How to Plant, Grow, and Care for Dahlias

Vibrant and dramatic in all the right ways, dahlias (dahlia spp.) will put on a show in your garden that’s sure to draw rave reviews. Featuring blooms of pink, red, orange, yellow, and everything in between, these stunning tubers vary wildly in size and form, with some varieties boasting flower heads as large as dinner plates.

If you’ve decided you’d like to grow dahlias this season, you’ve made an excellent choice for your garden. But these sun & heat loving flowers don’t come without their share of issues. They can be picky about soil type, and love to have their space. Making sure you are prepared for all aspects of their growth is crucial.

So, if you’ve already decided to order tubers from your favorite catalog, you’ve come to the right place. In this guide, we walk through everything you need to know about dahlias and their care. Let’s dig in!

About Dahlias

Blooming steadily from mid-summer to late-fall, dahlias deliver an extra-long burst of color at a time in the season when many of their early summer peers are beginning to fade. They are commonly grown as a winter-hardy perennial in zones 8-11.

There is some wiggle room in zone 7. They are typically treated as summer annuals in climates that get frost (zones 3-7). But their bulbs can be dug up easily, stored for winter, and replanted in spring. So, they’re a much more economical garden splurge than your typical one-season annual.

With 42 different species and thousands of cultivars, dahlias are a member of the Asteraceae (aster) family. they share many characteristics with their sunflower, chrysanthemum, and zinnia cousins.

Their petals may be toothy, and multi-layered or flat and daisy-like in form. Plants range in height from 10 inches to 7 feet, and their stems may be bushy or singular. This genus needs plenty of sunshine, well-drained soil, considerable maintenance, and regular watering to keep things popping all season long.

Mostly odorless and completely edible (as long as they have not been treated with chemicals), dahlias are often used to garnish salads and cakes. They are a cutting garden favorite, a pollinator magnet, a container garden stand-out, and a flower you should really get to know.

History

The flowers we now call dahlias are native to the high plains of Mexico and South America. They were originally named acocotli, which means ‘water stem.’ With the ancient genus being larger and tree-like in form, the plant’s tuberous roots provided a good way to store and transport water during long treks across the Mexican desert. They also provided the Aztecs with food and medicine.

In the late 1700s, some varieties were exported to Spain. They were then featured in the Gardens of Madrid and quickly spread throughout Europe. Renamed the dahlia after Swedish botanist Andreas Dahl, the genus was unique in that its DNA contained 64 chromosomes. This made the plant’s potential for genetic variety far greater than many of its peers.

Belgian botanists produced the first double flower blooms. They were credited with converting the genus from being utilitarian and edible to being ornamental and prized for beauty. Hybridization and experimentation led to an expansion of cultivars and an explosion in popularity due to their diverse color and size.

Said to be favored by Empress Josephine of France and Queen Victoria of England, the dahlia is currently the official flower of both San Francisco and Seattle. It is also still the national flower of Mexico.

Propagation

Dahlias can be propagated in several different ways, delivering more bang for your gardening buck year after year. With a little know-how and some attention to detail, new flowers can be grown from seed, cutting, or division.

Propagation From Seed

Yes, you can harvest seeds from your backyard blooms at the end of the season. You can then use them to grow new dahlias next year. However, it’s important to note that this process will not yield flowers that are identical to the varieties you are currently growing.

Having been carefully selected and cross-bred to produce the showiest blooms, the dahlias you’ve purchased from a nursery or catalog are most likely hybrid plants. This means their seeds will contain DNA from both parent species. In most cases, the new plants grown from your seeds will revert to characteristics possessed by their ancestors.

Additionally, your backyard dahlias will have been cross pollinated by butterflies and bees who’ve visited other plants and species. This means the DNA salad will be tossed even more.

But if you just want to roll the dice and see what happens, seeds can be easily harvested from most dahlia plants with just a few steps.

Propagation From Cuttings

Unlike seed harvesting, which produces a new plant with genetic variety, this method of propagation will produce a clone of the mother flower.

Because new dahlias are propagated from a cut piece of the original plant, they will share DNA and be mostly identical. But this method requires careful manipulation of light, humidity, and temperature to be successful, and that can be tricky for a novice gardener.

In cooler zones (3-7), overwintered tubers can be taken out of hibernation in late winter/early spring and forced into a growth state so their shoots can be harvested. There are several steps to this process:

Division

By far the easiest way to reproduce these flowers is through a process called division. A single plant’s tuberous roots will have compounded and grown dramatically in size as the season progressed. These clumps can now be separated into several pieces and planted directly in the garden.

Like the cutting process, division uses original DNA to produce a clone of the parent flower and your new blooms will be the spitting image of their mommas. Generally speaking, tubers should be divided using the following technique:

Planting

It bears repeating that they should never be planted outside until all danger of frost has passed. Many a flower has withered and died due to an overanxious gardener jumping the gun.  Check your hardiness zone for the average date of your region’s last frost and use that as a strict guideline.

Keeping that in mind, there are several ways to introduce them to your landscape, all with their unique benefits and challenges.

Starting Inside

For larger flowers with earlier blooms, you may want to start the seeding process about six weeks before you can safely plant outside. They typically take at least 100 days to flower, and in colder climates with shorter growing periods, you might not want to wait that long. Fortunately, seeds can be given a jump start inside using a variety of methods.

Paper Towel Starts

Sprouts can be forced between two wet paper towels before being transplanted to seed trays, and some gardeners swear by this method as the one that yields the most seedlings. It’s fairly easy, and it will likely give you elementary school science class flashbacks.

Seed Tray Starts

A paper towel start is not necessary, however, and not always practical when you are starting large batches of seed. If you can afford to lose a sprout here and there, seed trays are just fine for giving them a head start. The procedure is the same for store bought seeds as it was for harvested seeds.

Starting Outside

If you are a dahlia hoarder, you may have dug up and overwintered your tubers in a cool, dark place. When the time is right, bring your old friends into the light, divide if necessary, and get ready for a new season of spectacular blooms.

If you’ve purchased new tubers, pay careful attention to their size and space requirements, and determine the best location for planting. If there are no plants or shoots present on your tubers, look for the eyes and make sure they are pointing up during planting. These are the points from which new flowers will soon erupt.

How to Grow Dahlias

Now that you’re ready to give dahlias a green light in the garden, you’ll want to make sure the conditions are ideal for their healthy growth. Thoughtful consideration must be given to location, soil quality, technique, and care.

If you plant to companion plant with your dahlias, make sure proper plant selection that don’t compete for the same nutrients is taken into consideration.

Planting Depth

As most varieties require staking to maintain perfect posture throughout the growing season, knowing your variety’s mature height is an important first step.

The garden extension of Utah University suggests digging a hole 6-8 inches deep that is wide enough to accommodate both the root chunk and a garden stake or cane inserted six inches away. Roots should then be covered loosely with 2-3 inches of soil. Make sure the dirt over your tubers is not compacted, as this will inhibit new growth.

Once new shoots have breached the surface by six inches or so, backfill your tuber holes so that dirt is now level or slightly mounded around them.

If you are transplanting seedlings, you should dig a hole just deep enough to accommodate their young roots. This can be anywhere from 2-4 inches, depending on how mature your seedlings are. Soil can be slightly more compact and leveled off, as shoots are already breached.

With new sprouts, it might be tempting to hold off on staking until you are more established, but this would be a rookie mistake. Root systems will grow quickly when temperatures warm and inserting stakes at a later point may very well damage the plant’s growth points.

Spacing

A one-foot minimum is recommended for spacing, but again, pay close attention to the variety you are planting. Smaller pompon cultivars may need less room to grow, while dinnerplate darlings may require as much as 18 inches.

Light

Dahlias require abundant sunshine to thrive. Choose a location that will get at least six hours a day, and take care to avoid locations under tree limbs or shadows. If you have to choose between morning sun and afternoon sun, morning is preferable.

Water

Keep moist, but do not overwater during their first few weeks in the ground. They are prone to rot. Beds or pots where they are planted should never dry out but should also never be muddy. After shoots have emerged, it’s safe to assume that roots are established, and your tubers should be soaked more thoroughly from this point forward.

Aim for a total of 1-2 inches of water a week. In dryer climates, this might mean a good, deep watering 2-3 times a week, but gardeners in regions with more rainfall will want to adjust their watering schedules to be more in tune with Mother Nature. In times of drought, they will need supplemental irrigation to maintain consistent moisture.

While overhead watering might be ideal when new shoots are fragile, a soaker or dripline hose is recommended for established plants. Caution should be taken to avoid excess moisture on mature leaves and flower heads, as they are highly susceptible to fungal conditions.

Soil

Dahlias are somewhat particular when it comes to soil, so careful bed preparation is crucial to their healthy growth. They prefer loose, crumbly soil with good aeration and don’t generally do well in clay.

Working organic materials such as hummus, compost, peat moss, or manure into your existing garden soil will go a long way toward giving your dahlias a good start. Shoot for a ratio of 1/3 organic matter to 2/3 soil.

To discourage puddling, garden beds should be mounded slightly to raise the grade. Good drainage should be a top priority for soil preparation.

Dahlias prefer their roots to grow in slightly, but not highly acidic soil. If experience or a soil test reveals your beds to be highly acidic, introduce a product with some lime to bring it down a notch and these finicky flowers may give you a better performance.

Aim for a soil temperature of around 60 degrees before planting or transplanting.

Climate and Temperature

As natives to Mexico and South America, dahlias are partial to high heat and hot sunshine. Pay close attention to the last frost date in your growing zone and do not plant them before the time is right.

In southern climates, the safe date typically occurs in April or May. Some of you northern gardeners will have to wait until June, unfortunately, but the blooms will be worth it!

Dahlias are perennial flowers in parts of the world that do not get frost, generally anything higher than zone 8. In these hardiness zones, dahlias are considered perennial and tubers can be left in the ground year-round.

They will then flower seasonally. In more temperate climates, however, many cultivars are treated like a summer blooming annual, and will not survive the winter if left in place.

Fertilizing

Like most tubers, dahlias feed heavily and will take up nutrients efficiently from the soil. As long as you have amended your dirt with plenty of organic material, they will not need much in the way of supplemental fertilizer.

Newbie shoots may benefit from a little nitrogen while they are developing (about 30 days after planting outside), but generally speaking, they require a fertilizer with more potassium and phosphorous to keep their blooms optimal.

As with most flowering plants, too much nitrogen encourages leaf and stem growth rather than flower production. Fertilizing improperly can cause stunted blooms.

If you feel like your dahlias need a boost as the season progresses, select products that are designed for blooms and contain a ratio of 5-10-10 or the equivalent. In this universal fertilizer sequence, the formula represents nitrogen to phosphorous to potassium, you’ll want the nitrogen component to be roughly half of the other two.

Established plants will likely benefit from light fertilization once a month throughout the season and produce showier, more dramatic flower specimens.

Maintenance

Dahlias require a little more care than many of their flowering relatives, but the results are usually worth the extra effort.

Mulching

Their beds should be regularly mulched to prevent weeds, retain soil moisture, and keep ground temperatures even, but they will not perform well with heavy bark mulch or wood chips. Choose something light like straw, peat or pine needles that will allow shoots easy access to the above ground world.

Aim for a layer that is about 3-4 inches thick. Keep in mind that some mulch materials will break down and compress faster than others. You may want to add a slug or snail deterrent to your mulch, as these pests are known to take up residence in this protective layer.

Staking

All but the tiniest dwarf varieties will require staking to withstand wind, rain, and heavy blooms. At the time of planting, select a cane or stake that is equal in length to your variety’s mature height and drive it firmly into the soil about six inches away from your tuberous root.

Some gardeners use two canes per plant, but that can be an eyeful if you’re working in large quantities. Visit your blooms regularly to monitor new growth (which occurs from the plant’s top) and use garden ties to secure their stems to the stake.

Pinching

When new plants reach a foot in height and possess 3-4 sets of leaves, you may want to pinch or prune off their tops to encourage some side growth.

Dahlias are naturally single stem in nature, but performing this little garden trick early in the season will encourage multiple stems and therefore multiple flower heads.

Use a small pruning tool to cut the tops off just above the third or fourth set of leaves. This process can be repeated to encourage even bushier plants, if so desired.

Deadheading

To keep them bright and cheerful all through the summer and fall, spent blooms must be removed once a week or so. This process encourages roots to direct energy toward creating new blooms rather than spreading seed.

To properly deadhead your dahlias, use your fingers or a small pruning tool to snip off spent blooms just above a set of leaves or branch. Remember that spent blooms are pointed and new buds are rounded. Knowing this can help prevent a catastrophic maintenance error that rids your plants of their burgeoning flower heads!

Overwintering

In moderate zones, they can be cut back to the ground as soon as their leaves have blackened and withered. It’s not a bad idea to pile mulch, leaves, or dirt generously on top of your tubers to give them the warmest and most protected winter environment possible.

If your zone freezes and you’d like to keep your dahlia dreams alive while snow blankets your part of the world, you’ll need to dig up your tubers and prepare them for dormancy. Do this when the leaves have died or right after the first frost. This will be their signal to enter dormancy.

Cut the stems off just above the eyes from which they grew, clean the roots off with a dry towel and lay them out to dry out for a couple of days. Pack your tubers loosely in a basket, box, or burlap bag and cushion with peat moss, newspaper, or sand.

Select a container that is not air-tight, as a little bit of air circulation will help prevent molding. Store in an environment with a temperature that exceeds freezing but does not climb above 50 degrees and wait for spring.

Pests and Diseases

If you’ve given your dahlias a healthy start and planted them in the right conditions, they will likely be strong and resilient. The genus is not without its pests and diseases, however. Here are a few you should look out for and some ideas for addressing them:

Snails and Slugs

These guys can take up residence in your beds and cause a great deal of damage, especially since mulch layers are very attractive to things that slime and slither.

Look for leaves with irregular shaped holes or shredded margins to indicate their presence in your beds. If you suspect snail/slug damage, take a flashlight out to the garden at night and examine the undersides of your leaves. You’ll likely see them creeping around.

Dahlias are most at risk to these pests early in the season when foliage is tender and easily munched, so it’s important to get out in front of snails and slugs. Organic gardeners may choose to bait and detract these pests with a natural product featuring Spinosad.

You may also have some luck spreading coffee grounds, eggshells, or garlic around your dahlias. Snails and slugs can also be effectively managed through hand picking and diligent removal or by planting them alongside deterrent herbs like chives, mint, and chamomile.

Earwigs

Damage from earwigs can be recognized by frayed leaf edges that are often tinged with black as well as chewed buds and flowers. The best way to prevent earwigs is by keeping your beds tidy. Remove spent petals and leaves promptly and manage all weed types to make these pests less comfortable.

Spinosad sprays have proven to be effective in the organic garden along with leaf pile traps and disorienting materials like aluminum foil. Carbaryl-based insecticides can also be employed to reduce earwig feeding.

Thrips

Damage from thrips can be identified by discoloration and slightly warped leaves or blooms. Dahlias that have blotches of tiny black or cream-colored dots might be victims of these pests.

If left unchecked, thrips can cause systemic damage and plant death, so it’s important to keep an eye out for them.  Because they prefer a dry environment, keeping them moist will help discourage thrip inhabitance.

Some luck can also be had with organic leaf soaps or Imidacloprid-based pesticides.

Mites

If their foliage appears mottled and yellow, check for spider mites. While these sucking insects are often too small to see, their webs can sometimes be apparent on leaves and stems.

Since mites typically work from the ground up, removing the lowest leaves from your dahlias might help discourage an upward spread.

Apply horticultural oils or soaps on the undersides of leaves for a natural treatment, or try an abamectin-based insecticide. Plants that are overrun with mites should be destroyed to prevent spread.

Fungus

Always present in the soil, which is a good thing, fungus is known to wreak havoc in the dahlia garden. Not always apparent until the problem is advanced, fungus comes in many forms and can be difficult to treat.

If you’re seeing brown or scorched leaf margins, droopy leaves, and yellow to brown stems, Verticillium wilt may be the culprit. Unfortunately, once this fungus attacks a dahlia’s feeding system, the plant will most likely die and should be removed to prevent spread.

A common dahlia foe, powdery mildew presents later in the growing season with white or grayish mold clusters on leaves, buds, or blooms. While this condition won’t kill the plant and is mostly cosmetic, plants with powdery mildew should be promptly removed from their beds at the end of the season to prevent the fungus from overwintering.

Organic and chemical fungicides can be used preventatively, but sometimes they cause more problems than they solve. Fungal issues are best dealt with before they happen. Good watering practices will discourage puddling, and proper plant spacing will encourage air flow.

Bacteria

Stem rot and leafy gall are the most common bacterial diseases. If your plant stems turn brown and collapse, stem rot is likely the culprit. Leafy gall presents with multiple unsightly clusters of tissue at your dahlia’s base and causes distorted growth.

In both cases, bacteria have entered the plant’s system through a wound or opening and caused an infection. Unfortunately, most bacterial conditions are fatal, and plants should be destroyed to prevent further spread.

Frequently Asked Questions