How to Plant and Grow Dahlia Tubers (Tips for Earlier Blooms)

Learn how to plant and grow dahlia tubers in your cut flower garden. I’m also sharing simple tricks to get dahlias blooming weeks sooner.

Dahlias are known for their gorgeous, showy blooms and productivity in the garden.

In the summer, I can hardly wait for my dahlias to start blooming, and once they do, nearly every bouquet includes some of these beauties.

While dahlias are easy to grow, they do require special care to get plants that flourish until last frost.

Growing dahlias is definitely a labor of love, but it’s work that’s worth doing.

This dahlia growing guide will walk you through timing, planting depth, spacing, staking, and caring for your plants through the growing season.

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What Is a Dahlia Tuber?

A dahlia tuber is the underground storage organ that fuels the plant’s growth each season.

It looks kind of like a brown sweet potato, attached to a central crown at the top of the clump.

The crown is where the eyes (the small buds that’ll sprout new stems) are located.

Without a viable eye attached to the tuber, the dahlia won’t sprout. This is why identifying the eyes matters so much when dividing tubers.

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Additionally, if the neck (the part that connects the eyes to the tuber) is broken, the tuber also won’t sprout. Sometimes I wonder how my dahlias manage to survive all the manhandling I subject them to!

Dahlias are not true bulbs, even though you’ll find “dahlia bulbs” and “dahlia tubers” used interchangeably online and in casual conversation. I’m sticking to “dahlia tubers” for this post since that is the botanically correct term.

How to Identify a Healthy Dahlia Tuber

When you’re overwintering your own dahlias, you can pack them securely, but if you’re buying them from a big box store like Costco or having them shipped to your home, you’ll have to inspect them carefully to determine if the tuber is healthy enough to grow.

Signs of a good tuber include:

• Firmness and plumpness: A healthy tuber feels solid, like a fresh potato. It shouldn’t be mushy or lightweight.
• A visible eye: The eye is the growth point at the top of the neck (see photos). It may look like a small bump or faint reddish or greenish speck. If you don’t immediately see an eye, not all is lost. Give the tuber a few weeks in warmth and bright light before planting to be sure it has a viable eye.
• An intact neck: The neck is where the eyes meet the largest part of the tuber. It doesn’t matter how healthy a tuber looks. If the neck is broken, the dahlia will not sprout.
• No rot or mold: Scarring that has healed over is fine, as well as dark spots that are still firm. But you don’t want soft, mushy spots, white or gray mold, or a bad smell.

Learn to dig and store your dahlia tubers over the winter in this post.

It’s important to note that healthy dahlia tubers come in a variety of shapes and sizes ranging from as small as a double-A battery to as large as a grocery-store sweet potato.

Size doesn’t affect vigor. A small tuber will grow just as well as the large ones.

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What About Shriveled Tubers?

Sometimes tubers left in storage without adequate humidity will get shriveled. A little bit of shriveling isn’t going to keep the tuber from sprouting, however. As long as it still feels firm to touch and has a viable eye, it’ll likely grow once it’s been planted and watered. A tuber that feels hollow or papery, though, probably is a lost cause.

If you’re not sure, plant it in a pot. If it grows, that’s great! If not, you’re just out a pot and handful of soil. One thing’s for sure though, it defintely won’t grow if it’s tossed in the trash.

When to Plant Dahlia Tubers

Timing is the part that most beginners wonder about. When do I plant dahlias?

Dahlias are frost-sensitive. They shouldn’t go into the ground if you might get freezing temperatures, so it’s best to wait until your last frost date has passed before burying tubers in the ground.

But the earlier you get them growing, the earlier you’ll have blooms.

I’m sharing two options: potting up tubers indoors before last frost or planting directly into the ground once the soil has warmed.

Option 1: Pot Up Indoors for Early Blooms

Starting dahlia tubers in pots gives you a month or more head start on blooming season. Here in zone 6, I pot mine up around mid-March, which is 6 weeks before my last frost date of April 25.

I keep them in my seed starting room under grow lights and allow them to grow into small plants before hardening off and transplanting outside when my last frost date has passed.

Hardening off is the process of slowly exposing indoor-grown plants to outdoor conditions to acclimate them.

These potted dahlias typically begin blooming as early as July, over a month before tubers planted directly in the ground.

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One more bonus of potting up dahlias indoors is that you can easily make more plants by taking cuttings. Read my post on how to propagate dahlias from cuttings if you want to learn how.

Option 2: Bury Tubers Outdoors After Last Frost

If you don’t have space to grow tubers indoors, plant them directly in the ground once your last frost date has passed and the soil is warm and dry.

Soil moisture matters a lot when it comes to planting dahlias. Dahlias are very prone to rot with too much rain or water. Clay soil (like mine) is notorious for holding on to water and rotting tubers, but waiting until air and soil temperatures are warm helps the tubers sprout quickly so rotting doesn’t become an issue.

Dahlias planted after my last frost date (April 25) tend to bloom in mid- to late August all the way until first frost. It’s a shorter season, but it does give me something to look forward to in late summer!

When to Plant Dahlias by Zone

Planting times vary depending on where you live. I made this chart showing USDA zones 3-7, when to pot up tubers (6 weeks before last frost), and generally when it’s safe to plant outdoors.

Need help finding your zone or frost dates? Read my post on hardiness zones here.

What About Growing Dahlias from Seed?

You can absolutely grow dahlias from seed. The main difference between seed-grown dahlias and growing dahlias from tubers is: tubers produce the same types of flowers year after year, while seed-grown dahlias produce unpredictably different flowers.

Growing dahlias from seed can be an economical way to fill your garden with dahlias, but the trade-off is possibly getting “ugly” flowers.

I’ll cover this topic more in depth in a separate post.

How to Plant Dahlia Tubers

Once you’ve passed your last frost date, you can plant your dahlias outdoors.

Dahlias need three things to grow well: full sun, well-draining soil, regular maintenance.

Provide them with these three things and they’ll produce blooms until frost. Here’s how.

Choose the Right Location

Look for a spot in your garden that gets full sun or 6-8 hours of direct sunlight per day. Avoid low spots where water pools as the tubers will be more likely to rot. Provide dahlias with rich, well-draining soil.

Not sure if the location is a “well draining”? Try these:

1. Look at the garden after it rains. If you have standing water, do not plant dahlias there.
2. Pick up a handful of soil from your garden and squeeze. If it falls apart when you open your hand, it likely will drain well. If it holds shape like a lump of clay, then you may have poorly draining soil.
3. inspect your soil. The top layer should feel loose, be easy to poke a shovel through, and smell like dirt.

Take a look at this post for more on finding the right location for your garden.

Prepare the Soil

Remove weeds, rocks, and debris from the soil.

Work compost and slow-release all-purpose fertilizer into your garden bed. If you have limited quanitites and a large garden, you can simply bury handfuls of each in your planting hole.

Step-by-Step Planting Instructions for Dahlia Tubers

Check your weather forecast and ensure the next few days are rain-storm-free. Then plant your dahlias.

1. Space tubers about 12-18 inches apart. I like to lay them out first before digging so I can make sure I have room for everyone.
2. Dig a hole wide enough to fit the tuber or clump.
3. Place the tuber or clump with the eyes at the top. For single tubers, it’s ok to lay it on its side. Backfill with soil, covering the tuber completely, and gently tamp the soil down.
4. DO NOT water tubers or clumps unless your soil is very dry. This reduces the risk of tubers rotting before they’ve sprouted. Tubers have plenty of stored energy to sprout on their own without extra water.
5. Mark the spot so you don’t forget where tubers are planted.

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Starting Dahlia Tubers in Pots Indoors

If you want to have earlier blooms, pot dahlia tubers up indoors about six weeks before your last frost date.

You wil need:

• Pot large enough to fit each tuber
• Light potting mix
• A warm, bright location
• Labels (dahlias look nearly identical until blooming)

Step-by-Step Instructions for Potting Dahlias

1. Fill the pot half way with potting mix.
2. Place the tuber on top with the eyes facing up. Eyes should be level with or below the pot rim.
3. Bury the tuber with potting mix and water lightly.
4. Label the pot with the variety name.
5. DO NOT water again until you see sprouts emerging, typically within 2 weeks.
6. Keep the potted tubers in a warm spot with bright light. I keep them with my seed starts under grow lights.

Hardening off and Transplanting Outdoors

Harden off indoor-grown dahlias a week before planting out. To do this, set them outside in a sheltered spot in full sun for an hour the first day, then increasing by an hour or so each subsequent day until the end of the week. They should be ready for a full day of sun by then.

Space tubers 12-18 inches apart in your prepped garden bed. Dig a hole slightly larger than your pot.

Gently remove the plant from the pot and place it in the hole. Backfill with soil and tamp down gently. Make sure all plant roots are tucked under the soil and that the plant is buried up to the point where it has sprouted.

Water in the plant.

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Dahlia Plant Care

Watering

Once plants are actively growing (shoots have emerged from the soil), water dahlias deeply 2 to 3 times per week unless the rain does it for you.

Fertilizing

Once plants are a little under knee height, fertilize with an all-purpose fertilizer like Neptune’s Harvest fish and seaweed fertilizer. Feed once a month throughout the growing season. You can switch to a phosphorus-rich fertilizer like Espoma Bloom once plants are blooming if you like, but it’s not necessary.

Pinching for More Blooms

This is one of the best tips for productive dahlias. Once your plant is about knee height (or 12inches tall), snip out the central growing tip just above a leaf node.

This is called pinching. Pinching forces the plant to branch, resulting in more stems and blooms. Pinching low on the plant like this also results in a sturdier plant.

Deadheading

Deadheading, or removing spent blooms, keeps dahlia plants productive. Dahlias will bloom continuously, so the more you cut (or deadhead), the more the plants will bloom. In a cutting garden you’ll likely be harvesting plants for the vase, but if any blooms are left on the plant past their prime, deadhead them immediately.

Disbudding for Larger Blooms

If bloom size is important to you (think enormous dinnerplate dahlias), then you’ll want to disbud dahlias. This is best done when dahlia flowers are still tiny buds.

To do this, find the main or central bud on your plant. Remove the smaller secondary buds completely, allowing the central bed to remain. This redirects the plants energy into developing a larger flower rather than splitting its efforts in to multiple smaller flowers.

This video by Kristine Albrecht, a dahlia breeder, demonstrates disbudding.

Supporting Dahlias: Two Methods

Dahlias need support. Huge heavy blooms are often too much for the plants to keep upright.

A single strong wind can snap the brittle stems if you don’t stake your plants.

I’m a busy gardener, so I prefer these two low-effort support methods.

Option 1: Tomato Cage

Tomato cages are great supports for anything but tomatoes. I use mine to keep dahlias upright in the garden.

Install tomato cages at time of planting, being very careful not to poke the dahlia tubers with the stakes.

Center dahlia tubers right in the middle of the cage so the plant receives support on all sides.

This option is best for single dahlia plants.

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Option 2: The Corral Method

I love using this method because it’s quick to set up, economical, and scales well. No matter if you’re growing 6 plants or 60, you can use this method to support dahlias.

How to Set Up a Dahlia Corral

1. Drive sturdy T-posts into the ground at the corners and sides of your dahlia bed, roughly every 4- 6 feet.
2. Run strong twine around the perimeter of the posts about 18″ off the ground.
3. Pull the twine taught enough to support stems and tie off securely at each post.
4. I also like to criss-cross it over the middle of the garden bed in an X shape to support the interior plants.
5. Add another perimeter of twine 12″ above the first set as plants grow taller.

I use parachute cord for my corrals because its strong, reusable, and holds up through the weather. Baling twine is another alternative.

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Common Dahlia Pests and Problems

Dahlias are strong plants but unfortunately aren’t immune to pests and disease. These are a few pests and problems I’ve dealt with in my own garden and what I did to fix it.

Japanese Beetles

Japanese beetles are a frustrating pest. They arrive in midsummer and chew up whatever they can get their jaws on—including my dahlia flowers. If you see one, be certain that more are on their way. I recommend avoiding Japanese beetle traps. They tend to attract more beetles than they catch and we definitely don’t need any more help attracting beetles.

What to do: Hand pick them. It’s tedious but effective. Go out early in the morning when beetles are sluggish. Hold a cup of soapy water underneath a group of beetles and knock them in. They’ll drown pretty quickly.

Preventative measures: Some growers will bag every dahlia bud with large mesh bags to protect them from all insects. This is highly effective, but it is also labor-intensive.

For long-term organic Japanese beetle control, spread milky spore in the soil in late summer through early fall. It’s a bacteria that kills Japanese beetle grubs.

Cucumber Beetles

Cucumber beetles are small yellow-ish green beetles with black spots or stripes. Similar to Japanese beetles, they chew on flowers and leave holes in your blooms.

What to do: Hand pick cucumber beetles into soapy water as with Japanese beetles. You can also bag unopened buds as mentioned above.

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Aphids

Aphids love sucking on tender new dahlia growth and the undersides of leaves. If you see foliage curling or puckering, or a bunch of black or pale green dots on the growing tips of your plants, you may have aphids. Aphids spread disease and can weaken plants. A small population isn’t usually a big deal, but a heavy infestation may need intervention.

What to do: A strong blast of water from the hose knocks aphids off plants. You can also apply insecticidal soap (follow label instructions) every few days until the aphids are gone.

Note: Ladybugs and parasitic wasps are natural aphid predators. Check for these beneficial adult or larval insects before eliminating aphids (see photo above for black ladybug larvae). Planting one of these amazing companion flowers near your dahlia patch can help support and attract more of these beneficial insects.

Powdery Mildew

Powdery mildew shows up as a white, powdery substance on leaves (hence the name), typically late in the growing season. The good news is: powdery mildew is mostly a cosmetic problem.

If I notice powdery mildew on my dahlias, I typically leave it and the plants do fine until first frost.

Prevent powdery mildew by watering at soil level whenever possible and keeping foliage dry. Encourage good air circulation by spacing dahlias farther apart if powdery mildew is a problem where you live.

For more on treating powdery mildew (it affects zinnias and squash, too), read my post on how to deal with powdery mildew.

Stem Rot and Tuber Rot

Dahlia rot occurs with too much water. Whether that’s from heavy rainfall, poor drainage, or an overeager gardener, overly wet soil makes dahlias susceptible to fungal rots that can keep them from sprouting or continuing to grow if they’ve already sprouted.

Signs of possible rot:

• A tuber that fails to sprout when others do
• A plant that suddenly wilts and collapses
• A dark, mushy stem at soil level
• Soft dark spots on tubers when you dig them up in late fall

Prevention:

• Plant in well-draining soil and avoid low spots
• Don’t water until shoots have emerged
• Wait until spring rains have slowed and soil is warm to touch before planting dahlia tubers

Rotted tubers or stems cannot be saved. Remove and discard rather than composting to avoid spreading the fungus.

Leafy Gall

Leafy gall strikes terror in my heart. OK, I’m exaggerating a bit, but it’s a very serious problem in the dahlia growing industry.

Leafy gall appears like a dense, cauliflower-like mass of stunted, distorted shoots at the base of the plant or crown of a tuber. It looks extremely abnormal, so you’ll recognize it when you see it.

There is no cure for leafy gall and it can spread to your other tubers if you’re not careful.

I’ve never seen it, and I hope you never do, but if you do, here’s what to do:

• Remove the entire plant and its tubers and dispose in the trash.
• Mark the soil as contaminated (in a way you’ll remember) and avoid planting dahlias or other susceptible plants in that spot for a few years.
• Disinfect all gardening tools with bleach or rubbing alcohol.

The best way to avoid bringing it home is to buy tubers from reputable growers. Always disinfect cutting tools between plants, especially when dividing tubers or taking cuttings.

For more on leafy gall and to see more gnarly pics like the one below, check out the Dahlia Doctor.

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Dahlia Tuber FAQs

Ready to Keep Growing Dahlias?

Once you’ve got your dahlias planted and growing, you can enjoy those beautiful blooms… and think about next year.

Dahlias are perennial in zones 8 and above, meaning those of us in lower zones (zone 7 and below) will need to overwinter our tubers for the following year.

Learn to dig and overwinter dahlia tubers in this post.

A single dahlia tuber will produce a full clump of several tubers over the growing season. You’ll be able to divide dahlia tubers to get even more plants in the spring for free.

Or learn how to propagate dahlias from cuttings in this post.

Looking for more on growing cut flowers?

Try these posts next.

• 8 Simple Tips to Make Fresh Cut Flowers Last Longer
• Small Cutting Garden Plan for Homegrown Bouquets
• 21 Best Perennial Flowers and Shrubs for Cut Flower Gardens