Introduction
There is a special kind of magic in the late-summer garden when dahlias are at their peak. These spectacular bloomers, from the massive "dinnerplate" varieties like ‘Cafe au Lait’ to the perfectly symmetrical pompons like ‘Wizard of Oz,’ provide a bounty of color just as other plants begin to fade. One of the most rewarding parts of growing these flowers is knowing that the tubers nestled underground can be saved, stored, and replanted to bloom again next year.
At Longfield Gardens, we want to help you enjoy this cycle of growth year after year with our Dahlia Collections. Understanding when to dig your dahlia bulbs—technically called tubers—is the most important step in ensuring they survive the winter in great health. While it might seem like a complex task, the timing is actually governed by a few simple signals from nature.
This guide will walk you through the biological cues that tell you your dahlias are ready to come out of the ground. We will cover the impact of frost, how to handle the tubers in different climates, and the best way to prepare them for their winter nap. If you're not sure of your growing zone, the USDA Hardiness Zone Map is a helpful reference. By getting the timing right, you ensure that your tubers are mature, resilient, and ready for a head start next spring.
Timing your dahlia harvest correctly allows the tubers to develop a protective skin that keeps them fresh and viable during winter storage.
The Biological Clock: How Dahlias Prepare for Winter
To understand when to dig, it helps to understand what is happening inside the plant as the season changes. Dahlias are native to the high-altitude regions of Mexico and Central America, where the weather is consistently mild. In most parts of the United States, our winters are far too cold for these tropical plants to survive in the ground.
Throughout the summer, your dahlia plant is busy producing beautiful foliage and flowers. However, as the days grow shorter in late August and September, the plant’s focus shifts. When the day length drops to about 12 hours or less, the dahlia receives a hormonal signal to begin storing energy for the future.
This energy is sent downward into the root system, where it is stored in the form of starches within the dahlia tubers. These tubers are the "batteries" that will power next year’s growth. If you dig them up too early—while the plant is still in its peak flowering stage—the tubers may be small, watery, and lacking the energy reserves needed to survive months of storage. Giving the plant enough time to finish this energy transfer is the first "rule" of successful digging.
The Role of Frost in Timing the Harvest
For most gardeners in USDA hardiness zones 3 through 7, frost is the primary indicator that it is time to think about digging. Frost acts as a natural "off switch" for the dahlia plant. A light frost may only nip the top leaves, turning them black or brown while leaving the stems green. A hard frost—defined as temperatures dropping below 28°F for several hours—will kill the entire top of the plant, turning it into a dark, wilted mass.
The Two-Week Rule
The most common advice among experienced dahlia growers is to wait approximately 10 to 14 days after a hard frost before digging the tubers. This waiting period serves a very practical purpose. Once the foliage is killed, the plant stops sending water and nutrients upward. The energy remains in the tubers, and the lack of upward movement allows the tubers to "cure" while still in the protection of the soil.
During these two weeks, the outer skin of the tuber begins to toughen and thicken. In the gardening world, we call this "setting the skin." Much like a potato, a dahlia tuber with a thick, cured skin is much less likely to shrivel or rot during the winter. It also becomes more resistant to nicks and scratches from your shovel or garden fork.
For a more complete walkthrough, see How to Overwinter Dahlia Tubers.
Dealing with a Late Frost
In some years, autumn may stay unusually warm, and a hard frost might not arrive until very late. If your ground is in danger of freezing before a frost hits the foliage, you should go ahead and dig. While waiting for frost is ideal, the tubers must be out of the ground before the soil itself freezes. If the ground freezes several inches deep, it will turn the tubers to mush, and they will not be salvageable.
Managing Your Digging Schedule by Climate
Every garden has its own unique microclimate, and the right time to dig for a gardener in Maine will be very different from someone in Georgia. We recommend looking at your local weather patterns rather than a specific calendar date.
Cold Climates (Zones 3–6)
In these regions, the goal is to maximize the growing season without risking a frozen ground. You can usually wait for that first hard frost in October or early November. If the forecast predicts a sudden, deep freeze (well below 25°F) and your dahlias haven't seen a light frost yet, it is safer to cut the plants back manually and dig them immediately rather than risking damage to the tubers.
Transitional Zones (Zone 7)
Gardeners in Zone 7 often have a choice. In a mild winter, dahlias might survive in the ground if they are buried under a thick layer of mulch (6–10 inches of straw or wood chips). However, heavy winter rains can often cause tubers to rot in the ground even if it doesn’t get too cold. Most growers in this zone still choose to dig and store them to be safe. You can wait until late November to dig, as the ground stays warmer longer.
Warm Climates (Zones 8–10)
In the South and along the West Coast, a killing frost may never arrive. In these areas, the dahlia’s biological clock still slows down as day lengths shorten. You can mimic the effects of frost by cutting the plants down to about 4 inches above the soil in late November or early December. Wait two weeks for the skins to cure in the soil, and then proceed with digging. This gives the tubers a necessary "rest period" before they are replanted in the spring.
What to do next:
- Monitor your local 10-day forecast starting in mid-September.
- Note the date of the first light frost in your garden journal.
- If a hard frost is predicted, prepare your labels and tools for the "big dig" two weeks later.
Preparing the Plants for Extraction
Once the frost has arrived or you have reached your target date, the first step is to "knock down" the garden. Use a pair of sharp loppers or garden shears to cut the dahlia stems down to about 4 to 6 inches above the soil line.
Cutting the stems serves two purposes. First, it makes the plants much easier to manage. Dahlias can grow to be 5 feet tall and quite heavy; removing the bulk of the foliage allows you to see exactly where the base of the plant is. For support and staking tips, see How to Pinch and Stake Dahlias. Second, the remaining few inches of stem act as a convenient handle when you are lifting the tubers out of the earth.
Be careful when cutting! Dahlia stems are often hollow and can hold water. Some gardeners like to cover the cut stems with a piece of aluminum foil if it is going to rain during the two-week waiting period. This prevents water from traveling down the hollow stem into the "crown" of the tuber clump, which can sometimes lead to rot.
The Right Tools for the Job
Digging dahlias is much easier when you have the right equipment. While a standard shovel can work, it has a sharp, flat edge that is very likely to slice through a tuber. Dahlia clumps can grow much wider than you might expect, sometimes reaching 12 to 18 inches across.
We find that a sturdy garden fork (sometimes called a digging fork or pitchfork) is the best tool for the job. The tines of the fork can slide between the tubers rather than cutting through them. If you do use a shovel, make sure to give the plant plenty of space.
Step-by-Step: Lifting the Tubers
Lifting a large dahlia clump like those produced by Thomas Edison or ‘Emory Paul’ requires a gentle touch. Follow these steps to ensure you don't damage the "necks" of the tubers, which is where the eyes (the growth points) are located.
- Clear the Area: Remove any mulch or decorative stakes from around the base of the plant so you can see the soil clearly.
- Start at a Distance: Place your garden fork in the soil about 12 inches away from the main stem. This ensures you are outside the main mass of tubers.
- Circle the Plant: Push the fork deep into the ground and gently pry upward. Do this on all four sides of the plant to loosen the soil and break the long, thin feeder roots.
- Lift Gently: Once the soil is loose all the way around, slide the fork under the center of the clump and lift slowly.
- Remove Soil: Gently shake the clump or use your hands to brush away large chunks of soil. Do not bang the tubers against a hard surface, as they are surprisingly brittle when first dug.
A dahlia tuber's "neck"—the narrow part where it connects to the main stem—is its most fragile point. If the neck is broken or badly creased, the tuber usually won't grow, even if it looks healthy.
Post-Digging: Cleaning and Curing
Once your tubers are out of the ground, they need a bit of cleaning before they can go into storage. There are two schools of thought on washing tubers: some gardeners swear by it, while others prefer to leave a little soil on.
To Wash or Not to Wash?
Washing your tubers with a gentle stream from a garden hose allows you to see the "eyes" more clearly and makes it easier to spot any signs of rot or insect damage. It also prevents you from bringing soil-borne pests into your storage area. However, if you wash them, you must be absolutely certain they are dry before you pack them away.
Leaving the soil on can act as a slight insulator, but it can also hide problems. If you have heavy clay soil, we generally recommend washing them, as clay can dry into a hard "brick" that is difficult to remove in the spring. If you have loose, sandy soil, you can often just brush the dirt off once it dries.
The Curing Process
After cleaning, the tubers need to dry in a protected area for 24 to 48 hours. This is the final stage of curing. Place them in a cool, shaded spot with good air circulation—a garage or a covered porch works perfectly. Avoid direct sunlight, which can cause the tubers to shrivel too quickly.
When the tubers feel dry to the touch and the skin looks slightly matte, they are ready for storage. If you plan to divide your tubers into smaller pieces, the fall is a great time to do it because the stems are still soft. However, many beginners find it easier to wait until spring when the "eyes" begin to swell and become easier to see.
Identifying and Labeling Your Varieties
One of the most common mistakes in dahlia gardening is forgetting which tuber is which. Once the flowers are gone and the stems are cut, all tubers look remarkably similar. A Maarn tuber looks just like a ‘Kelvin Floodlight’ tuber.
We recommend labeling your tubers immediately after digging. You can use a soft pencil or a permanent marker to write the variety name directly on the skin of the tuber. Alternatively, you can use plastic nursery tags tied securely to the stem with garden twine. If you are digging multiple varieties, work with one plant at a time to avoid mix-ups.
Final Steps: Preparing for Winter Storage
Your tubers are now cured, cleaned, and labeled. Their survival through the winter depends on two factors: temperature and moisture balance.
- Temperature: The ideal storage temperature is between 40°F and 50°F. A basement, a cool closet, or a crawl space usually fits the bill. You want to keep them cold enough to stay dormant but never so cold that they freeze.
- Moisture: The storage medium should keep the tubers from drying out without making them damp. Popular choices include slightly damp peat moss, vermiculite, or wood shavings (the kind used for animal bedding).
Place your tubers in a breathable container, such as a cardboard box, a plastic bin with holes drilled in the sides, or a paper bag. Layer the tubers with your chosen medium, ensuring they aren't touching each other. This prevents a single spoiled tuber from affecting the entire batch.
Storage Quick-Check:
- Choose a medium like peat moss or vermiculite.
- Ensure the storage spot stays between 40°F and 50°F.
- Check your tubers once a month during winter. If they look shriveled, lightly mist the medium with water. If they look fuzzy or moldy, increase the airflow.
Realistic Expectations for Tuber Survival
Even for the most experienced gardeners, not every dahlia tuber will make it through the winter. Some varieties are naturally better "keepers" than others. Small, thin tubers are more prone to drying out, while very large, thick tubers can sometimes hold too much moisture and rot.
If you lose a few tubers over the winter, don't be discouraged! It is a normal part of the gardening process. Factors like the humidity in your home, the weather during the curing period, and the age of the tuber all play a role. The goal is to save the majority of your collection so you can start the next season with a healthy, vibrant garden.
Conclusion
Digging and storing dahlia bulbs is a rewarding ritual that marks the end of a successful growing season. By waiting for the natural cues of shorter days and the first frost, you give your plants the time they need to store energy and prepare for their winter rest. Whether you are growing dinnerplate dahlias for their size or pompon varieties like Wizard of Oz for their intricate shapes, the extra care you take in the fall ensures a spectacular display next year.
At Longfield Gardens, we believe that gardening should be a joyful and accessible experience. Taking these simple steps to preserve your tubers is one of the best ways to build a garden that grows more beautiful every year. Once your tubers are safely tucked away, you can spend the winter months dreaming of the vibrant colors and lush blooms that will return with the spring sun. If you find you have extra space in the spring, it is the perfect time to explore new colors and shapes like Kelvin Floodlight to add to your collection.
"Saving your dahlia tubers is like putting a little bit of summer on hold; it’s a simple act of hope that pays off in spectacular blooms the following year."
For your next steps, we recommend checking your storage area to ensure it stays within the ideal temperature range. If you find you have extra space in the spring, it is the perfect time to explore new colors and shapes to add to your collection.
FAQ
Can I dig my dahlias before the first frost?
Yes, you can dig them before the frost if your growing season is short or if a deep freeze is predicted before a light frost occurs. Aim to wait at least 100 days after planting, or until the plant begins to show signs of slowing down and the foliage starts to yellow naturally. This ensures the tubers have had enough time to store energy for the winter.
What happens if I dig my dahlias too early?
If tubers are dug while the plant is still actively growing and flowering vigorously, they may be "immature." Immature tubers have very thin skins and low starch reserves, which makes them much more likely to shrivel and die during winter storage. They also lack the "eyes" necessary for growth the following year, which may not have developed yet.
How long should I let dahlia tubers dry before storing them?
After digging and cleaning, tubers should air-dry in a cool, shaded spot for about 24 to 48 hours. You want the exterior to be dry to the touch and the cut ends of the stems to feel calloused. Avoid drying them for more than a few days, as they can begin to lose too much internal moisture and become shriveled.
Should I divide my dahlia tubers in the fall or the spring?
You can do either, but each has its benefits. Fall division is easier because the tubers are softer and easier to cut, and it takes up less storage space. However, spring division is often better for beginners because the "eyes" (growth buds) become much more visible as the weather warms, making it easier to ensure each division is viable.
