How to Grow Dahlias

Dahlias are known for their vibrant colors and magnificent blooms. Are you raising it correctly?

Flowers are revered for their beauty, but dahlias are in a class of their own. When gazing upon a bloom, you can’t help but be mesmerized by its brilliant colors and exquisite petal arrangement. Their tropical beauty stems from the intricate, geometric-like pattern that looks too perfect for forming naturally.

With some basic care, you can fill your yard with a bounty of these stunning flowers, creating a picture-worthy landscape to enjoy.

Why Plant Dahlias?

Without a doubt, the best reason to plant dahlias in your garden is because of their wow factor. You can fill your yard with awe-inspiring blooms ranging from 2 inches in diameter to the size of dinner plates. 

Dahlias are available in thousands of varieties—over 57,000 worldwide—in almost any color, size, and floral form you can imagine. 

The Many Different Types Of Dahlias

The sheer amount of dahlia cultivars registered through the Royal Horticultural Society is mind-boggling at more than 57,000 (the Global Biodiversity Information Facility can back this number up). With just 42 different species in the genus, this explosion of hybrids is possible because, unlike most other plants with two sets of homologous chromosomes, dahlias have eight.

An assortment of colorful dahlias in a garden

Plants Are Classified Into Official Dahlia Divisions

In an attempt to organize the massive number of cultivars, numerous associations have arranged them into groups by flower characteristics. But even this isn’t very clear because international approaches are different, and there are sub-divisions within the groups. One of the most commonly recognized systems is the UK’s National Dahlia Society (NDS) Classification with fourteen different dahlia groups or divisions.

Group 1: Single-flowered Dahlias
Group 2: Anemone-flowered Dahlias
Group 3: Collerette Dahlias
Group 4: Waterlily Dahlias
Group 5: Formal Decorative Dahlias
Group 6: Ball Dahlias
Group 7: Pompon Dahlias
Group 8: Cactus Dahlias
Group 9: Semi-Cactus Dahlias
Group 10: Miscellaneous Dahlias
Group 11: Fimbriated Dahlias
Group 12: Star Dahlias
Group 13: Double Orchid Dahlias
Group 14: Paeony Dahlias

Do Dahlias Come Back Every Year?

Dahlias are classified as tender perennials, which is code for “it’s complicated in gardening terms.” In most areas in zones 8, 9, and 10, you can leave them in the ground over winter. But in areas colder than zone 8, you should dig dahlia tubers in the fall and store them indoors, protecting them from freezing temperatures and preventing rot.

How To Find Your USDA Plant Hardiness Zone

Are you unsure of your growing zone? If you don’t know your zone—or have forgotten it—use the USDA Hardiness Zone Finder to determine what number you’re in, so you can figure out if you can overwinter tubers in the ground. This map is based on annual minimum winter temperature, separating the United States into 10°F zones.

How Long Do Dahlias Bloom?

One of the great things about dahlias is they bloom for a long time. Once you plant tubers in the ground, plants take about two months to begin blooming. For most USDA hardiness zones, this means you’ll see flowers in late June or early July. Once flowers start opening, they bloom for about a lengthy 4-month stretch until a hard frost.

Planting Dahlia Tubers VS Dahlia Seedsvs

Your second decision, behind choosing from the many, many, many dahlia cultivars, is determining how to start plants. Many gardeners opt to buy tubers to plant, but you can purchase seeds. Seeds germinate well, making them a viable choice, but it’s essential to understand a significant difference between the two methods.

  • Using dahlia tubers to start plants is a method of asexual propagation. The sprouted plants are identical clones, and you’ll know exactly what flowers will grow.
  • Using seeds to start dahlias employs sexual propagation. The seeds get their genetic makeup from the two flower parents, and no two plants will be 100% identical, so you don’t know exactly what flowers will sprout and grow.
The tubers of a dahlia plant exposed on a soil before planting

What Time Of Year Should I Plant Dahlias Outside?

Exact planting dates vary from one growing zone to another, but the recommending timing is always after the danger of frost has passed and the soil temperature reaches at least 60°F. It is tempting to plant earlier, but tubers do not like cool, moist soil. The climate in most areas is suitable in mid-April to May for planting.

Determining Your Local Frost Free Day

Frost dates are a little more challenging to calculate than growing zones. Use The Old Farmer’s Almanac First and Last Frost Dates Calculator if you’re unsure of yours. A frost-free date is the average date of the last light freeze (29° to 32°F) in spring and is based on historical climate data. These dates are only estimates and not a guarantee.

Start With Good Soil For Beautiful Dahlias

Getting your plants to produce beautiful flowers starts with well-draining, fertile soil. Plants like slightly acidic (6.3 – 6.8) soils and plenty of nutrients. A soil test helps check the pH level and determine its fertility. To improve the soil, work 3-4″ of finished compost into the soil, loosening the top 12″ of ground.

Starting Dahlias From Seeds Two Different Ways

If you’re looking to add many plants to your garden, and don’t mind various (unexpected) flowers, starting dahlia seeds is the way to go since seeds are much cheaper than tubers. Start seeds indoors 6 to 8 weeks before the last frost date by directly sowing them in potting soil or germinating seeds in paper towels.

Option 1: Direct Sowing Seeds In Seed Trays

  1. Fill seed starting trays or containers with premoistened potting soil.
  2. Using a dowel, poke a small hole in the center of each cell or container.
  3. Plant one seed in each hole, about ¼” deep, gently covering the seed with potting mix.
  4. Bottom water the containers or mist them with a spray bottle, and cover with the clear plastic dome or a clear zip-top bag.
  5. Place the seeds on a seed starting mat or someplace warm like on top of the refrigerator.

Option 2: Germinating Seeds in Damp Paper Towels

  1. Dampen two paper towels and lay one flat on a hard surface.
  2. Spread seeds across the paper towel and cover them with the other piece.
  3. Put the paper towel sandwich inside a zip-top bag.
  4. Place the bag on a seed starting mat or on top of your refrigerator where it’s warm.
  5. After seeds germinate, plant them in containers or a seed tray.

Potting Seedlings Up To Prevent Rootbound Plants

After a couple of weeks, your seedlings will develop true leaves, and the roots will begin growing vigorously. To keep the plants from becoming rootbound, you will want to move them from the seed starting trays into larger containers. Continue raising the seedlings indoors until the frost danger has passed and you can safely move your dahlias outside.

A young dahlia seedling planted on a small black tin can

Preparing Your Dahlia Seedlings To Grow Outdoors

Before moving your seedlings outside to the garden, they need to be acclimated to the outdoor conditions. If you plant them without adjusting them, there is a high chance they’ll experience transplant shock because of the difference in light and temperature. So, a week or ten days before transplanting, start moving them outdoors for periods of time. 

This acclimation process is known as “hardening off.”

Set the seedlings outside in a sunny spot protected from strong winds. After a few hours, bring them inside. Gradually increase their time outside every day until it’s time to transplant them outdoors. If the weather is temperate, you can leave them outside for a night or two.

Transplanting Seedlings To The Garden

Dahlias need a little bit of elbow room when you move them to the garden. Plant seedlings 12″ apart in the flowerbed or garden, with 18″ between the rows. Set plants in their planting holes so the potting soil is even with the soil surface. Try to avoid disturbing the roots.

Tips On Planting Dahlia Tubers

Should I Soak Tubers Before Planting?

There are mixed opinions on soaking dahlia tubers before planting. They only need a little moisture to break dormancy and start active growth, so soil moisture should provide enough. But soaking can speed up sprouting, but it does increase susceptibility to rot. If you choose to soak them, leave them in tepid water for no more than an hour.

How Far Apart To Space Tubers

You should plant tubers the same as your seedlings for most varieties, with 12″ between plants and 18″ between the different rows. This spacing allows for good flower and foliage growth, plus lets air circulate freely through the plants. When plants are too close together and airflow is minimal, you’ll see more problems with pests and diseases. 

How Deep To Plant Dahlia Tubers

The ideal planting depth for tubers is about 4 to 6″ beneath the soil surface. If you are planting many at once, it may be easiest to dig a trench (or trenches) instead of individual holes. Lay each tuber on its side in the bottom of the hole with the eye facing upwards towards the sky.

A dahlia plant with its tubers exposed on the ground

How To Keep Your Dahlias Beautiful All Season

Watering Dahlias While Preventing Tuber Rot

Watering dahlias is a little different than other plants. After planting, most soils have enough moisture that you can skip watering until plants develop their first true leaves. Waiting helps minimize tuber rot. Then water the soil deeply, less frequently to keep the soil damp but not soggy. If possible, water the ground around plants, keeping moisture off the foliage.

Giving Dahlias Their Required Nutrients With Fertilizer

Because they are forming large potato-like tubers, plants respond well to fertilizer applications and, in turn, grow larger tubers and produce bigger flowers. Starting 30 days after planting, feed plants monthly with a high-quality, water-soluble plant food low in nitrogen yet high in phosphorus and potassium to promote blooms over vegetative growth. You can also fertilize every 14 days at half-strength.

Pro-Actively Support Your Plants With Stakes

Dahlia plants appreciate a little extra support, especially if you’re growing varieties with dinner-plant flowers. The stems are somewhat brittle, and heavy flowers, wind, and rain can cause them to snap. It’s best to be proactive and stake plants early on to minimize losing your big beautiful blooms later in the season.

The easiest time to stake plants is when you plant the tubers. Set one or two wooden or metal stakes within a couple of inches of each tuber, making sure they are hammered in deep enough they don’t wobble. As plants grow, secure the stems to the stakes. The foliage should hide the supports as leaves develop.

Blooming orange dahlias in a very healthy garden during autumn season

When, How, & Why To Pinch Dahlias

You’ve likely heard that pinching plants back helps improve growth but may not understand why it works. Plants grow from a terminal bud on the top of the central stem. Removing this central bud triggers lateral plant growth instead of vertical, making your plant bushier and encouraging more bud development. 

You can pinch your plants back when they are 12-18″ tall with four sets of leaves on the main stem. Locate the center stem on the plant and, using either your fingernails or disinfected plant shears, remove the top 3-4″ of the stem, down to the next set of leaves. So after cutting, you have three sets of leaves remaining. 

Controlling Weeds For Vigorous Plant Growth

Keeping the area around your dahlias free of weeds not only minimizes the competition for water and nutrients, but a weed-free zone also keeps pests and disease problems down. Gently weed around plants with a three-tine cultivator hand pull pesky weeds to avoid damaging the tubers or roots. You can also spread mulch around plants to prevent weed seed germination.

Keeping Your Dahlias Pest Free

In a perfect world, pests wouldn’t both any of your beautiful flowers. Unfortunately, though, this isn’t a perfect world, and there are numerous gardens pests attracted to dahlias. Continuously scout your plants, watching for slugs, snails, caterpillars, and insects, including earwigs, aphids, spider mites, thrips, potato leafhoppers, and tarnished plant bugs. 

Create Barriers Around Plants

One of the best ways to keep pests away from your plants is to create a barrier they won’t want to cross to reach your dahlias. Companion plants with intense aromatics—marigolds, thyme, coriander, anise—naturally ward off insects pests. And sprinkling diatomaceous earth or crushed eggshells on the soil keeps snails, slugs, and caterpillars at bay.

Use Good Insects for Pest Control

Another great natural way to control nuisance pests is by releasing beneficial insects in your garden. These helpful bugs feed on the bad ones, deterring them from moving in, and keeping numbers down if they infest the garden. Some of the best beneficial insects are ladybugs, beneficial thrips, green lacewings, and minute pirate bugs.

A full grown red dahlia under the heat of the sun with a fly and bug

Fight Pests With All-Natural Neem Oil

If you end up with insect pests, neem oil is excellent, organic pest control. It is a naturally-occurring pesticide made from the seeds of a neem tree. Just keep in mind that neem oil affects all flying insects, including bees, so spray neem oil pre-dawn or at dusk when honeybees aren’t active. 

Diseases To Watch For and How To Treat Them 

Powdery mildew, gray mold, leafy gall, botrytis, verticillium wilt, and bacterial stem rot are common problems in dahlia plantings. Prevent problems—which is much easier than treating diseases—by keeping good spacing between plants and always using disinfected gardening tools. If you observe disease symptoms, treat your plants immediately with an appropriate fungicide to prevent spreading. 

What is Deadheading And Why It’s Important

Deadheading is similar to pinching plants back in that it removes flower buds. But in this case, it removes spent blooms to encourage new growth and new bud development on your plants. As flowers die, cut them off with disinfected shears or pinch them back to the next set of leaves with your fingernails.

A person cutting a dried dahlia using a small plant trimmer

Digging Up Dahlia Tubers For The Winter

If you live in one of the colder hardiness zones, you must dig up your tubers for the winter and store them someplace where they can stay dormant until planting time the following spring. But even in zones 8 and warm, it may be beneficial to lift them and store them through the winter to keep them from rotting in cool, wet soil. 

When Should I Dig Up Dahlia Tubers?

The goal with dahlias is to enjoy flowers as long as possible but get them out of the ground before the tubers rot. In areas that experience fall frost, you can wait until that first killing frost to dig them out of the bed. You can dig tubers up in warmer climates once they’ve spent 120 days in the ground.

Cleaning Dahlia Tubers For Winter Storage

After you dig up your plants:

  1. Cut the stems off a few inches above the fleshy, swollen tuber.
  2. Use your garden hose to wash off the excess soil, being careful not to spray so forcefully you break or damage them.
  3. Set them in the fall sun for a couple of days to dry.

How To Properly Store Dahlia Tubers Through The Winter

Now that the tubers are dry, it’s time to put them away for the winter. Ideally, your want to store them in a cool, dark spot (40-50°F), keeping them dry until spring. You can either put them in some peat moss or sawdust in a paper bag or pack them eye-side up in a bin layered with vermiculite or peat moss.

Divide Your Tubers To Multiply Your Flower Stock

Regardless of whether you started plants from seed or planted tubers, the mother tuber produces up to twenty new tubers each year. Buying new tubers is expensive, but dividing your plants is an inexpensive way to increase your plant numbers. Before planting tubers in the spring, break them into clumps with at least three eyes, and then plant each cluster like normal.

Carley Miller
Carley Miller is a horticultural expert at Bustling Nest. She previously owned a landscaping business for 25 years and worked at a local garden center for 10 years.
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