When should I plant dahlias? Find out the best time to plant the tubers and how to care for your dahlias throughout the growing season.
For a quick overview for beginners, also see 7 Best Tips for Growing Dahlias (Year After Year).
Dahlia Growing Tips
It’s hard to resist the beauty of flowering dahlias. And, with so many colors, flower formations, and sizes, most gardeners grow a lot of them.
These tips walk you through everything starting with how to plant your dahlia tubers in early spring.
You will also find tips for tagging your plants to keep track of each variety, container growing, pinching and grooming, watering, and fertilizing.
At the end of the growing season, use these tips on How to Store Dahlias For the Winter to save the tubers and use them again next year.
Dahlia | Genus: Dahlia | Asteraceae family
Dahlia Growing Tips
Tuberous herbaceous perennial native to Mexico and Central America
• 42 species | 57k cultivars
• USDA Hardiness Zone: 8 up
Grow with winter storage: zone 2 up
• Sun: 6+ hours full morning sun
• Soil: pH 6.5, does not like damp or dry
• Start indoors: 4 to 6 weeks before last frost
• Plant outdoors: after last frost, soil 60°F (16°C)
• Propagation: seed, tubers, cuttings, grafting
• Flowers: mid-summer through to fall frosts
• Cold climates: store tubers for winter at 40-45°F (4-7°C)
• Pollinators: mainly bees and beetles
• Larval host plant: various Lepidoptera, specifically moths
Shop Online: Buy Dahlias at Burpees (US Shipping)
More Dahlia Growing Tips
- 7 Best Tips for Growing Dahlias (Year After Year)
- How to Store Dahlias for the Winter (Overwintering)
1How Do Dahlias Grow?
Once you know how dahlias grow, it is much easier to understand how to plant and care for them.
Here is a quick overview.
You can grow dahlias at home from seed, tubers, or cuttings.
The most common and quickest way to get started is growing from tubers (also known as root tubers or tuberous roots).
These are similar in appearance to sweet potatoes and get planted in spring. You will see them for sale in stores in winter and early spring.
When you buy tubers there might be one tuber total or a cluster of them growing together. Either way is fine.
Dahlias grow their main stem from the ‘eye’ of the tuber. Unlike potatoes which form eyes in several locations, these eyes only form on the crown (located on the ‘neck’ of the tuber).
It’s the main stem that produces leaves and lateral shoots which in turn form buds which become blooms.
While dahlias are perennial, they are not winter hardy below USDA zone 8. But exposure to some cold is necessary for their development each year so we wait until early frosts have blackened the foliage before digging up the tubers and storing them for the winter.
Come spring, they can be started a few weeks early indoors or planted directly outdoors after last frost.
2How Do I Know a Dahlia Tuber is Good?
Beware of Mush and Mold
If shopping in-person, have a look at the tuber (if you can) before purchasing.
Otherwise, look it over when it arrives in the mail.
Always have clean hands and handle with care.
You want to be sure the tuber is disease-free and firm like a good sweet potato, not shriveled or squishy.
You may also notice the eye which grows on the crown located on the neck. A tuber without an eye can’t grow a stem or flowers.
Check the plant tag for any special planting needs and what the mature dahlia will be like. Flowers size and height can vary dramatically from one-foot to six-feet tall.
The fine print may also say whether or not that particular variety is a good candidate for winter storage. Some dahlias are best just as annuals—one and done. Others can keep producing flowers for years when stored properly.
3How to Tag and Track Dahlias
Because we can save the tubers each fall for growing again the following spring, it’s smart to have a way to track the variety so you’ll know what you’re planting. All the tubers look alike when dormant.
There are tens of thousands of varieties and tremendous range of flower shapes, colors, sizes, and heights.
Some varieties like single or collarettes do not even look like dahlias. They are still beautiful but often mistaken for other flowers.
You’ll want to know what you’re planting each year so you can provide the best growing conditions and put that particular dahlia where it will look just right.
Marking Pen on the Tuber
Before planting, write the name or a code for the name (remember to write down what your code means) directly on the dry tuber with a permanent marking pen. This is an easy tracking method while in storage. The marker will not harm it but it may fade or distort as the tuber grows.
Also have corresponding plant tags or markers to keep with your dahlias as they grow. I attach mine to the support stakes. For extra insurance you can also bury a plastic name tag with the tuber and retrieve it in fall.
4How to Plant Dahlias
When to Plant Dahlias
Start dahlia tubers indoors 4 to 6 weeks before last frost or wait until the risk of frost has passed and plant directly outdoors. The longer it takes to flower (check your plant tag), the earlier you can start it.
Starting Dahlias Indoors
The advantage to starting your tubers indoors is you get a jump start on the growing season and may see flowers up to a month earlier than you would if you planted directly outdoors later.
Approximately 4 to 6 weeks before last frost, plant your tubers in sterile potting mix in the largest containers you can manage. Start the ones that take longest to flower the earliest.
Choose a container large enough for the tuber to fit laying flat and deep enough to be able to plant it 6-inches deep. If I have room, I use larger pots and may continue container growing outdoors as well.
If your plant tag has other instructions, follow those ones.
After planting, provide full sunlight or place 6-inches below grow lights and water sparingly—but never let it dry out. To start growing, there must be moisture in the potting mix.
After last frost, gradually introduce your dahlias to life outdoors (harden them off). You can continue growing dwarf varieties in pots or transplant the tubers to the garden.
Will Dahlias Bloom Indoors?
As a one-season annual, maybe. And it could be a long flowering season before it finishes.
I could not find an example online but theoretically you should be able to plant your tuber in a pot and, with enough light and consistent moisture, get it to bloom indoors. And the keyword is light: it takes a lot to get them going.
So, assuming you got the dahlia to flower indoors, it will still need exposure to fall frosts eventually to prepare for winter storage, so that will require sorting out.
If you have tried this or know some examples, send me an email. It’s definitely worth experimenting.
Planting Dahlias Outdoors
It’s outdoor planting time after last frost in spring or early summer when the soil temperature is consistently 60°F (16°C) over several days.
A handy rule is, if it’s the right time to plant tomatoes, it’s the right time to plant dahlias.
Ideally, choose a location with full morning sun and part-shade in the afternoon. This prevents the hot afternoon sun from potentially drying out the tubers.
The soil should be rich and well-draining.
Dahlias should be placed approximately 1 to 3 feet apart, depending on the cultivar (check your label for instructions) and avoid placing them in competition with other plant roots nearby.
Most instructions recommend planting the tuber around 6-inches deep, laid flat, with the eye or shoots aiming up. Make the planting hole at least 12-inches in diameter and (usually) six-inches deep.
If your dahlia already has a shoot and leaves, remove the lowest set of leaves (the cotyledon or embryonic leaves) and plant so the first true leaves are just above soil level.
If your soil is already moist and you get regular rainfall, avoid the impulse to water your newly planted tubers. It’s during this stage that the tubers tend to rot if over-watered.
Later on, as the plant gets big and flower form, you will water more often.
Because dahlias need support—the flowers are very top-heavy—place the taller varieties behind smaller plants in your garden beds so you can enjoy the flowers without seeing the stakes.
Always put your stakes (or whatever supports you are using) in place at planting time.
Use any very sturdy metal like rebar or pipes or wood rods or posts tall enough for the mature height of the dahlia plus a foot to submerge in the ground or container.
Position the stake right beside the eye on the tuber (at planting time). This way the main stem will grow next to the stake and you don’t run the risk of puncturing the tuber by adding the support later.
If you are growing masses of dahlias in a bed, look into horizontal mesh netting options.
As the dahlia grows you will add twine loops, loose around the plant, tight around the stake, to support it every foot or so.
5Growing Dahlias in Containers
As mentioned, dahlias can be started in containers indoors in early spring or planted directly in containers outdoors after last frost.
Some advice says to only choose shorter dwarf or miniature varieties because the container may otherwise stunt its growth.
Personally, I have grown really large dahlias (5-feet tall with big flowers) in 2-gallon pots without any problem. I’m sure success will vary depending on the specific variety and growing conditions.
Using containers makes it very easy to dig up the tubers at the end of the season for winter storage. Plus, I like being able to move the plants around to avoid heat waves and rain storms that may damage the plants.
The goal is even moisture. We don’t want the soil so dry that the tubers shrivel up, but neither do we want it so moist that they rot.
Your watering routine will depend on your growing conditions and weather.
If you get really hot afternoons like we do, mulch around the base (without the mulch touching the stem) to retain moisture and water as needed.
Dahlias need more water as they grow.
Some avid growers use drip lines or tapes to get water to the roots. Soaker hoses may only penetrate the first inch or two of soil.
7How Long Do Dahlias Take to Flower?
Dahlias need 90 to 120 days to flower
Flowering time depends on the variety. Some take approximately 90 days (3 months) from your planting date. Other larger cultivars may take as long as four months or 120 days.
Check your product package. This may determine whether it’s worthwhile starting them early indoors.
8Pinching, Grooming, and Topping
I grew dahlias for years without any grooming, but, as you gain experience, you may want to ‘pinch’ (remove the top of) the main stem for a better outcome.
The purpose of topping or pinching is to create a bushier plant by removing the upper section of the main stem from the tip down to a leaf set, which forces the dahlia to branch out more.
This is done once in the season and some experienced growers say the sooner the better.
If you are really keen, look up your specific variety for instructions, otherwise, wait until there are between 2 to 4 sets of true leaves and the plant is at least a foot tall. The lowest set of leaves doesn’t count because they are cotyledons (embryonic leaves not true leaves).
That may be all the grooming you want to do.
Disbudding is a little more advanced method for growers wanting the biggest and the best flowers.
On each plant, dahlias produce a number of buds that can become flowers. You can remove all but one bud on each stem for greater but fewer flowers.
Disbranching is just what it sounds like: removing entire stems (branches, not the main stem). It’s the opposite of topping or pinching really because instead of encouraging side growth we’re halting it entirely.
Whatever you do, always wash your hands, and disinfect your gear before and after handling each plant. Dahlias are susceptible to various transmittable diseases.
9Should I Fertilize My Dahlias?
This may come down to how environmentally-conscious you wish to be. The quest to produce show-worthy flowers tends to ignore the environmental impact of our choices. A home grower may be very happy with their dahlias without extra care.
There are successful dahlia growers who swear by their fertilizers (the right one at the right time) and those who don’t find it necessary.
Personally, I’m just a casual grower and I just rely on good old compost in my soil for nutrients, avoiding synthetic fertilizers and never tempted by commercial organic ones.
Those who do fertilize have different timings: some say to do the first application a month before planting, others say at planting time and then a month later.
All agree you should avoid high nitrogen, otherwise your dahlia will be all foliage and no flowers.
For specific N-P-K amounts, I’ve seen water soluble or granular fertilizers with 3-5-5 and 5-10-10 recommended.
Dahlias are heavy feeders so do investigate this further if you think your soil or potting mix doesn’t have what it takes.
As mentioned, dahlias can be propagated by seed, tubers, or cuttings. Grafting is also possible.
Growing from tubers (as described here) is the most popular method.
You can also raise your tubers just for cuttings, to get many plants from one, or take cuttings from your dahlias during the growing season.
Growing from cuttings is quite economical, sometimes generating as many as five new plants from each tuber.
If you want to root dahlia cuttings from an existing plant, the method is the same as the softwood cutting technique shown here.
If growing from seed, follow the instructions on the seed packet.
And don’t forget to divide your tubers when they multiply: so long a there is an eye on each section, you can grow a new dahlia. This can be done when you dig them up for winter storage.
11Dahlia Pests and Diseases
- Watch for slugs and snails, especially around the young plants.
- Earwigs disfigure the blooms.
- Aphids go after the stems and immature flower buds.
- Red spider mites dine on the foliage.
- Capsid bugs create holes on the stem tips.
On a brighter note, while not entirely deer-proof, dahlias are not their first food choice when other plants are available.
Possible diseases include powdery mildew, grey mould (Botrytis cinereal), verticillium wilt, dahlia smut (Entyloma calendulae f. Dahliae), phytopthora, and other plant viruses.
There is a list here on Wikipedia: Dahlia Diseases
~Melissa the Empress of Dirt ♛
How to Plant Dahlias
Supplies & Materials
- 1 Dahlia tuber
Starting Dahlia Tubers Indoors for Planting Outdoors
- Four to six weeks before last frost, choose flower pot wide enough to fit tuber horizontally.
- Add just enough moistened potting mix to lay tuber in pot six inches deep (or at depth recommended on your plant tag).
- Cover with moistened potting mix up to one inch below lip of pot.
- Place in full sun or six-inches below grow lights.
- Water sparingly but never allow to dry out.
- After last frost, gradually transition dahlia to life outdoors over a two-week period.
- Transplant into ground when soil temperature is consistently 60°F (16°C) minimum or continue growing in pot (best only for dwarf varieties) in sunny location.
- Add support rod at transplant time if dahlia variety is known to be top-heavy.