Table of Contents
- Introduction
- The Short Answer: It Depends on Your Zone
- Why Soil Drainage Matters More Than Temperature
- When to Leave Dahlias in the Ground
- When Digging Up Tubers is the Better Choice
- How to Dig Up Dahlia Tubers: A Step-by-Step Guide
- The Secret to Successful Winter Storage
- Dividing Your Dahlias: When and How
- Growing Dahlias in Containers
- Realistic Expectations for Winter Survival
- Preparing for Spring
- Conclusion
- FAQ
Introduction
There is nothing quite like the late-summer magic of a garden filled with dahlias. From the massive, shaggy petals of dinnerplate varieties to the perfectly symmetrical pompons, these plants provide a spectacular finale to the growing season. At Longfield Gardens, we believe every gardener should experience the joy of cutting a fresh bouquet of these vibrant blooms from their own backyard.
As the first frost approaches, many gardeners find themselves asking the same question: do you have to dig up dahlias every year? The answer isn't a simple "yes" or "no," as it depends on your local climate, your soil, and how much effort you want to put into your garden's winter nap. For some, lifting tubers is a ritual that ensures years of beauty; for others, a thick layer of mulch is enough to keep the plants safe until spring.
This guide will help you determine the best path for your specific garden. We will look at USDA hardiness zones, soil conditions, and the simple steps you can take to protect your dahlias through the cold months. Whether you decide to lift them or leave them, our goal is to help you enjoy even more flowers next year.
The Short Answer: It Depends on Your Zone
The most important factor in deciding whether to dig up your dahlia tubers is your USDA hardiness zone. Dahlias are native to the high plains of Mexico and Central America, where the ground does not freeze solid. Because they are tropical perennials, they have a limited tolerance for cold temperatures. For a broader overview of dahlia care, see All About Dahlias.
In warmer climates, specifically USDA zones 8 through 11, dahlia tubers can usually stay in the ground year-round. In these areas, the soil temperature typically stays high enough that the tubers do not freeze. As long as your soil has excellent drainage, your dahlias will likely return in the spring with minimal intervention.
If you live in USDA zones 7 or colder, the general rule is that you should dig up your tubers. In these regions, the ground freezes deep enough to reach the tubers, which causes the water inside them to expand and rupture the plant cells. This results in a mushy, dead tuber that will not grow back. However, even in zone 7, some gardeners find success leaving tubers in the ground if they provide extra protection and have a sheltered microclimate.
Key Takeaway Most gardeners in zones 8 and above can leave dahlias in the ground. In zones 7 and below, digging the tubers up for indoor storage is the most reliable way to ensure they survive the winter.
Why Soil Drainage Matters More Than Temperature
While cold is a major factor, moisture is the "silent killer" of dahlia tubers during the winter. A dahlia tuber that is cold but dry has a much better chance of survival than one that is sitting in cold, wet soil. If your garden has heavy clay soil that holds onto water, your tubers are at a high risk of rotting, even in warmer zones. For more on tuber anatomy, see Dahlia Tubers: What You Need to Know.
Dahlias are starchy and porous, much like a potato. When they sit in soggy soil during their dormant period, they easily succumb to fungal diseases and rot. If you live in an area with high winter rainfall, you may want to dig up your tubers regardless of your temperature zone. Alternatively, planting your dahlias in raised beds or on a slope can provide the drainage they need to stay healthy through a wet winter.
If you aren't sure how your soil drains, you can do a simple test. Dig a small hole and fill it with water. If the water drains away within an hour, you have good drainage. If it takes several hours or even a full day, your soil is heavy and may require you to lift your tubers to keep them safe from rot.
When to Leave Dahlias in the Ground
If you are in a warm enough zone and have well-draining soil, leaving your dahlias in the ground can save you a lot of time and effort. It also allows the plants to establish larger, more complex tuber clumps over several years. This can sometimes lead to earlier blooms and sturdier plants in the following season.
To give your dahlias the best chance of surviving the winter in the ground, follow these simple steps:
- Wait for the first frost: Allow the foliage to turn brown or black after a light frost. This signals the plant to go dormant and move its energy into the tubers.
- Cut back the stems: Use sharp pruners to cut the stems down to about 2 to 4 inches above the soil line.
- Apply a heavy mulch: This is the most critical step. Apply 6 to 12 inches of mulch over the crown of the plant. You can use wood chips, shredded leaves, or straw. This insulation helps keep the soil temperature stable.
- Mark the spot: It is easy to forget where your dahlias are once the stems are gone. Use a sturdy garden stake to mark the location so you don't accidentally dig into them when planting spring bulbs.
Using a Tarp for Extra Protection
If you live in a borderline zone or expect a particularly wet winter, you can add a layer of protection by covering your mulched area with a waterproof tarp. Secure the edges with bricks or landscape staples. This prevents heavy rain and melting snow from saturating the soil directly over your tubers. Just remember to remove the tarp in early spring so the soil can warm up and the new shoots can emerge.
When Digging Up Tubers is the Better Choice
Even if you live in a region where dahlias can survive the winter in the ground, there are several reasons why you might choose to dig them up anyway. At Longfield Gardens, we often find that lifting tubers provides more control over the health and quantity of your plants.
1. You Want to Divide Your Plants
One of the best things about dahlias is their ability to multiply. A single tuber planted in the spring will grow into a large clump of 5 to 20 tubers by the end of the season. If you leave these clumps in the ground for years without dividing them, the plants can become overcrowded. This leads to smaller flowers and weaker stems. By digging them up, you can divide the clump and turn one plant into five or ten new ones for the next season. For a step-by-step breakdown of the process, see How to Divide Dahlia Tubers.
2. You Have Rare or Expensive Varieties
If you have invested in a specific dinnerplate variety that you absolutely love, digging it up is the only way to guarantee its survival. Winter weather is unpredictable. An unusually deep freeze or a record-breaking rainy season can kill even the hardiest dahlias. If losing a particular plant would be heartbreaking, it is worth the hour of work to lift and store it safely indoors.
3. You Want to Rearrange Your Garden
Gardens are dynamic, and your vision for next year might be different from this year's layout. Digging up your tubers gives you the freedom to move your dahlias to a sunnier spot or a different color grouping next spring. It is much easier to move a dormant tuber than it is to transplant a growing plant in the middle of summer.
What to do next
- Check your USDA zone to see if your climate allows for overwintering.
- Observe your garden's drainage during a heavy rain.
- Decide if you want to multiply your plants by dividing them this year.
How to Dig Up Dahlia Tubers: A Step-by-Step Guide
If you have decided that digging is the right choice for your garden, the process is straightforward. It doesn't require specialized equipment—just a bit of patience and a gentle hand.
Step 1: Timing is Everything
The best time to dig up dahlias is after the first "killing frost." This is a frost that is cold enough to turn the green foliage black or brown. This frost tells the plant that the growing season is over. We recommend waiting about a week after this frost before you start digging. This short waiting period allows the tubers to "cure" slightly in the ground, which toughens their skin for storage. However, do not wait so long that the ground itself begins to freeze solid.
Step 2: Cut Back the Foliage
Cut the stems down to about 4 to 6 inches above the ground. These short stems act as a handle and help you locate the center of the tuber clump. Avoid cutting them all the way to the soil line, as it can be difficult to see where the tubers are once they are covered in dirt.
Step 3: Dig Wide and Deep
Dahlia tubers grow outward from the center stem, often much further than you might expect. To avoid slicing through a tuber, start digging at least 12 inches away from the main stem. Use a garden fork rather than a shovel if possible. A fork is less likely to slice through a tuber and allows the soil to fall through the tines.
Gently pry up the soil from all sides of the plant until the clump feels loose. Once it is free, lift the clump by the stems and the base. Never pull hard on the stems, as they can snap off and damage the "neck" of the tubers, where next year's growth will emerge.
Step 4: Clean and Inspect
Gently shake or brush off the excess soil. You can use a garden hose to wash the tubers clean, but be careful not to use high pressure, which can nick the delicate skin. Inspect the clump for any signs of rot or insect damage. Healthy tubers should feel firm, like a fresh potato. If you find any mushy or clearly rotten tubers, trim them off with clean pruners and discard them.
Step 5: Curing the Tubers
Before you pack your dahlias away for the winter, they need to dry out. Place the cleaned clumps in a cool, dry, frost-free area with good air circulation. A garage or a covered porch works well. Let them sit for 2 to 3 days. This drying process helps prevent mold from forming during storage. Do not leave them in direct sunlight or in a very hot area, as this can cause them to shrivel.
The Secret to Successful Winter Storage
Storage is where many gardeners feel the most uncertainty, but it is actually quite simple. The goal is to keep the tubers in a "Goldilocks" environment: not too cold (which freezes them), not too warm (which wakes them up), not too wet (which rots them), and not too dry (which shrivels them).
Choosing the Right Temperature
The ideal temperature for storing dahlia tubers is between 40°F and 50°F. A cool basement, an unheated crawl space, or a corner of a garage that doesn't freeze are perfect spots. If the temperature stays above 55°F, the tubers may try to start growing too early or may dry out completely.
Packing Materials
To maintain the right moisture balance, we recommend packing the tubers in a medium that breathes but holds a tiny bit of humidity. Popular choices include:
- Vermiculite or Perlite: Excellent for moisture control.
- Peat Moss: A classic choice that is easy to find.
- Wood Shavings: Often sold as pet bedding, these are inexpensive and effective.
- Dry Sand: Good for heavy clumps, though it can be heavy to move.
Place a layer of your chosen material in a cardboard box or a plastic bin with holes drilled for ventilation. Lay the tubers inside, making sure they aren't touching each other, and cover them with more material.
Checking on Your Tubers
It is a good idea to check your stored tubers once a month during the winter. If they look shriveled, give the packing material a very light misting of water. If you see any signs of mold or rot, remove the affected tuber immediately to prevent it from spreading to the rest of the clump.
Dividing Your Dahlias: When and How
One of the rewards of digging up your dahlias is the chance to divide them. You can do this in the fall before you store them, or in the spring before you plant them. Many beginners find it easier to divide in the spring because the "eyes" (the small bumps that become new stems) are more visible.
To be a viable plant, a divided tuber must have three parts:
- The Body: The plump part that stores energy.
- The Neck: The thin part connecting the body to the stem.
- An Eye: A small growth point located on the "crown" (the area where the neck meets the old stem).
If a tuber breaks off and doesn't have a piece of the crown with an eye, it will not grow, even if the tuber itself looks healthy. When dividing, use a sharp, clean knife or garden snips to ensure clean cuts.
Key Takeaway Dividing your dahlias every 2–3 years keeps the plants vigorous and gives you plenty of extras to share with friends or expand your own garden beds.
Growing Dahlias in Containers
If you grow your dahlias in pots or containers, you have a slightly different set of rules. Because the soil in a container is above ground, it freezes much faster and more deeply than the soil in the garden. Even in zone 8, a dahlia in a pot might freeze if left outside. For a fuller step-by-step on potted plants, see Can You Grow Dahlias in a Container? Yes! Here’s How.
The easiest way to overwinter container dahlias is to move the entire pot into a frost-free area, like a shed or garage, once the foliage has died back. Stop watering the pot and let the soil dry out completely. In the spring, you can tip the pot over, refresh the soil, divide the tubers if needed, and start the cycle over again.
Realistic Expectations for Winter Survival
Whether you leave your dahlias in the ground or store them in a box, it is important to remember that nature is in charge. Even professional growers expect to lose a small percentage of tubers over the winter. Some varieties are naturally hardier than others, and sometimes a specific tuber simply doesn't have enough stored energy to make it through the dormant season.
Don't be discouraged if a few tubers don't make it. Most of the time, the ones that do survive will be more than enough to fill your garden with color. Gardening is a constant learning process, and every winter will teach you a little more about the microclimate of your own backyard.
Preparing for Spring
As the days get longer and the ground begins to thaw, the excitement for dahlia season returns. If you left your tubers in the ground, you can begin to pull back the heavy mulch in mid-to-late spring once the danger of a hard freeze has passed. This allows the sun to warm the soil and encourages the tubers to sprout.
If you stored your tubers indoors, wait until the soil temperature reaches about 60°F before planting them outside. Dahlias love warm soil and will sit dormant if planted too early in cold, wet ground. Starting them in small pots indoors about 4 to 6 weeks before the last frost can give you a head start on the blooming season, especially in northern climates with short summers. For zone-based shipping details, see Shipping Information.
Conclusion
Deciding whether you have to dig up dahlia tubers every year is one of those gardening choices that depends on your location and your goals. In warm zones with sandy, well-draining soil, you can often leave them to their own devices with just a warm blanket of mulch. In colder regions, the small effort of lifting and storing them ensures that your favorite varieties will return to dazzle you year after year.
At Longfield Gardens, we want to make gardening as successful and enjoyable as possible. Dahlias are a generous crop, rewarding a little bit of winter care with an explosion of color that lasts from mid-summer until the first frost.
- Know your zone: Use your USDA hardiness zone as your primary guide for digging versus mulching.
- Prioritize drainage: Ensure your soil doesn't stay soggy, which is the leading cause of winter rot.
- Store with care: If you dig, keep your tubers in a cool, dark, and slightly humid environment.
- Embrace the bonus: Use the digging process as an opportunity to divide your plants and grow your dahlia collection.
Growing dahlias is a rewarding journey that connects you to the seasons. Whether you tuck them into the ground or store them in the cellar, the anticipation of those first green shoots in the spring is one of the greatest joys of the gardening year.
FAQ
Can I leave dahlias in the ground in Zone 7?
It is possible, but risky. If you have very well-drained soil and a sheltered spot (like near a south-facing wall), you can try leaving them with a very thick layer of mulch (12 inches) and a waterproof cover. However, in a particularly cold or wet winter, you may still lose them. Many gardeners in Zone 7 choose to dig up their most prized varieties and experiment with leaving the hardier ones.
What happens if I don't dig up my dahlias in a cold climate?
If the ground freezes down to the depth of the tubers, the water inside the tubers will freeze and expand. This destroys the plant's tissue. When the ground thaws, the tubers will turn into a soft, mushy mess and will eventually rot away. They will not sprout or grow back in the spring.
Do I need to wash the dirt off before storing dahlia tubers?
It isn't strictly necessary, but it is helpful. Washing the tubers allows you to see the "eyes" and check for any hidden rot or damage. It also makes the tubers cleaner to handle in the spring. If you do wash them, just ensure they are completely dry before you pack them away in storage medium to prevent mold.
My stored tubers look shriveled in mid-winter. Are they dead?
Not necessarily! It is normal for tubers to lose a little moisture in storage. If they are slightly shriveled but still feel firm when squeezed, they are likely still alive. You can lightly mist your packing material (like peat moss or shavings) with water to increase the humidity. If they are as flat as a pancake and feel brittle or hollow, they have likely dried out too much and may not be viable.