Table of Contents
- Introduction
- Deciding to Dig or Overwinter
- The Importance of the First Frost
- How to Dig Dahlia Tubers Step-by-Step
- Understanding Tuber Anatomy
- Dividing Dahlias: Fall vs. Spring
- Creating the Ideal Storage Environment
- Winter Maintenance and Monitoring
- Special Cases: Containers and Raised Beds
- Planning for Spring Success
- Safety and Care Reminders
- Conclusion
- FAQ
Introduction
There is a special kind of magic in the late-summer garden when dahlias are at their peak. These spectacular plants provide some of the most vibrant colors and intricate shapes in the floral world, often blooming right up until the first frost arrives. At Longfield Gardens, we believe the end of the blooming season is just the start of an exciting cycle that allows you to enjoy these beautiful flowers year after year.
As the days grow shorter and the air turns crisp, you may be wondering how to protect your investment for the next growing season. While dahlias are often called "bulbs," they are actually tubers with thin skins that need specific care to survive the winter. Whether you live in a cold climate where the ground freezes or a mild area where winters stay gentle, there is a simple path to follow.
This guide is designed for home gardeners who want to ensure their dahlias return bigger and better next year. We will cover how to decide if you should dig your dahlia tubers, the best timing for the harvest, and how to store them so they stay plump and healthy until spring. By following a few basic steps, you can successfully overwinter your dahlias and even increase your collection for free.
Deciding to Dig or Overwinter
The first step in fall dahlia care is determining whether your tubers need to come out of the ground or if they can stay put. This decision mostly depends on your local climate and how much risk you are comfortable taking with your plants.
In most parts of the United States, dahlias are considered "tender perennials." This means they can live for many years, but they cannot survive being frozen solid. If you live in USDA hardiness zones 8 through 11, your ground likely stays warm enough that the tubers can stay in the garden. In these warmer areas, you can simply cut the stems back and apply a thick layer of mulch to protect them from occasional chilly nights.
For gardeners in zones 7 and colder, the safest approach is to lift the tubers and store them indoors. When the soil freezes several inches deep, the water inside the dahlia tuber expands, which causes the cells to burst. This leads to a soft, rotten tuber that will not grow in the spring. Even in zone 7, where some winters are mild, a sudden deep freeze can be a surprise. If you have a variety you absolutely love, we recommend digging it up to be safe.
There is also the factor of soil drainage. Dahlias are like potatoes; if they sit in cold, soggy soil for months, they are prone to rot. If your garden has heavy clay soil that stays wet all winter, lifting the tubers is usually a better choice regardless of your temperature zone.
The Importance of the First Frost
Timing is everything when it comes to fall gardening. For dahlias, the first killing frost is a helpful signal from nature. A killing frost is when temperatures drop low enough to turn the green foliage black or brown.
While it might be tempting to tidy up the garden as soon as the weather cools, we suggest waiting for this frost. When the plants experience a cold snap, they receive a signal to stop sending energy to the leaves and flowers. Instead, they begin to pull all those nutrients down into the tubers. This process "cures" the tubers, toughening their skins and preparing them for a long winter nap.
If you live in an area that does not get a hard frost until very late in the year, you can manually start the process in mid-November. Simply stop watering the plants and allow them to begin yellowing on their own. However, if a frost is in the forecast, let it happen. The plants will look a bit messy for a few days, but the tubers will be much more resilient in storage because of it.
Key Takeaway: Waiting for the first frost helps the tubers develop a thicker skin, which prevents them from drying out or rotting during the winter months.
How to Dig Dahlia Tubers Step-by-Step
Once the foliage has turned dark and the plants have sat for about a week or two after the frost, it is time to get to work. Lifting tubers is a rewarding task that lets you see exactly how much your plants have grown over the summer.
Step 1: Cut Back the Stems
Use a clean pair of garden loppers or hand pruners to cut the main stalks down. Leave about 4 to 6 inches of the stem sticking out of the ground. This "handle" makes it easier to see where the center of the clump is and gives you something to hold onto, though you should never pull the plant up by this handle alone.
Step 2: Loosen the Soil
Dahlia tubers grow outward from the center, often reaching further than you might expect. Use a garden fork or a sturdy spade and start digging at least 12 inches away from the main stem. Gently push the fork into the soil and pry upward to loosen the earth. Work your way in a circle around the plant.
Step 3: Lift the Clump
Once the soil is loose all the way around, gently slide your fork under the center of the clump and lift. It is important to be patient here. Dahlia tubers have narrow "necks" where they connect to the main stem. If these necks break, the tuber cannot grow next year. Support the weight of the clump from underneath as you lift it out of the garden.
Step 4: Initial Cleaning
Shake the clump gently to remove large chunks of soil. You can use a small stick or a soft brush to poke away dirt from between the individual tubers. We generally recommend avoiding the use of a high-pressure hose to wash them. Adding extra moisture right before storage can increase the risk of rot. If your soil is very heavy clay and you must wash them, make sure they dry completely in a protected area before you pack them away.
What to do next:
- Label each clump immediately using a waterproof marker or a tag tied to the stem.
- Remove any tubers that are obviously damaged or mushy.
- Check for "mother" tubers (the original one you planted), which may look shriveled; these can be discarded if they look spent.
- Place the clumps in a cool, dry area (like a garage or shed) for 24 hours to let the surface moisture evaporate.
Understanding Tuber Anatomy
Before you move on to storage or dividing, it helps to know what you are looking at. A viable dahlia tuber needs three specific parts to grow a new plant in the spring:
- The Body: This is the fat, sausage-like part of the tuber that stores food and energy.
- The Neck: This is the thin part that connects the body to the crown. It must remain intact and unbroken.
- The Crown (and the Eye): The crown is the base of the old stem. This is where the "eyes" are located. An eye is a small bump that will eventually become a new sprout.
If a tuber breaks off and loses its connection to the crown, it will not grow, even if the body looks healthy. This is why gentle handling is the most important part of the digging process.
Dividing Dahlias: Fall vs. Spring
One of the best things about dahlias is that one tuber planted in May often turns into a clump of five to ten tubers by October. You can choose to divide these into individual plants now, or you can store the entire clump as it is.
Dividing in the fall has its benefits. The tubers are soft and easy to cut, like a fresh potato. However, the "eyes" are often very difficult to see in the fall. If you are a beginner, we suggest waiting until spring to do your dividing. By March or April, the eyes will begin to swell and turn pink or white, making it very obvious where to make your cuts.
If you do choose to divide in the fall, use a sharp, sterilized knife. Each piece you cut must have a piece of the crown and at least one eye. After cutting, let the pieces sit out overnight so the cut surface can "callus" over. This creates a dry seal that prevents bacteria from entering the tuber during storage.
Creating the Ideal Storage Environment
The goal of winter storage is to keep the tubers dormant, cool, and just moist enough that they don't shrivel up. If they get too warm, they will try to grow; if they get too cold, they will freeze; and if they get too wet, they will rot.
The Right Temperature
The "sweet spot" for dahlia storage is between 40°F and 50°F. An attached garage, an unheated basement, or a root cellar is usually perfect. Avoid storing them in a living area of your house, as the indoor heating will dry them out too quickly. Also, keep them off concrete floors, which can pull moisture out of the tubers or get too cold during a deep freeze.
Choosing a Storage Medium
Dahlias should be tucked into a material that regulates moisture. At Longfield Gardens, we have found that slightly dampened peat moss is one of the most reliable options. Other gardeners have success with:
- Pine Shavings: Often sold as pet bedding, these provide good airflow.
- Vermiculite: A mineral that holds moisture well without being "wet."
- Coarse Sand: Good for very dry environments but can be heavy to move.
- Newspaper: Wrapping individual tubers in several layers of dry newspaper can work well if your storage area has high natural humidity.
Packing the Tubers
Use a breathable container like a cardboard box, a plastic bin with the lid left cracked, or a wooden crate. Start with a 2-inch layer of your chosen medium at the bottom. Lay the tubers in a single layer so they aren't touching each other. Cover them with more medium and repeat until the box is full. Using a breathable container ensures that if one tuber starts to rot, the moisture can escape rather than spreading to the entire batch.
Key Takeaway: The perfect storage setup balances insulation and ventilation. Peat moss in a cardboard box provides a stable environment that mimics the protection of the soil.
Winter Maintenance and Monitoring
Overwintering dahlias isn't a "set it and forget it" task. To ensure a high success rate, you should check on your tubers about once a month. This small effort can save your entire collection.
When you open your storage boxes, look for two things: shriveling and rot.
If the tubers look wrinkled like a prune, they are losing too much moisture. You can fix this by lightly misting the storage medium with a spray bottle of water. You don't want the medium to feel wet—just slightly damp to the touch. This extra humidity will help the tubers plump back up.
If you see any fuzzy mold or soft, black spots, that is a sign of rot. Use a clean knife to cut away the affected area immediately. If a whole tuber has turned to mush, remove it from the box so the rot doesn't spread to its neighbors. After removing rot, you can leave the box open for a day to improve airflow before tucking them back in.
Special Cases: Containers and Raised Beds
If you grew your dahlias in pots or raised beds this year, they need a little extra attention. Soil in containers and raised beds cools down much faster than the soil in the ground because it is exposed to the air on the sides.
For dahlias in pots, you have a very easy option. Once the foliage has died back, you can simply move the entire pot into a frost-free garage or basement. Stop watering entirely and let the soil dry out. The soil itself acts as the storage medium. In the spring, you can tip the pot over, harvest the tubers, divide them, and give them fresh soil for the new season.
Raised beds are more susceptible to freezing than the ground. If you live in a borderline zone (like Zone 7) where you would normally leave dahlias in the ground, you should still consider digging up those in raised beds. The elevated soil is more likely to reach those dangerous freezing temperatures that turn tubers to mush.
Planning for Spring Success
While fall is about preservation, it is also the best time to think about next year's garden. As you dig and store your tubers, take a moment to reflect on which varieties performed best. Did the "Cafe au Lait" dinnerplate dahlias steal the show?
Did the "HS Date" attract the most bees?
If you found that some of your plants were too crowded or didn't get enough sun, make a note in a garden journal now while the memory is fresh. This information is invaluable when you go to replant in the spring.
Most gardeners find that their dahlia collection grows naturally over time.
By saving your tubers each fall, you can eventually fill larger garden beds or have extra plants to swap with friends and neighbors. It is a rewarding hobby that turns a single spring purchase into years of floral beauty.
Safety and Care Reminders
As you handle your dahlia tubers, keep a few safety tips in mind. Dahlia tubers are not considered edible and can cause stomach upset if ingested by humans or pets. When storing them, make sure they are in a location where curious pets or young children cannot reach them.
Additionally, always use clean tools. If you are dividing multiple varieties, it is a good idea to wipe your blades with a bit of rubbing alcohol between plants. This prevents the spread of any soil-borne diseases from one clump to another.
Gardening is a learning process, and every winter is a little different. If you lose a few tubers to rot or shriveling, don't be discouraged. Even professional growers expect a small amount of loss during the dormant season. The more you practice the cycle of digging and storing, the more "tuned in" you will become to the needs of your plants and the unique microclimate of your storage space.
Conclusion
Taking care of your dahlia tubers in the fall is one of the most satisfying parts of the gardening year. It marks the transition from the busy growing season to a time of rest and reflection. By waiting for the frost, digging with care, and providing a cool, stable home for your tubers, you ensure that the vibrant colors of summer will return to your yard next year.
At Longfield Gardens, we want every gardener to feel confident in their ability to overwinter these spectacular plants. It doesn't require expensive equipment or a green thumb—just a little bit of time and some basic attention to the needs of the tubers.
Final Steps for Fall Dahlia Care:
- Wait for the foliage to turn black after the first frost before digging.
- Lift the clumps gently to avoid breaking the delicate necks of the tubers.
- Store in a cool, dark place (40-50°F) in a medium like peat moss or wood shavings.
- Check your stored tubers once a month to manage moisture and prevent rot.
The effort you put into your dahlias this fall is an investment in a more beautiful garden for next spring. There is nothing quite like the feeling of pulling healthy, plump tubers out of storage in April and seeing those first pink eyes begin to sprout.
We wish you a wonderful and relaxing winter season, and we look forward to helping you grow a more beautiful world when the ground warms up again.
FAQ
Can I leave my dahlias in the ground if I live in Zone 7?
In Zone 7, leaving dahlias in the ground is a calculated risk. If the winter is mild and your soil drains well, they may survive if covered with 6 inches of mulch and a layer of plastic to keep out excess rain. However, a particularly wet or cold winter can cause them to rot. If the variety is rare or sentimental, we recommend digging it up to be safe.
Do I have to wash the dirt off the tubers before storing them?
It is not strictly necessary to wash them. In fact, many gardeners find that leaving a little bit of dry soil on the tubers helps protect them. If you have sandy or loose soil, simply brushing it off is enough. If you have heavy clay that sticks to the tubers, you can wash them, but you must ensure they are 100% dry before putting them in a storage box to prevent mold. For more basics on growing and handling dahlias, see our 8 Tips for Growing Better Dahlias.
What should I do if my tubers look shriveled in mid-January?
Shriveling is a sign that the storage environment is too dry. To fix this, use a spray bottle to lightly mist the storage medium (like peat moss or shavings) with water. You can also move the tubers to a slightly less ventilated container or close the lid of the bin more tightly to trap moisture. Most tubers that are only slightly shriveled will still grow just fine in the spring.
Why did my dahlia tubers turn to mush in storage?
Mushy tubers are usually the result of rot caused by too much moisture or freezing temperatures. If the storage area dropped below 32°F, the tubers may have frozen. If the temperature was fine, the tubers may have been too wet when they were packed or the storage medium was too damp. Always remove mushy tubers immediately to prevent the rot from spreading to healthy ones.