Dahlia plants are easy to grow and they will reward you with spectacular blooms for many months, from mid-summer until fall or even early winter.
While the flowers will die off completely in winter, a bonus is that there will be tubers underground that will keep your dahlias going.
Our 2021 planting guide has a wealth of information on planting these colorful garden gems.
When to plant dahlia bulbs? Generally, spring is the time to plant dahlia tubers, but your local climate will dictate safe planting dates more accurately. Remember that they don’t like frost and soil temperature is important.
Planting Dahlia Bulb in Different Climates
Dahlias are native to Central America and Mexico, which is a country with a diverse climate. But if you think that studying the climate of Mexico will help you work out where dahlias grow best, you couldn’t be more mistaken.
The internationally recognized Köppen climate classification highlights 5 climate types: tropical, dry, temperate, continental, and polar. Numerous sub-categories make the classification more precise.
Mexico is divided into temperate and tropical zones and different regions experience a range of climatic conditions. These vary from desert and semi-arid conditions to those that are subtropical and humid, and Mediterranean in type.
So, you might experience most climate types in this area other than polar. But that doesn’t help your dahlia-growing efforts much, does it?
Originally, dahlias grew like weeds in the mountainous regions of Mexico (they still do). Their seeds were shipped to Spain by explorers who admired their beauty.
Today there are at least 36 dahlia species and 10s of thousands of different cultivars. Most grow best in warm weather and they don’t like frost, so be aware of frost dates in your area.
Growing Dahlias in Different Zones
Chances are if you are a keen gardener you know which gardening zone you live in. If you don’t, now is the time to find out.
The United States Department of Agriculture (USDA) has an interactive map on its website. The Canadian government has plant hardiness zone maps and other gardening resources on its Natural Resources Canada website.
Remember that in North America, the lower the zone number, the colder the area.
Dahlia plants are considered to be a vulnerable perennial in colder North American regions. They are, though, winter-hardy in Zones 8-11.
We will talk about whether tubers should be left in the ground or dug up before winter in more detail later on. But if you live in Zones 2 to 7, you can either dig them up or treat them as annuals that will re-emerge in spring.
Even though they do well in warm weather, dahlias aren’t well suited to very hot climates like those in Texas and southern Florida. There is some hope though because there are some heat-resistant varieties available.
Wherever you are, be aware that dahlias will produce more blooms if they have about six to eight hours of direct sunlight. They will also do best if the soil has warmed before you plant them.
Let’s see whether your climate is well-suited to growing dahlias.
Tropical climates have temperatures that average 18°C (64.4°F) or more. They also have a lot of rain.
If you live in a tropical climate, dahlia plants aren’t going to grow in the intense summer heat. Rather grow them in later summer and autumn.
Dry climates don’t have much rain, but the temperature is also relevant. For instance, there are hot and cold desert climates as well as hot and cold semi-arid climates.
Whichever type of dry climate you experience, irrigation is going to be as important as planting time.
Temperate climates have an average that is between about 0°C (32°F) in the coldest month. But rainfall is very variable, and there are nine sub-categories of climate type.
It is probably safe to assume that late spring or early summer will be a good time to plant dahlia tubers.
The temperatures experienced in continental climatic regions are similar to temperate regions, but there’s considerably more rain.
There are 12 sub-categories in this climate type, so it isn’t surprising that many U.S. states experience a continental climate at least in some parts.
This is where average temperatures are always below 10°C (50°F). Dahlias won’t cope here unless you have a greenhouse or some other environment that is heated.
Choosing What Type of Dahlia Bulbs to Buy
Since there are more than 36 dahlia species and tens of thousands of different cultivars, the choice of what to buy and what to grow is mind-boggling.
It might also be confusing when you realize that most suppliers offer dahlia tubers rather than dahlia bulbs. Don’t worry about it, both tubers and bulbs are fleshy and they grow into plants.
The main difference is that bulbs are more regular in shape and tubers aren’t true bulbs! There’s nothing regular about the shape of dahlia bulbs.
What you need to do is ascertain which dahlias grow best in your area. There’s a huge choice and you can buy them online or from your local garden center.
You can, of course, also grow dahlias from seeds, but these will take a lot longer to grow. You can buy seeds from commercial companies or harvest your own from dahlia plants.
Whichever route you take, choosing which type of dahlia to buy can be a daunting prospect. You will find that there is a multitude of colors and sizes.
Most grow about 4-5 feet tall.
How to Plant Dahlia Bulb Seeds
Your dahlia plants will grow from seeds or bulbs (tubers). The planting process of each is a little different.
How to Plant Dahlia Seeds
If you decide to grow your dahlias from seeds, you are going to have to wait a long time before the tubers form and sprout. Then it’s going to take another couple of years for the plants to produce anything vaguely worthwhile.
If you have the time and want a challenge, here’s what you need to do:
- Sow the seeds in suitable soil
- Cover the seeds lightly
- Keep the soil moist while the seeds germinate
- When the seedlings are big enough, transfer to pots
- Let the dahlia grow so that the plants establish themselves before planting out
Once your seedlings are in your garden bed, they will grow, but it will take a while before they form tubers.
How to Plant Dahlia Tubers
You can buy tubers or dig out the tubers that have already produced dahlia flowers. Do this in winter before the first hard freeze.
There is some debate on whether to leave dahlia tubers in the ground.
Dahlias are perennials, but in many regions, the ground gets so cold in winter that the tubers will die if they are left in the ground. There is also a danger that they might rot during winter or be dug up by squirrels and other critters.
But, if you live in the warmer hardiness Zones 8-10, you can usually leave the tubers safely in the ground. Cut the plants back to a few inches above soil level and they will sprout again in spring.
If you have bought tubers or dug them out of the ground, you can plant them in spring when the ground is warm and there’s no chance of further frost.
An American Dahlia Society (ADS) course on how to prepare for a dahlia garden points out that dahlias prefer slightly acidic soil. Take steps to make sure that the soil is also enriched with organic material like compost, peat moss, manure that is well-rotted, oak leaves, and leaf humus.
Ideally, plant in raised beds in well-drained soil.
Dig holes for the tubers that are 6-8 inches deep (15-20 cm) and 9-12 inches (23-30.5 cm) apart. Mix the soil you have dug out with compost or peat moss.
You can put a handful of bone meal into the hole, but otherwise don’t fertilize when you plant dahlia tubers. Pop the tubers into the holes with the eyes, which are the growing points, facing up, and cover with between 1 and 3 inches (2.5-7.5 cm) of soil.
Tall species will need to be staked to give the dahlia plants support.
If deer or other critters like groundhogs are a potential problem, fence your garden to protect it and make it groundhog and deer resistant.
You can also grow companion plants that some find to be deer resistant. These include toxic plants like foxgloves, poppies, and daffodils, as well as strongly fragrant plants including peonies, lavender, and ornamental salvias.
How to Water Dahlia Bulbs
Dahlia plants don’t need a lot of water. While it’s important to water regularly, it’s more important to be careful not to give them too much than too little otherwise the young plants can rot.
They do not like wet soil. End of story!
Generally, you don’t need to water them at all unless you’ve had less than an inch (2.5 cm) of rain in 7 days. If this is the case, it’s best to give them a deep watering once a week rather than watering them a little more often.
If there isn’t enough rainfall in your area, you should consider installing drip irrigation. It’s controlled and the water will filter through the soil to the roots slowly and evenly without any unnecessary runoff.
If you are going to water by hand, try and avoid wetting the leaves and flowers. Rather keep your hose pipe directed at soil level.
Water regularly throughout the growing season and water slowly so that the water doesn’t run off or pool. Once the first few inches of soil are moist, the water will soak in more easily.
If you grow your dahlia bulbs in pots, you might need to water them more regularly, unless the pots are out in the open. Be sure to let the soil dry out between waterings.
How to Grow Dahlia Bulbs
The experts, including those at the ADS, reckon that growing dahlias is a lot like growing tomatoes. So, it stands to reason that if you can grow tomatoes, you can grow dahlias.
Use the same time frame you would use when growing tomatoes. If you want to see your dahlias bloom early, start the bulbs (tubers) inside about a month before the recommended planting time.
In most parts of North America, it’s okay to plant dahlias as late as mid-June, which is late spring. Then let the plant grow through summer and fall.
While dahlias bloom from mid-summer, the best show is in fall and early winter. Not only will you be rewarded with spectacular blooms in your garden from mid-summer onwards, but you will also have a source of beautiful cut flowers.
How Long Do Dahlia Bulbs Take to Grow?
It usually takes about five weeks for dahlia tubers (bulbs) to sprout. But this will depend on the variety you have planted as well as the temperature of the soil, and how deep you planted the dahlia bulbs.
It usually takes about 8 weeks before dahlias bloom.
Dahlias are easy to grow and make stunning cut flowers. They are perennials and will continue to flower season after season, even if you lift the tubers out of the ground in winter once they have finished flowering.
Our plant guide is packed with useful information and tips that will help you produce dahlias that will be the envy of your neighborhood. How can you resist?