Introduction
Few flowers offer the sheer variety and late-season drama of dahlias. These garden stars bloom from midsummer until the first frost, providing armfuls of color for your vases and garden beds. At Longfield Gardens, we love how a single tuber can transform into a stunning focal point. However, because dahlias are native to the warm climates of Mexico and Central America, they have specific needs when winter approaches.
Whether you must dig up your dahlia tubers—often called bulbs by home gardeners—depends largely on where you live. For some, they can stay in the ground with a bit of extra protection. For others, lifting them for winter storage is the only way to ensure they return next year. This guide will help you determine the best strategy for your specific climate and soil.
We will cover the role of hardiness zones, how to protect dahlias left in the ground, and the steps for successful digging and storage. By understanding these basics, you can enjoy these spectacular blooms year after year.
Our goal is to make dahlia care straightforward and rewarding. Choosing the right path for your garden ensures you get the most out of your investment and your time outdoors.
Understanding Your USDA Hardiness Zone
The most important factor in deciding whether to dig up your dahlias is your USDA hardiness zone. These zones are based on the average minimum winter temperature in your area. Because dahlias are tropical plants, they cannot survive if the ground freezes solid around their tubers.
In general, gardeners in zones 8 through 11 can often leave their dahlias in the ground year-round. In these warmer regions, the soil rarely freezes deep enough to reach the tubers. If you live in zone 7, you are in a "swing" area where success depends on your specific microclimate and how well you protect the soil. If you live in zone 6 or colder, the ground will almost certainly freeze deep enough to kill the tubers, making digging a necessity.
If you are unsure of your zone, you can easily find it by looking up your zip code on a hardiness zone map. This simple step takes the guesswork out of your fall garden chores. Knowing your zone is the first rule of "right plant, right place," ensuring your dahlias have the environment they need to thrive.
Key Takeaway Your local winter temperatures are the primary guide for dahlia care. In zones 8 and warmer, they usually stay in the ground. In zones 6 and colder, they must be dug up and stored indoors.
When You Can Safely Leave Dahlias in the Ground
If you live in a mild climate, leaving dahlias in the ground can save a significant amount of time and effort. There are even some benefits to this approach. Dahlias left in place often emerge earlier in the spring and grow into larger, more robust clumps than those that are replanted every year.
However, survival in the ground isn't just about temperature. It also depends on drainage, which is how fast water leaves the soil. Dahlias hate "wet feet" during their dormant winter period. If your soil stays soggy or waterlogged throughout the winter, the tubers are likely to rot, even if the temperature stays warm.
To give your dahlias the best chance of surviving the winter in the ground, consider these factors:
- Soil Type: Sandy or loamy soils that drain quickly are much safer for overwintering than heavy clay soils.
- Location: Dahlias planted near the foundation of a house often benefit from "leaked" heat, which keeps the soil slightly warmer.
- Shelter: A spot protected from cold winter winds can help maintain a more stable soil temperature.
Winterizing Dahlias in Warm Climates
If you decide to leave your tubers in the ground, you shouldn't just walk away and hope for the best. A few simple steps will "tuck them in" for the winter. This process is often called winterizing.
First, wait for the first frost to blacken the foliage. This signal tells the plant to stop growing and enter its dormant phase. Once the foliage has turned brown or black, cut the stems down to about 3 to 4 inches above the soil line. This makes the garden look tidy and prevents the long, dead stems from acting like a straw that pulls cold air down to the tuber.
Next, apply a thick layer of mulch. Use 4 to 6 inches of wood chips, straw, or shredded leaves. This mulch acts like a blanket, insulating the soil and preventing it from swinging between freezing and thawing. It also helps keep the tubers dry by shedding some of the winter rain.
What to Do Next: Leaving Dahlias in the Ground
- Check your USDA zone to confirm it is zone 8 or warmer.
- Evaluate your soil drainage to ensure it doesn't stay waterlogged.
- Wait for the first frost before cutting back the stems.
- Apply at least 4 inches of insulating mulch over the planting area.
When Digging Dahlias is Necessary
For gardeners in the northern half of the United States, digging is a mandatory part of the dahlia cycle. If the ground freezes to the depth of the tuber, the water inside the tuber will expand and burst the plant's cells. When the ground thaws, you will be left with a mushy, rotten mess instead of a healthy plant.
Even if you live in a borderline zone like zone 7, you might choose to dig your dahlias anyway. This is a great "insurance policy" if you have a particularly rare or favorite variety that you don't want to risk losing to an unusually cold winter. It is also the only way to divide your dahlias, which is how you turn one plant into three or four for the following season.
Wet winters are another reason to dig. If your garden is in a low-lying area or has heavy clay soil, the risk of rot is high. Digging the tubers and storing them in a controlled, dry environment is the safest way to ensure they survive until spring.
Key Takeaway Digging is essential in cold climates and helpful in wet ones. It protects your investment from freezing and rot while giving you the chance to multiply your flower collection.
How to Dig Up Dahlia Tubers
The process of lifting dahlias is straightforward, but it requires a gentle touch. You want to avoid piercing or snapping the tubers, as wounds can lead to rot during storage.
Timing Your Harvest
Timing is everything when it comes to a successful harvest. We recommend waiting until after the first "killing frost" has turned the foliage black. This frost sends a signal to the plant to move all its remaining energy down into the tubers for the winter. This makes the tubers "ripen" and better prepared for storage.
If a frost hasn't arrived by mid-November, you can go ahead and cut the plants back anyway. Once the stems are cut, wait about a week before digging. This short waiting period helps the skins of the tubers toughen up slightly, making them less prone to damage.
For additional timing guidance, see when to dig up dahlia bulbs.
The Lifting Process
When you are ready to dig, don't reach for a narrow shovel. A garden fork is a much better tool for this job. It is less likely to slice through a tuber than a solid spade blade.
Start by digging about 12 inches away from the main stem. You want to loosen the soil all the way around the plant before you try to lift it. This ensures you aren't accidentally prying against a hidden tuber. Once the soil is loose, gently lift the entire clump from underneath.
Once the clump is out of the ground, shake off the excess soil. Do not bang the tubers against a hard surface or each other, as the "necks" (the thin part where the tuber connects to the main stem) are very fragile. If the neck breaks, the tuber will not grow next year.
What to Do Next: Digging Successfully
- Wait for the first frost to blacken the leaves.
- Cut stems down to 4 inches and wait one week.
- Use a garden fork to loosen the soil 12 inches away from the stem.
- Lift the entire clump gently and shake off loose dirt.
Cleaning and Curing Tubers for Storage
Before your dahlias go into their winter home, they need a little preparation. This involves cleaning away soil and "curing" the skins to prevent rot and dehydration.
To Wash or Not to Wash?
Gardeners have different opinions on washing tubers. Some prefer to leave the dirt on, much like you would store a potato. Others prefer to wash them clean so they can inspect for pests or rot. At Longfield Gardens, we generally recommend a light washing with a garden hose to remove most of the soil. This makes it easier to see the "eyes" (the growth points) and any signs of damage.
If you do wash them, be very gentle. Use a low-pressure spray. Avoid scrubbing the tubers, as you don't want to peel away the skin. After washing, turn the clumps upside down for a few hours. This allows any water trapped inside the hollow stems to drain out, which is a key step in preventing crown rot.
The Curing Phase
Curing is the process of letting the tubers dry slightly in a protected area before they are packed away. Move your cleaned clumps to a cool, dry spot out of direct sunlight. A garage, basement, or shed works well, as long as it is frost-free.
Let the tubers sit for 1 to 3 days. You want the surface of the skin to feel dry to the touch, but you don't want the tuber to start shriveling. Shriveled tubers are dehydrated and may not have enough energy to sprout in the spring.
Key Takeaway Cleaning and curing prepares the tubers for a long winter sleep. Removing excess moisture while preventing total dehydration is the balance you are looking for.
The Best Environment for Winter Storage
Successful storage is about finding the "Goldilocks" zone: not too cold, not too warm, not too wet, and not too dry. The ideal temperature for storing dahlia tubers is between 40°F and 50°F. If the temperature stays below 40°F, they may freeze. If it stays above 50°F, they may wake up too early and start to sprout or shrivel from the heat.
For a complete walkthrough, read this guide to storing dahlia bulbs over winter.
Choosing Your Storage Container
There are many ways to pack your dahlias. The goal is to provide a medium that maintains a tiny bit of moisture while still allowing the tubers to breathe. Common choices include:
- Cardboard Boxes or Paper Bags: These are breathable and easy to store.
- Plastic Bins: If you use plastic, do not seal the lid completely. Leave it cracked to allow for air circulation.
- Peat Moss or Vermiculite: Packing the tubers in these materials helps regulate moisture and keeps the tubers from touching each other, which prevents rot from spreading.
Some gardeners simply wrap individual tubers in plastic wrap, while others prefer the "nesting" method in a box of wood shavings. Experiment to see what works best in your specific storage area. A dry basement might require more packing material to prevent shriveling, while a damp cellar might require more air circulation.
Monitoring During Winter
Don't just pack your dahlias away and forget them until spring. We suggest checking on them once a month. If you see any soft, mushy spots, cut those sections out immediately and throw them away so the rot doesn't spread to the rest of the clump.
If the tubers look very wrinkled and shriveled, they are too dry. You can give them a light misting of water with a spray bottle to help them plump back up. If you see mold or condensation inside your container, they are too wet. Open the lid or move them to a spot with better air flow.
What to Do Next: Optimal Storage
- Find a storage spot that stays between 40°F and 50°F.
- Select a breathable container like a cardboard box or paper bag.
- Use a packing medium like vermiculite or dry peat moss.
- Set a calendar reminder to check for rot or shriveling once a month.
Dividing Dahlias: The Hidden Benefit of Digging
One of the most exciting parts of digging up your dahlias is the opportunity to divide them. Over a single growing season, one tuber can grow into a large clump containing five, ten, or even fifteen new tubers. If you leave the clump whole, the plant can become overcrowded, leading to smaller flowers and weaker stems.
By dividing the clump, you create multiple new plants. Each new tuber must have three things to be successful:
- A Tuber: The fleshy part that stores energy.
- A Neck: The narrow part connecting the tuber to the crown.
- An Eye: A small bump on the crown (near the stem) where the new sprout will emerge.
If a tuber breaks off without a piece of the crown and an eye, it will not grow. Think of the eye like the eye on a potato; without it, there is no growth point.
Many gardeners find it easiest to divide in the spring when the eyes are starting to swell and are easier to see. However, you can also do it in the fall if you are experienced at spotting those tiny bumps. Dividing is a simple, cost-effective way to expand your garden or share your favorite flowers with friends.
For more detail, follow this step-by-step dahlia tuber dividing guide.
Key Takeaway Dividing your dahlias every year or two keeps the plants healthy and multiplies your collection. It is the reward for the effort of digging.
Choosing the Best Path for Your Garden
So, do you have to dig up your dahlia bulbs every year? For many, the answer is a practical "yes" to ensure survival. For those in the South or West, it is a choice based on how much work you want to do and whether you want to divide your plants.
Gardening is a series of choices based on your local environment and your personal goals. If you love the routine of the seasons and want to ensure 100% survival, digging is a rewarding autumn ritual. If you prefer a lower-maintenance approach and live in a warm zone, leaving them in the ground with a heavy blanket of mulch is a perfectly valid strategy.
At Longfield Gardens, we want you to feel confident in whichever path you choose. Dahlias are remarkably resilient plants. As long as you protect them from their two main enemies—deep freezes and soggy rot—they will continue to reward you with their spectacular beauty year after year.
Ready to start your dahlia journey or add to your collection? Browse dahlia collections and find the right varieties for your garden. Gardening should be a source of joy, and with a few simple steps, your dahlias will be the highlight of your summer landscape.
Final Thought Whether you lift your tubers or mulch them in place, the effort is well worth the payoff of those first midsummer blooms. Success comes from matching your care to your climate.
FAQ
Can I leave dahlias in the ground in zone 7?
In zone 7, leaving dahlias in the ground is a calculated risk that often pays off if you provide extra protection. Success depends on having well-drained soil and applying a very thick layer of mulch (6 inches or more) to prevent the ground from freezing. If your winter is unusually harsh or very wet, you may experience some losses, so many gardeners in zone 7 choose to dig their most prized varieties.
How do I know if my dahlia tubers have rotted?
Rotted tubers will feel soft, mushy, or hollow when you squeeze them gently. They may also give off a sour or unpleasant odor and appear dark or discolored compared to healthy, firm tubers. If you find rot, it is best to discard the affected portions immediately to prevent it from spreading to healthy tissue.
What is the best material to store dahlia tubers in?
The most popular materials for storage are slightly damp peat moss, vermiculite, or wood shavings (the kind used for animal bedding). These materials are excellent at regulating moisture levels, preventing the tubers from drying out completely while still allowing for air circulation. Some gardeners also find success using simple brown paper bags kept in a cool, dark place.
Can I treat dahlias as annuals and leave them to die?
Yes, you can certainly treat dahlias as annuals if you do not wish to dig and store them or if you live in a cold climate. Many gardeners enjoy the flexibility of buying new varieties each spring to change their garden's color palette. While it is more expensive than saving your own tubers, it eliminates the need for fall digging and winter storage.
