Dahlia flowers are a joy in the garden! They thrive in warm climates and attract pollinators through hot summers. They make stunning cut flowers, and care for dahlias is not hard at all. If you haven’t planted tubers in your garden to this point, now may be the time.
Dahlias thrive in areas with warm weather, but they can be grown as annuals in cold climates. When you decide which you’d like to grow, you’ll have a slew of ball dahlias, cactus dahlia types, pompon dahlias, and more to choose from.
In this piece, we’ll discuss how to provide dahlias with all the basics they need in and out of the growing season. We’ll touch on how to plant them, too.
Quick Care Guide
|Scientific Name||Dahlia spp.|
|Height & Spread||15 inches to 6 feet tall, 2 to 10 inches wide|
|Soil||Fertile, moist, well-drained soil|
|Water||1 inch per week before blooming, 2 inches per week during blooming|
|Pests & Diseases||Powdery mildew, fusarium wilt, botrytis, necrotic spot virus, root-knot nematodes, Asiatic garden beetles, spider mites, four-lined plant bug, leafhopper, thrips|
All About Dahlia Flowers
Dahlia cultivars like ‘Dark Spirit’ are a beautiful addition to any garden.
Dahlias grow wild in Mexico and Central America. They’re herbaceous flowering plants that grow from tuberous roots. You might wonder, are dahlias perennials?
Well, that depends on where you’re planting them. In areas close to their native range or those with similar climates, they are perennial. If you grow dahlias in cooler climates, they’re annual.
Dahlias have either long, herbaceous stems or woody ones, depending on how old the plant is. As a member of the aster family, the dahlia flower has a composite, rayed structure of petals around a center disk that is most often yellow or orange. A dahlia blooms in midsummer through the fall season, and they die back in winter to emerge again in spring.
Indigenous American and Central American peoples have cultivated dahlias for ages. Dahlias were found by Spanish conquistadors to Mexico in the early 1500s.
Then in the late 1700s, French colonists were captivated by the lovely flower forms found in the same place. They were brought back to Europe, and initial cultivation of more flowers began in Spain in the 1800s.
Species of the plant were classed by various European scientists who named the genus after a student of Carl Linnaeus, Anders Dahl.
From there, Dahlia pinnata, Dahlia rosea (with its dark pink blooms), and Dahlia coccinea were the premiere species beloved in Europe. The American Dahlia Society was formed in New York in 1915 and dedicated its activities to this lovely plant.
You should have no problem finding dahlias at your local nursery. Most carry single cultivar tubers, which means it’s easy to opt for purely dark red blooms if that’s what you want. However, you can also find blends, and go for either a dark blend or a light blend. Incorporating both means you’ve got light-colored and deeply-colored petals to look forward to.
Dahlia tubers are fairly easy to plant.
Aside from knowing when to plant dahlias, planting dahlia tubers is fairly easy! The trick is to plant them in containers about a month before the last frost in preparation for transplant from mid-spring to mid-June in some areas.
Prepare your planting site and your dahlia containers with rich, well-draining soil that can hold some moisture. The containers you use should be at least 5 gallons with at least 12 inches of diameter.
Dig a planting hole and plant your dahlia tubers indoors about 4 to 5 inches deep. Ensure the eye is facing upward when planting, and space them 12 to 18 inches apart (you can see this done in detail in the video below).
Cover your dahlia tubers with soil, and give them a sprinkling of water. In a few weeks, dahlias grow, and you’ll have plants you can transplant in your garden. Wait to do this until after the danger of frost has passed in the spring, as cold soil can kill your tubers.
If planting your dahlias directly into the garden, you’ll want to still maintain that same depth of 4-5 inches with the eye pointed upward, 12-18 inches apart.
A Power Planter auger bit can really help with speedy planting hole preparation and breaking up the surrounding soil.
Caring for Dahlia Flowers
Dahlias require regular maintenance and support when grown in the garden.
Now that you know how to plant dahlias, let’s discuss caring for them as the plants grow. Then you can enjoy them as cut flowers – though they don’t have a long vase life – or simply appreciate their beauty as they grow in your garden.
Sun and Temperature
As natives to Central Mexico, these beautiful blooms thrive in the sun.
Dahlias need good light in the form of direct sun for best growth, with at least 6 to 8 hours of direct sunlight daily. In areas with very hot summers, dahlias can benefit from some partial shade in the hard afternoon sun.
Ideal temperatures range from 68° to 72°F (20° to 22°C). In prolonged periods of heat above that range, blooms occur more quickly, and the plant’s overall vitality may reduce.
To keep the plants growing healthily, give those in hot weather afternoon shade. You can plant them near obstructions or use a shade cloth to help them bloom less frequently in warm weather.
Cold weather will cause the dahlia plant to die back, but as long as there is adequate mulch around it, the tuber should survive. In areas where the winters are very cold and those that have a lot of hard frost, dahlias are annuals. Most are hardy and perennials in USDA zones 8 to 10.
Water and Humidity
A drip irrigation system or soaker hoses are best for watering.
On the most basic level, dahlias need at least 1 inch of water per week. At their initial planting time, when you first plant them in the garden, give them 2 inches per week until they are established. Consistent watering is key here, as these plants tend to enjoy temperate weather and moist soil.
Drip irrigation or soaker hoses are the best mode of watering dahlia plants. This is because the slow drip of moisture doesn’t overwhelm the tuber below ground, which can rot if it sits in excess moisture for too long. When the large flowers bloom in the garden, increase watering to support them.
In times of drought, more water will be needed, and in times of heavy rains, your dahlias won’t need extra. Remember to water in the morning before the sun dries and evaporates all of the moisture out of the soil. High humidity in the area they’re planted is a plus as well.
Ensure you plant in rich, well-draining soil.
Gardeners should provide their dahlias with rich, well-draining soil that is slightly moisture retentive. Heavy soil and clay soil should be amended with some grit, like agricultural sand or perlite. These improve soil texture. Dahlia plants can survive in poor soil as long as it drains well.
Additions of peat moss will promote moisture retention. Aged manure will support nutrient requirements for your dahlias as well, but don’t add too much as the plants thrive in a soil pH of 6.5 to 7.0. Too much manure can make the soil very acidic.
Fertilizing Dahlia Flowers
You can use a slow release fertilizer to enhance your dahlia’s blooms.
Side dressings of manure and regular applications of organic fertilizers will keep your dahlias happy and promote plenty of lovely blooms. Too much nitrogen in your fertilizer will push out more foliage and green growth rather than the flowers you love.
For those gigantic blooms, dahlias require a planting site that is already amended with some manure. Then as the small plant grows, apply a 10-20-20 or 6-24-24 organic slow-release fertilizer once per month until the end of the bloom period.
It’s at this time your dahlias go dormant. There is no need to fertilize the tuber during dormancy as it contains enough nutrients to push out new foliage in spring.
Pruning can help promote healthier bloom production.
There are a few different ways gardeners can prune dahlia plants to promote healthier growth. One involves pinching back the tips of your plant’s leaves when they grow about 1 foot to 1.5 feet tall. This promotes bushier growth around the central stem.
Another way to support the development of buds is to prune back all but one of the flowering stems. Doing this gives you one giant bloom on each plant. If you’re into lots of blooms, and you’re ok with them being smaller, prune all but 10 of the stems.
When your dahlias bloom their flowers, feel free to cut stems and pop them in a vase with a flower arrangement. If you choose to leave them on the plant, you can deadhead spent flowers to promote more blooms later in the season. In some cases, it’s possible to prune for a third set of blooms in one season.
Sometimes it’s hard to tell spent blooms from side buds. If you avoid cutting those with a cup shape and focus on those with a cone shape, you’re set!
Most dahlias have strong stems, but if the stem of your dahlia gets tall and floppy, you can stake it to provide some extra support.
Propagation is quite easy and tubers can be split in the fall or spring.
When your dahlia flowers fade and the foliage dies back to the ground, you can remove the tubers from the year’s plants for overwintering. This usually occurs after the first frost. Take them from the ground, clean them gently with a dry rag, and let them air dry in an area away from exposure to sunlight.
They’ll need to be stored in a cool place with high relative humidity, around 80% to 90%. In areas where dahlias are hardy, tubers can be left in the garden and covered with mulch. The mulch protects the plant’s root system in colder seasons.
Planting seed is possible, and the dahlia seeds can be extracted from spent flowers during the process of deadheading. Wait for these to dry, and then crack them open to collect the seed. Directly sow dahlias in areas with a long growing season, or start them indoors to be planted out in late spring.
To start seeds, sprinkle them in a flat, in containers, or directly on the ground somewhat densely. Then lightly cover the planting area with soil. Water the area, and after 7 to 10 days, you should have sprouts, which will continue to proliferate over the course of several days.
If you’re sowing indoors, you’ll have a small plant ready for transplant in a few weeks.
Repotting Dahlia Plants
Container grown plants can be separated and repotted if they are container-crowded.
Annually, you can unearth container-grown tubers to check and see if they need to be separated due to overcrowding. This allows you to grow more flowers in other pots if you so choose. Use the same process for harvesting tubers as you would for overwintering them.
Look for damaged or rotten tubers and remove them. Then separate the clusters of tubers into smaller sections. Prepare your pots, and plant the tubers 4 to 5 inches deep. If you plan to overwinter them, bring the containers indoors and put them back outside when the last frost passes.
Growing dahlias is pretty simple, but they do have their fair share of issues that can arise in the process. Let’s discuss those so you know what to look out for when you’re growing them.
If grown in conditions not suitable for their growth, dahlias will struggle.
Dahlias may not flower if they are grown in improper conditions. Full sunlight and well-draining soil that is rich with nutrients is required. Water regularly to keep the soil moist but not overly wet, which can create conditions that support the development of rot.
If you water your dahlias too much, though, the stem can become too tall and floppy. In heavy rains, there’s no getting away from this. Staking can help your dahlias stay upright.
Beetles and other pests are a fairly common problem.
Root knot nematodes can cause issues with your dahlia tubers, especially if the tubers have remained in an in-ground garden for a long time. One way to prevent their damage is to regularly divide tubers and move them to other areas of the garden. You can also treat your garden with beneficial nematodes twice in a temperate season about two weeks apart.
Asiatic garden beetles are another below-ground pest that like to feed on tubers as the larvae pupate in the ground. Use the same prevention methods, and treat them with beneficial nematodes where necessary.
Dahlias also deal with feeding from spider mites, the four-lined plant bug, leafhoppers, and thrips. All of these can be blasted from your plant with a strong stream of water. If this doesn’t get rid of them, you can use insecticidal soap to wipe them out. Horticultural oil spray can also treat spider mites.
Diseases can be common, including powdery mildew.
Thrips tend to spread necrotic spot virus as they feed on the stem of your dahlia plants. The disease causes bubbling leaves and yellow mottling along leaf veins. There is no cure for this virus, so treat for thrips to prevent it. Thankfully, the virus relies on living plant tissue to proliferate. Remove the infected dahlia and grow a new one in its place.
Powdery mildew, fusarium wilt, and botrytis are all fungal infections that can strike dahlias. Powdery mildew presents as a white film on the upper surface of leaves. Remove infected leaves and prune your plants to promote good air circulation to prevent illness. The same treatment can be used for botrytis.
Fusarium wilt can’t be treated and causes death eventually. It strikes above ground first and then spreads to tubers. Evidence of the disease can be seen in one side of the dahlia as it yellows, wilts, and then dies. If you dissect tubers and notice a rotten center, fusarium wilt is the likely cause. Do not grow in that area for a couple of seasons to prevent spreading the disease.
Frequently Asked Questions
Q: Do dahlias come back every year?
A: If you grow them in their hardiness range, yes! They will be perennial. Elsewhere they are annual.
Q: What month do you plant dahlias?
A: You’ll want to transplant them in mid-spring through early summer (as long as it doesn’t get too hot in summer).
Q: How long do dahlia flowers last?
A: Most of the flowers last for 3 to 5 days. Some can last 10 days. It depends on the cultivar.
Q: Can you leave dahlias in the ground over winter?
A: Yes, you can, especially if you live in zones 8 to 10. Otherwise, you may want to overwinter tubers.
Q: Do dahlias grow well in pots?
A: They do! Ensure you’ve got a pot large enough to hold them. One that’s at least 5 gallons and 12 inches wide works.
Q: Do dahlias multiply?
A: Over the course of a season, the underground tubers will multiply. These can be divided and planted elsewhere.
Q: Do dahlias need a lot of water?
A: A lot of water isn’t necessary, but consistent water is. Give them 1 inch of water per week and 2 inches per week during their blooming period.
Q: Are dahlias poisonous to dogs?
A: They can cause gastrointestinal distress and dermatitis in dogs and cats. Keep your furry friends away from them.