Dahlias are beautiful summer-blooming tuberous plants that add vibrant pops of color to any garden. Their flowers come in a diverse range of sizes shapes and colors. Dahlias grow from tubers which are enlarged stems that store nutrients over the winter.
A common question gardeners have is whether you can leave dahlia tubers in the ground over winter. The short answer is yes, you can leave dahlia tubers in the ground over winter in some climates. However, there are important factors to consider when overwintering dahlias in the garden.
Overview of Overwintering Dahlias
Overwintering refers to leaving dahlia tubers in the ground through the winter months instead of digging them up in fall for storage
The main benefit of overwintering dahlias is convenience – you avoid the labor-intensive process of carefully digging up the tubers in fall and properly storing them over winter. Overwintered dahlias also bloom earlier and grow into larger, more vigorous plants the following season.
However, overwintering does carry some risks, including potential tuber loss from frost damage, rotting, pests, etc. You’ll also likely get less blooms in the late summer and fall since overwintered plants will be more mature and starting to decline in productivity.
So can you leave dahlia tubers in the ground over winter? Let’s take a deeper look at the key factors to weigh.
Key Factors for Overwintering Dahlias in the Ground
Successful overwintering relies heavily on your gardening zone and winter temperatures. Proper drainage and mulching are also critical to prevent tubers from rotting over the wet winter months.
Here are the key factors to consider:
Climate and Hardiness Zone
The most important factor is your gardening zone and winter climate. Dahlia tubers will be killed if the ground freezes where they are planted.
Most experts recommend only overwintering dahlias in zones 6 and warmer. In cooler climates, it’s best to dig up and store the tubers over winter.
In zones 8-10, dahlias can be left in the ground year-round. Zone 7 gardeners sometimes “oversummer” dahlias during hot summers.
Drainage
Excellent drainage is crucial for overwintering dahlias outside. Standing water or overly wet soil will cause the tubers to rot over the winter.
Planting dahlias on slopes or raised beds improves drainage. Amending dense soil with compost also helps.
Mulching
Applying a thick layer of mulch over the dahlia tubers helps moderate soil temperatures and prevents frost penetration. Shredded leaves, straw or evergreen branches work well.
Plastic tarps can also be used to create a greenhouse effect and shield tubers from heavy frost or snow. Just remove any covers in early spring to avoid premature growth.
Tuber Depth
Planting the tubers slightly deeper, 8-10 inches, can help protect them from freezing temperatures. Staking the plants is recommended to prevent frost damage to the foliage.
Rodent Protection
Mice, voles and other rodents may dig up and eat overwintered tubers. Surrounding them with hardware cloth can help deter pests.
Tuber Health
Start only with disease-free tubers. Weak or damaged tubers likely won’t overwinter successfully. Carefully inspect tubers in spring for any signs of rot or decay.
Supplemental Mulch
Cover beds with an extra 4-6 inches of shredded leaves or straw at the end of the season. This helps insulate the soil and protect tubers from wide temperature swings.
Spring Removal
Remove any supplemental winter mulch in early spring to allow soil to warm up. Watch for emerging sprouts so tubers aren’t damaged during removal.
Overwintering Tuber Preparation
To give your overwintered dahlias the best chance of survival:
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Plant tubers slightly deeper, 8-10 inches below the soil surface. This helps shield them from frost.
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Water tubers well after a killing frost. Moist soil retains warmth better than dry soil.
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Cut back stems to about 6 inches after the first hard frost.
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Consider surrounding tubers with hardware cloth cylinders to protect against rodents.
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Add a layer of shredded leaves, straw or evergreen boughs over the beds.
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In very cold climates, also cover beds with breathable plastic tarps.
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Check for sprouting tubers and remove winter mulch in early spring.
Pros and Cons of Overwintering Dahlias
Overwintering dahlias in the ground has both advantages and disadvantages:
Pros:
- Avoid digging up and storing tubers indoors over winter
- Earlier spring growth and summer blooms
- Bigger, more vigorous plants with more blooms
- No need to divide or re-plant tubers in spring
Cons:
- Higher risk of losing tubers to frost, pests or rot
- Fewer blooms late summer and fall as plants age
- No tuber dividing or propagating new plants
- Earlier bloom means more exposure to certain pests
Weigh these pros and cons carefully to decide if overwintering is right for your specific climate and garden conditions.
Storing Dahlia Tubers Over Winter
If overwintering dahlia tubers in the ground is not a good option in your location, they will need to be dug up in fall and stored over the winter months. Here is a quick overview of fall dahlia tuber harvesting and storage:
When to dig up tubers:
- After the first killing frost blackens the foliage
- Before a hard freeze reaches the tubers below ground
How to dig and divide tubers:
- Lift clumps carefully with a garden fork, keeping the varieties separate
- Allow to dry out of the sun for a few hours
- Cut stems back to about 6 inches above the tuber
- Gently separate tubers, leaving a few stems attached to each tuber
- Divide the largest tubers into smaller sections, ensuring an eye/sprout is present
- Dust any cuts with sulfur to prevent rotting
Storing conditions:
- Ideal storage temperature is 45-50°F
- High humidity around 90%
- Store in plastic bags or crates with peat moss
- Check periodically for any rotting tubers
Proper fall harvesting and winter storage provides the best chance of getting your dahlia tubers through the winter months in colder climates.
Key Tips for Overwintering Dahlias
Here are some key tips to remember if overwintering dahlias in the ground:
- Only overwinter in zones 6+ where ground doesn’t freeze
- Ensure excellent drainage and amend soil with compost
- Plant tubers a bit deeper, 8-10 inches below surface
- Mulch heavily with leaves, straw or evergreen branches
- Cover beds with plastic sheeting in very cold climates
- Check for sprouting tubers and remove mulch in early spring
- Stake plants to prevent frost damage to foliage
- Surround with hardware cloth to deter pests
- Start with healthy, disease-free tubers
- Remove any winter protection once sprouting begins
Follow these tips diligently to get a head start on growth and enjoy months of earlier blooms from your overwintered dahlia tubers.
Many gardeners wonder if they can leave dahlia tubers in the ground over winter. While not suitable everywhere, overwintering is possible in zone 6 and warmer as long as the ground doesn’t freeze. Excellent drainage and heavy mulching are critical. The trade-off is earlier blooms but decreased productivity in late summer/fall. In colder zones, it’s safer to dig up tubers in fall and store them over winter. Carefully weigh your climate, soil conditions and willingness to commit to added winter protection to decide if overwintering dahlias is right for your garden.
Do I have to dig my dahlias up every year?
You should dig up dahlias if your ground gets very cold and wet. If your weather is often dry, your soil is well drained and/or your winters rarely freeze, then it’s worth leaving your dahlias in the ground.
Because even if you do dig your dahlias up and store them, there’s no guarantee that they’ll survive the winter.
The autumn of 2022 was unusually wet in some places. Many people, including professional dahlia growers, dug up their dahlias in very wet weather. They discovered that it was almost impossible to get them dry enough to store safely.
A huge number of dahlia tubers rotted away in storage.
Yet many of my dahlias have been left in the ground, year after year. They have filled our late-summer borders with glorious colour.
Although, perhaps the borders are not exactly the colours I originally planted. But more of that later.
And if you want another time-saving autumn gardening tip, see this video about what to do (or not do) about your autumn leaves.
In what climate or zone can you leave dahlias in the ground?
Officially, people whose winters are no colder than minus 12C/10F (USDA Zones 8 and above) can leave their dahlias in. That’s pretty much the whole of the UK.
But if your winters are below minus 15C/5F (USDA Zones 7 and below), you need to dig them up.
However, if your area is very dry or you have a very sheltered spot, you may be able to over-winter dahlias in the ground in temperatures lower than minus 12C/10F.
And if your winters are warmer than minus 12C/10F, but are very wet, then you may lose dahlias in the ground. Dahlias hate being in cold, wet soil. They will almost always rot.
If you get long periods of freezing temperatures, you should probably dig up your dahlias for winter. And make sure that you store them somewhere frost-free.
Can you leave dahlias in the ground over winter?
FAQ
What happens if you leave dahlia tubers in the ground over the winter?
Dahlias may be left in the ground over the winter; however, dahlias are susceptible to rot and/or freeze. Dahlias are not hardy since they are a tuber (thin-skinned) and not a bulb. If you wish to move or transplant your dahlias to a new location, you may do so in the spring. Dahlias cannot be transplanted in the fall.
How to winterize dahlias in the ground?
If you live in hardiness zones 8-10, where winter temperatures rarely fall below 20° F, the soil provides enough insulation to protect the tubers and it’s safe to leave them right in the ground. In late fall, simply cut the plants back to several inches above soil level. They will start growing again in spring.
What do you do with dahlias in the winter?
How to protect tender plants. Cannas, dahlias and pelargoniums (commonly known as geraniums) can be lifted and stored in a semi-dormant state over winter.
What happens if you don’t separate dahlia tubers?
Each plant will produce a clump of new tubers which, undivided, will produce smaller plants with poorer quality blooms.