damp material to keep them from drying out completely. Common choices include vermiculite, coarse sand, peat moss, or wood shavings.
- Checking In: Check your tubers once a month during the winter. If they look shriveled, mist the packing material with a little water. If you see any soft spots or mold, cut those parts away and leave the bin open for a day to improve airflow.
Dividing Dahlias: Fall or Spring?
A common question is whether you should divide the clumps into individual tubers immediately after digging or wait until the spring.
Dividing in the Fall
- Pros: The tubers are soft and easy to cut. They take up much less space in storage.
- Cons: It can be very difficult to see the "eyes" (the small bumps where next year's stems will grow). If a tuber doesn't have an eye, it will never grow, even if the tuber itself looks healthy.
Dividing in the Spring
- Pros: The eyes are much easier to see as they begin to swell or sprout. You can be 100% sure which tubers are viable.
- Cons: The tubers have toughened up over the winter and can be very woody and difficult to cut.
For beginners, we often suggest storing the clumps whole and dividing them in the spring when the growth points are obvious.
Realistic Expectations and Troubleshooting
Gardening always involves a bit of trial and error. Even if you follow every step perfectly, you might lose a few tubers over the winter. This is normal. Weather fluctuations, unexpected humidity changes, or even the natural lifespan of a tuber can affect survival.
If you find that your tubers have rotted in storage, it usually means they were too wet when they were packed or the storage area was too humid. If they have shriveled into hard, dry "mummies," they were likely too dry. Aim for the texture of a firm carrot.
If you are trying to overwinter them in the ground and they don't come back, the most likely culprits are a deep freeze that reached the tubers or soil that stayed too wet for too long. In these cases, you might consider planting in raised beds next year to improve drainage or switching to the dig-and-store method.
What to do next:
- Identify your USDA zone.
- Check your soil drainage after the next rain.
- Decide on your strategy: mulch for Zone 8+, or prepare storage bins for Zone 7 and below.
- Gather your supplies (labels, packing material, garden fork) before the first frost arrives.
Conclusion
Dahlias bring a level of drama and beauty to the garden that is hard to overstate. While the question of whether you have to dig them up depends on your local climate, the effort of protecting them is always worth the reward. Whether you choose to mulch them heavily in a warm southern garden or tuck them into a cool basement in the north, you are preserving a living investment that will pay off in buckets of blooms.
We at Longfield Gardens are here to support your gardening journey every step of the way. From providing premium dahlia collections to offering practical advice on care, our goal is to make your garden a place of joy and success. Remember that every dahlia grower started exactly where you are, and every season is a new opportunity to learn and grow.
"The secret to successful dahlia gardening isn't found in a magic trick; it's simply about matching the plant's need for a dry, frost-free winter with the right storage strategy for your home."
Now that you have a plan for your tubers, you can enjoy those final autumn blooms with confidence, knowing you are ready for whatever the winter brings. If you're ordering more plants for next season, our Shipping Information page explains how orders move through the year.
FAQ
What happens if I don't dig up my dahlias in a cold climate?
If you live in Zone 6 or colder and leave your dahlias in the ground without extreme protection, the tubers will likely freeze. When dahlia tubers freeze, the water inside their cells expands and breaks the cell walls. Once the ground thaws, the tubers turn into a soft, rotten mush and will not grow back in the spring.
Can I leave dahlias in pots over the winter?
Yes, but you must move the pots to a frost-free location like a garage or basement. Because pots are above ground, the soil inside them gets much colder than the ground soil. For a closer look at container growing, see Can Dahlia Bulbs Be Planted in Pots? Tips for Success. If left outside in a cold climate, the tubers in the pot will freeze quickly. Simply cut the stems back, let the soil dry out, and move the entire pot into a cool, dark corner until spring.
How long do dahlia tubers need to dry before I store them?
Dahlias should "cure" or air-dry for about 1 to 3 days after being washed. You want the surface of the tuber to feel dry and the skin to look slightly matte, but you don't want the tuber to start shriveling. Drying them upside down is helpful to ensure any water inside the hollow stems drains out completely, preventing crown rot.
When is the best time to divide the clumps?
The best time depends on your experience level. Many professional growers divide in the fall because the tubers are easier to cut. However, for most home gardeners, waiting until spring is easier because the "eyes" (growth buds) become much more visible. Dividing in the spring ensures you only save and plant tubers that are guaranteed to grow.
