How to Grow and Care for Dahlias

Dahlias are late-season flowers that bloom from midsummer through the first frost, offering a vast variety of colors, patterns, sizes, and flower forms that can be small border plants or giant plate-sized blooms atop 6-foot plants. Dahlias are perennials in warmer climates and considered annuals below USDA hardiness zone 8, and these flowers grow best in full sunlight with well-drained soil and frequent waterings during the growing season. Because this species of flower is toxic to both dogs and cats, it's best to plant dahlias away from areas that pets can access.

Dahlia Care

• Plant dahlia tubers outdoors in the early spring, after the last frost and once the soil has warmed.
• Depending on the variety, plant dahlia tubers 2 to 6 inches deep.
• Choose a planting location that receives full sunlight.
• Place smaller varieties at least 12 inches apart and larger varieties up to 3 feet apart to allow for air circulation.
• After the tubers have sprouted, water the plants at least once per week, increasing waterings to twice per week or more during dry spells.
• Once tubers sprout, pinch off the top stem just above the highest set of branches to encourage more blooms.
• Deadhead faded blooms to keep the plant looking tidy and encourage flowering.

Light

In order to produce abundant blooms, dahlias require full sun, preferably 6 to 8 hours a day. In climates similar to their native growing regions (USDA hardiness zones 8 and up), this plant will benefit from shade mid afternoon when the sun is especially hot.

Soil

Dahlias prefer rich, loamy soil with plenty of organic matter that drains well. If you're unsure of your soil being rich enough, mix in some compost. Also, if your backyard soil tends towards a denser clay, add sand, peat moss, or manure to loosen the soil texture for better drainage. Dahlias thrive in a neutral soil pH of around 6.5.

Water

After planting dahlia tubers in the spring, do not water them until green growth sprouts above the surface. These flowers do not need water before their root system develops.

Once sprouted, water your dahlias once or twice a week. Make sure to water deeply, as large tubers may be planted six inches deep. If summer days are especially hot and dry, you may need to water more frequently and never let the soil dry out.

Temperature and Humidity

Timing is especially important when it comes to planting dahlias, as they'll struggle to establish in cold soil. Wait until the final spring frost has passed and the ground temperatures have reached 60 degrees Fahrenheit.

You can start tubers in containers indoors—perhaps in a garage or greenhouse—to get them off to a quick start. It's safe to plant dahlias outdoors when all danger of frost has passed. If you plan to dig up tubers and store them indoors for winter, make sure your storage area has a little humidity so they don't shrivel up and dry out.

Fertilizer

Dahlias are heavy feeders—the more food they receive, the larger the plant will grow. Fertilizing your dahlias makes the flowers grow larger and in higher numbers. Use a fertilizer with a high percentage of phosphorus (perhaps a 10-30-20 ratio) to promote blooming. For the amount, follow the product label instructions. Do not use fertilizer with a high percentage of nitrogen. Too much nitrogen creates lush foliage, but few blooms.

If you plan to dig up and store your tubers for winter, stop fertilizing your plants at the end of August. This discourages more growth late in the season as you prepare the tubers for dormancy.

Types of Dahlias

Native to Mexico and Central America, dahlias boast over 20,000 cultivars and 30 species and are the prized darlings of plant breeders and florists alike. Dahlias come in many different patterns, textures, and colors. A few popular types of dahlias take on different shapes and flower patterns and include the following:

• Dahlia ‘Pianella’: This pink variety is considered a "cactus dahlia" with double-flowering blooms and long, rolled petals that make it look spiny like a cactus.
• Dahlia ‘Kelvin Floodlight’: This variety is yellow in color and has broad, flat petals that are slightly rolled at the tip. Classified as a "decorative informal," the petals of this flower are irregularly placed, giving the bloom its full look.
• Dahlia ‘Magenta Star’: Some types of dahlias like 'Magenta Star' feature only one row of slightly overlapping petals, taking on a much different look than fuller varieties. This particular variety is named for both its color and appearance.
• Dahlia ‘Moor Place’: This striking red plant is deemed a "pompom dahlia" with small, double-flowering blooms. The petals on this variety are round and tightly rolled, giving it a pompom look.

Pruning

Prune dahlias once the plant reaches about 16 inches in height by pinching the top tips off above a set of healthy leaves. This will help the plant grow multiple new stems in place of the pinched central stem. Deadheading flowers after they die back also encourages your dahlias to grow fuller and with more blooms. Without regular pruning, dahlias can still grow healthy, but they won't reach the largest possible size and will produce fewer and smaller flowers.

Propagating Dahlias

You can propagate dahlias either from cuttings or from overwintered tubers. Propagating from cuttings requires waiting until your tubers sprout in the spring. However, you can get a jump start on the growing season by dividing your dahlia tubers and planting them in containers indoors before outdoor temperatures warm up. Come summer, your plants will be fully mature and may flower earlier. Plus, dividing your tubers before planting yields more plants and, ultimately, more flowers.

How to Propagate Dahlias by Division

1. Gather your overwintered dahlia tubers, a trowel, garden shears or pruners, potting soil with vermiculite, peat moss, large growing containers, and a 5-gallon bucket.
2. Mix the soil and peat in the bucket and water until barely moist. Transfer the soil into several different growing containers.
3. Examine a clump of tubers and identify those with eyes (if you can't find them, put the clump in a warm, moist area for several days until eyes begin to swell or sprout). Cut those tubers from the clump at the neck. Depending on the size of the clump, you can remove several tubers from each one.
4. Dig a hole in the soil of each container (2 to 3 inches deep for small tubers or 6 inches deep for large tubers). Lay single tubers horizontally in the planting hole with the eye pointing upwards or plant tuber clumps upright and vertically, with at least one inch of soil covering last year's stem.
5. Allow the tubers to sprout in a sunny window.
6. Never let the soil completely dry out, but only keep it slightly moist. Dahlia tubers are especially prone to rot before sprouting.
7. Once sprouted with three sets of branches, pinch off the top.
8. When the soil warms in late spring, plant small varieties at least 12 inches apart and large varieties 3 feet apart. Allow 2 to 3 feet between rows.

How to Propagate Dahlias From Cuttings

1. Wait until your tubers sprout and grow at least 3 inches tall.
2. Gather a sharp knife, alcohol pads, potting soil, rooting hormone powder, and a 4-inch pot. Clean your knife with the alcohol pads and allow it to dry.
3. Make your cut just below the sprout and partially into the tuber.
4. Lay your cutting on a hard surface and trim away the lower leaves.
5. Prepare your pots with potting soil, then poke three or four small holes along the edge of the pot.
6. Dip the end of your cutting into the rooting hormone powder and place it into the hole, backfilling it with soil. Repeat with other cuttings and holes.
7. Water the pot, allow it to drain, and place it in a sunny window. Keep the soil moist.
8. In two to three weeks, your cuttings should root. Once they are growing well and temperatures have warmed, plant them outdoors in your garden bed using the recommended spacing.

How to Grow Dahlias From Seed

Dahlias can be grown from seeds purchased at your local nursery or from seeds collected from last year's plants. Here's how:

1. Fill a seeding tray with seed starting mix and sow seeds indoors, directly in the mix, four to five weeks before the last frost.
2. Move the tray to a sunny window and keep the soil moist.
3. Once sprouted, allow the seedlings to form one true set of leaves before transplanting each seedling into its own cell or small pot.
4. Once outdoor soil temperatures reach 65 to 70 degrees Fahrenheit, transplant the seedlings directly into an outdoor garden bed.

Potting and Repotting

If you're growing dahlias in pots, larger containers are best. A good rule of thumb is to use a container that is at least 12 inches deep and wide. If you use containers this size or larger, there should be no need to repot your dahlias during the season. Bring your containers outside in the summer to assure full growth and plentiful blooms, and stake the stems so that they stay upright. You can also grow compact dahlia varieties, which require less space.

Overwintering

When grown as annuals, you'll need to dig up your dahlias, store the tubers indoors for winter, then replant them in the spring. These flowers are too tender to leave in the ground all winter long in most zones.

To do so, select the healthiest plants from your garden, then wait for the first hard frost. Cut the plant back to 4 inches above the ground. Leave the tubers in the ground for one week before digging them up.

Dig up each root ball starting at least 1 foot away from the stem. Carefully remove the tubers (taking care not to damage them) and clean off excess dirt. Allow the root ball to air dry in a place that is sheltered from frost and out of direct light.

At this point, you can either bag and store the whole root ball or carefully separate the tubers and store them individually. Store your tuber pots, boxes, or bags in a cool, dark, humid place with temperatures between 40 and 50 degrees Fahrenheit. A basement or root cellar works best; just don't let them freeze.

Common Pests & Plant Diseases

Common pests like slugs, earwigs, caterpillars, and thrips adore dahlias. Slugs are especially problematic when the foliage is young and tender. Once the plants mature, slugs are usually not a problem. Some gardeners have trouble with deer, while others claim the deer avoid their dahlias. This may simply depend on the variety of other plants in your garden. Keep your flowers protected, just in case.

Dahlias are also prone to powdery mildew and other fungal diseases. Keep the foliage as dry as possible and space out your plants to provide good air circulation. If you notice an infection, treat it with neem oil or another natural solution.

How to Get Dahlias to Bloom

Bloom Months

Dahlias bloom from midsummer until fall, typically between the months of June and September or October (depending on the growing region). Dahlias reach maturity and bloom about eight to ten weeks after planting in late spring.

How Long Do Dahlias Bloom?

Dahlias can bloom for up to 120 days (or three months) from the time they begin blooming in summer until fall.

What Do Dahlia Flowers Look and Smell Like?

Dahlia flowers vary widely in appearance. Some dahlias have double-flowering blooms, while others have flowers that are slightly rolled at the tip. Dahlias can also have slightly overlapping petals. In general, most dahlia flowers are circular in shape. Dahlias do not typically have a strong fragrance, but their scent can range from slightly sweet and floral to buttery.

How to Encourage More Blooms

Dahlias bloom best when the plant is not flopping to the ground, so be sure to use stakes and twine to keep them upright. Fertilize your dahlias every two weeks to allow flowers to proliferate. An organic fertilizer that is high in phosphorous will assure a good flowering rate and strong stems. Provide ample water for your dahlia bed, especially those containing large varietals, and mulch around the bottom to retain moisture.

Deadheading Dahlia Flowers

Always deadhead your dahlia plants as soon as the flowers are spent. Removing dead flowers promptly will encourage more blooms.

Common Problems with Dahlias

Dahlias can be temperamental and fussy about their growing conditions, yet many of the newer cultivars are more reliable and easy to grow. There are a few common problems you may encounter when growing this species.

White Bases on Stems

Stem rot can occur in dahlia beds that have been watered too profusely or in those that have poor drainage and heavy soil. To avoid this, always amend your soil before planting, and never allow for standing water in your garden beds.

Wilting Flowers

Dahlias may topple over and wilt during the heat of a midsummer day. This is not necessarily a problem, as it's the plant's way of adapting to stress. If the bed is moist, your dahlias should perk back up once the sun goes down.

Yellowed Leaves

Dahlia leaves may become yellowed and feel mushy to the touch if the plants are overwatered. The flowers may even feel dry or crispy if the roots are overwatered, as they will not absorb more water. If your plant's soil still feels wet when it's experiencing these symptoms, cut back on waterings until it perks back up again.