Are Dahlias Perennials or Annuals?
Are dahlias perennials, or are they annuals? Depending on where you're growing dahlias, the answer may change. Gardening expert Kelli Klein explains why that is and what it means for your flower garden.
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The interest in growing dahlias is on the rise and it’s easy to see why. Dahlias produce beautiful and unique multi-petal blooms that make excellent cut flowers. This includes popular options like dinnerplate dahlias which showcase 10-12 inch diameter double flowers.
With this rise in popularity, many new gardeners are asking themselves, are dahlias perennials or annuals? Well, it turns out that whether dahlias are annuals or perennials is not an easy yes or no.
The Answer: They Can Be Both
Dahlias can be grown as both annuals and perennials! How you achieve this will heavily depend on which USDA growing zone you are in. In colder regions, extreme temperatures will kill off the tubers in winter, meaning you can only grow them as annuals outdoors (unless you dig them up in fall). In warmer regions, they will pop up again year after year.
It also helps to understand their life cycle and how they grow in their native habitat. Dahlias are native to Central America and Mexico, so it makes sense that they would prefer naturally frost-free areas.
Dahlias grow and bloom best in full sun with 6-8 hours of sun per day. This also helps underground tubers survive the winter. Full-sun locations maintain a higher soil temperature in the winter as well. They do not tolerate frosty conditions and will die back to the ground in the winter.
Growing dahlias as perennials is possible, but it takes a fair bit of skill, trial, and error in certain regions. There are also steps to follow to ensure they not only grow again but flower repeatedly.
If you’d like to consider the more straightforward approach of growing dahlias as annuals, tubers at the beginning of each growing season rather than attempting to get them to survive winter.
Winterizing Dahlias in Hardiness Zones 8-10
It is relatively easy to grow dahlias as perennials if you’re located in a warm climate. There are several options for winterizing the plants within these zones. In hardiness zone 10, the plant grows easily with no winter protection.
In zones 8-9, dahlia plants are tender perennials. Wait until the first frost in fall when the dahlia foliage dies back. At this point, cut the dead foliage to 2-4 inches above the ground.
Since the stems are hollow, it’s best to cover them to stop moisture from entering the stems. This can cause rotting.
Protect the tubers by covering the ground with several inches of bark chips, pine needles, straw, or other mulch. This thick layer of mulch will provide each tender perennial protection from cold winter temperatures. This method works best in these zones because the temperatures rarely fall below 20°F (-7°C) during the winter.
This will not work in cold climates with consistent freezing temperatures. In colder regions, even mulch cannot keep the soil temperatures warm enough for the tubers to survive.
Winterizing in Hardiness Zones 7 and Below
In hardiness zone 7 and below, either grow dahlias as annuals or dig up the tubers and store them to plant the following spring.
The soil temperature in these zones does not stay high enough for the dahlia tubers to reliably overwinter in the garden. Tender perennials do not tolerate freezes, so this method of storing the dormant tubers indoors will guarantee they survive winter.
If you’ve chosen to dig up the dahlia tubers and store them over winter, there are a few steps to follow.
Step 1: Wait For Dormancy
Firstly, you’ll want to wait until the first frost of fall. This frost causes the dahlia flower and foliage to die back. Once the dahlia plant has gone through this stage, the dahlia tubers will naturally enter a state of dormancy. Wait a few weeks after this before you dig them to be sure they are dormant.
Dig up dahlias before hard frosts and cold temperatures begin to freeze the soil. They need a frost-free place for storage.
Step 2: Cut the Foliage
Next, cut the foliage down to the soil surface. Prune just before you plan to dig them up. Leaving the plant cut and uncovered can cause moisture to travel down the hollow stem, leading to rot.
Once pruned, use a spade or garden fork to carefully dig up the tubers. Remove clumps by digging on all four sides of the plant.
Step 3: Dry Them Out
After the tubers have been removed from the garden, allow them to air dry before storage. The purpose of air drying is to avoid excess moisture, which can cause your tubers to rot over winter.
Step 4: Storage
It takes trial and error to find the right conditions for storage. Light, temperature, and humidity all have an effect. Some gardeners store their dahlias in a peat or coconut coir to balance the moisture until it’s planting time.
Store them in a frost-free, cool, dry, and dark area with good air circulation. You can store them in a milk crate, cardboard box, or paper bag. Avoid areas with fluctuating temperatures and too much light, like a sunny window.
Check regularly throughout dormancy for signs of damage or rot. Remove those tubers before the rot spreads to the remainder.
If you live in a cold climate and would rather not deal with digging and storing, buy new plants at the beginning of each growing season and grow them as annuals instead.
Key Takeaways
Dahlias are considered annuals or tender perennials in hardiness zone 7. In USDA hardiness zones 8-10, the plants grow as perennials. The plant still benefits from winter protection in these areas, but the tubers can be left in the ground year-round.
In areas that receive consistently freezing temperatures over winter (USDA zones 7 and below), you may choose to grow dahlias as an annual. You can also overwinter them by digging up tubers and storing them in your garage or basement during the winter.
Frequently Asked Questions
Do dahlias come back every year?
This depends on your USDA hardiness zone! Dahlias are not frost-hardy and die back at the first sign of frost. The tubers, however, can survive below the soil surface and come back to life in the spring.
In zones 8-10, they are perennial and survive in the earth with little winter protection. In zones 7 and below, the tubers need to be removed and stored indoors during the winter to survive.
Can you leave dahlias in the ground over winter?
Yes, in zones 8-10, they can be left in their place to overwinter. In USDA hardiness zone 7 and below, they need to be dug up for storage if you want to grow them as perennials.
Should I cut back dahlias in the fall?
Cut back to 6 inches above the ground after the foliage has died back. Since the stems are hollow, they will need some cover to avoid water reaching the tubers. If you’re digging up the tubers, wait to cut back until just before you dig the tubers up for winter storage.
Do dahlias multiply?
Yes, the mother tuber can produce 5-20 new tubers. This perennial can be divided in the cooler temperatures of winter and will grow an identical flower from each division.
What temperature is too cold to grow dahlias?
The flowers and the foliage die back at the first light frost. The tubers, however, can survive in the soil in zones 8-10. In the colder regions of zones 7 and above, they need to be removed from the ground and stored in an area that stays above freezing. You can commence planting in the garden soil the following spring.
