The interest in growing dahlia plants is on the rise and it’s easy to see why. Dahlias produce beautiful and unique multi-petal blooms that make excellent cut flowers and provide forage for pollinators. This includes varieties like the dinnerplate dahlias that showcase 10-12 inch diameter double flowers.
With this rise in popularity, many dahlia growers are asking themselves, are dahlias perennials or annuals? Well, it turns out that whether dahlias are annuals or perennials is not so easily answered with a yes or no.
To answer this question, we first have to understand the complete life cycle of how dahlias grow and bloom. In their native habitat, they are tropical plants. It’s also important to know that these flowers grow from underground tubers.
The Quick Answer
Dahlias are considered annuals or tender perennials in hardiness zone 7. In USDA hardiness zones 8-10, the plants can more reliably be grown as perennials. These warm climates offer the flowers a good imitation of their natural habitat for most of the growing season. 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 growing zones 7 and below, you may choose to grow dahlias as an annual. If you grow and plant dahlias this way, there is no need to offer winter protection, and you can purchase new plants at the beginning of each new growing season. You can also overwinter them by digging up tubers and storing them in your garage or basement during the winter.
The Detailed Answer
As mentioned above, they can be grown as both! How you achieve this will heavily depend on which USDA growing zone you are in. You can use a simple search to determine your growing zone by using your zip code. Once you know your growing zone, you have a better idea of the best way to grow dahlias in your garden.
It also helps to understand their life cycle and how they grow in their native habitat. Dahlias are native to Central America and Mexico, which makes sense that they would prefer naturally frost-free areas.
Dahlias are even the national flower of Mexico. It is believed that they were first grown by the Aztecs as animal fodder and as a medicinal plant.
Dahlias grow and bloom best in a full-sun location that receives 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. The tubers can multiply over a single growing season! They do not tolerate frosty conditions and will die back to the ground in the winter.
No matter your growing zone, you’ll want to plant dahlia tubers in a fully lit garden location. Whether you can leave the tuberous roots in the ground or grow them as bedding dahlias and dig them up for storage, allow the dahlia foliage to die back completely before pruning them.
Growing dahlias as perennials that produce their striking flower takes a fair bit of skill, trial, and error, and sometimes luck that comes with ideal weather conditions.
If you’d like to consider the more straightforward approach of growing dahlias as annuals in the garden, you may want to purchase tubers at the beginning of each growing season for planting 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 one of these warm climate areas. 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 perennial plants. You’ll want to wait until the first frost in autumn when the dahlia flowers and 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 avoid too much moisture entering the stems and potentially reaching the dahlia tubers a few inches below. 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 for perennials works best in these zones because the temperatures rarely fall below 20 degrees Fahrenheit (-6 degrees Celsius) during the winter.
This will not work in cold climates that receive consistent freezing temperatures. In colder regions, even a layer of mulch cannot keep the garden soil temperatures warm enough for the tubers to survive. However, following these methods in hardiness zones 8-10 will ensure that your plants grow as perennials rather than annuals.
Winterizing in Hardiness Zones 7 and Below
Since dahlias are tender perennials in hardiness zone 7 and below, they can either be grown as annuals, or the tubers can be dug up and stored to be planted directly in the garden the following spring.
Tender perennials do not tolerate freezes, so this method of storing the dormant tubers indoors will guarantee that they survive winters in these zones that regularly receive frosts. Unlike their warmer climate counterparts, the soil temperature in these zones does not stay high enough for the dahlia tubers to reliably overwinter in the garden.
If you’ve chosen to dig up the dahlia tubers and store them over winter until planting time, then there are a few things to know. Overwintering dahlias this way takes a bit of skill. Firstly, you’ll want to wait until the first killing frost of fall. This frost causes the dahlia flower and foliage to die back.
Step 1: Wait For Dormancy
Once the dahlia plant has gone through this stage, the dahlia tubers will naturally enter a state of dormancy. You can wait a few weeks after this before you dig them to be sure that they are dormant.
Make sure to dig them 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 of the dahlia plants down to the soil surface. Make sure the pruning occurs just before you plan to dig them up. Leaving the plant pruned and uncovered can cause moisture to travel down the hollow stem, which can lead to the dahlia tubers rotting.
Once pruned, you can 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. Now, this is where it gets a bit tricky.
The purpose of air drying is to avoid too much moisture, which can cause your tubers to rot over winter. On the other hand, too little moisture over the winter, and they can shrivel up and die.
Step 4: Storage
Sometimes this takes a bit of trial and error to find the right conditions that are affected by the relative humidity in your storage space. Light, temperature, etc., also have an effect on them. Some gardeners store their dahlias in a peat or coconut coir medium to help balance the moisture until it’s garden planting time.
In general, you want to store them in a frost-free, cool, 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 them regularly throughout dormancy for signs of damage or rot, and 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 then you may opt for buying new plants at the beginning of each growing season and grow them as annuals instead.
However, if you are already planning on making a purchase in the spring and have nothing to lose, then why not give overwinter tuber storage a try? You might be surprised with some extra flowers in late spring and mid-summer!
Frequently Asked Questions
Q: Do dahlias come back every year?
A: 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 will need to be removed and stored indoors during the winter in order to survive.
Q: Can you leave dahlias in the ground over winter?
A: Yes, in zones 8-10, they can be left in their place to overwinter. Dahlias grow and bloom as annual plants in USDA hardiness zone 7 and below.
Q: How do you winterize dahlias?
A: This also depends on your growing zone. In zones 8-10, they benefit from in-ground winter cover and protections. In zone 7 and below, the tuberous roots will need to be removed from the ground and stored indoors until planting time.
Q: Should I cut back dahlias in the fall?
A: The plants can be cut back to 6 inches above the ground. However, since the stems are hollow, they will need some cover to avoid water reaching the tubers and causing rotting. If you’re digging up the tubers, wait to cut back Dahlias until just before you dig the tubers up for winter storage.
Q: Do dahlias multiply?
A: 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.
Q: What temperature is too cold dahlia?
A: The flowers and the foliage die back at the first light frost. The tubers, however, can survive in the soil in zones 8-10, where winter temperatures rarely fall below 20 degrees Fahrenheit (-6 degrees Celsius) during winter.
In the colder regions of zones 7 and above, they will 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.
