How to Grow Dahlia Flowers

The dahlia plant produces flowers in a range of colors, shapes and sizes.1 The flowers are often loved by gardeners as they have one of the longest blooming periods compared to other plants.2 Dahlias will start blooming in June and many continue to flower until the first frost.

The dahlia plant is native to the high plains of Mexico. Some species have also been propagated in Guatemala, Honduras and Costa Rica.3 The dahlia has been grown in Europe since arriving at the botanical gardens of Madrid during the 18th century. The original flowers had an open center but it wasn't long before gardeners found they could hybridize the plant and change both form and color.

You can find dahlia plants in the form of water lilies, peonies, chrysanthemums and anemone.4 They're grown as flat broad petals, rolled pointed petals or globular flowers. The flowers may be the size of a dinnerplate or a small pom pom.5 In other words, no matter your preference for shape and form, there's likely a dahlia plant to suit your taste.

The plants are often grown as ornamental flowers and belong to the aster family,6 which also includes zinnias, safflower, ragweed and dandelions.7 The dahlia is the national flower of Mexico.

Familiar with cultivating the plant, the Aztecs were the first to discover a dahlia tree known to grow approximately 20 feet high with a hollow interior.8 They were able to use the flowers for water. When the Spaniards conquered the Aztecs, they brought seeds back to Spain, where they were cultivated.

Planting dahlias in your garden offers you some of the most spectacular views throughout the summer. According to The American Dahlia Society,9 there are now 42 recognizable species and several more subspecies. Growing dahlias is easy and they often yield beautiful blooms beginning in the middle of summer. This can go on well into the fall months.

The Fundamentals of Growing Dahlias

Learning how to grow dahlias requires a few steps before you plant. It's important to choose the right type and size for your garden. Large plants can grow as high as 5 feet tall10 with flowers of up to 1 foot in diameter. Their height and blooms look best at the back of a perennial border. The larger plants require some staking to support the stems during wind and rain.11

Medium dahlias grow up to 2 feet in height and can be grown in containers. The smaller ones reach 20 inches and remain fairly compact, which makes them a perfect addition for a window box, container or the edge of a border.12

Each dahlia grows from a tuber and can be found at most nurseries and garden centers.13 If you're interested in a plant not commonly found in the garden center, nearly every local dahlia society has events where guests are welcome. The benefit to purchasing tubers or plants from a club is the ability to speak to experts who can answer your questions and help you through your first season.

The dahlia is a perennial plant in USDA Hardiness Zones 8 to 11. In cooler areas, gardeners may plant the tubers and either allow them to die at the end of the year or dig them up and store them for winter.14 When choosing your tubers, avoid those that look wrinkled and instead choose the ones that have a little bit of green growth or buds.15

Consider Soil, Water and Sun in Growing Dahlias

Dahlias appreciate well-drained, rich soil with a neutral pH near 6.5.16 If your soil is clay or heavier, consider enriching it with plenty of organic matter, even when planting in containers or window boxes. You may also add sand, aged manure or peat moss for better drainage and to keep the texture loose.17

Bone meal may be added as you plant the tuber, but if you have animals in the yard that are attracted to bone meal, they may dig up your tubers!18 In containers, use two parts garden soil with one part potting soil,19 which helps with moisture retention as potting soil dries quickly.20 Dahlias grow from tubers that form at the base of the stem in the fall.21

In addition to rich soil, your growing dahlias will do best when regularly watered.22 Although the plants appreciate a drip irrigation system, if you’re hand watering, water at the base of the plants to keep the foliage dry and reduce the risk of fungal growth.

Select a site in full sun as your plants will produce more blooms when they have up to eight hours of direct sunlight.23 In a less sunny location the plants will grow taller but have fewer blooms.24 Look for a location protected from the wind as the larger plants may take a beating during inclement weather.25

Planting and Caring for Your Dahlias

Your dahlias will do best in plain garden soil without using potting soil mixes. Weed killer can burn your tubers before they sprout and cause them not to grow at all.26 If you're planting in a container, it should be no smaller than 12 inches by 12 inches for each one you plant. The larger dahlias could be grown in a pot the size of a whiskey barrel. Do not use prefertilized potting mix because it may burn the tubers.27

If you're planting directly in the garden, the soil should be at least 60 degrees Fahrenheit.28 If you live in cooler climates, you can start your plants indoors four to six weeks before you expect the last frost. Start them in containers with the same proportion of garden soil to potting soil.

Whether planting in a container or in the ground, be sure the soil is loosened at least 1 foot deep. As you plant the tubers, keep the stems or sprouts upright and position the top of the tuber no more than 2 inches below the surface.29 They should be 18 to 24 inches apart.30 If the soil is dry, water it sparingly but do not soak it.

Most outdoor areas have enough rain in the spring to meet the water needs until your dahlia plant is fully established. Be careful because overwatering can cause rot.31 If you add mulch to the base of the plants it helps retain moisture and keep the soil cool.32

Experts disagree on the amount of fertilizer dahlias require. Most recommend using a low if you choose to use any at all. It’s also commonly suggested to be careful not to overfertilize because this can make the plant develop weak tubers at the end of the season and have smaller blooms as well as fewer blooms.33 The American Dahlia Association34 believes the best strategy is to test the soil to determine the specific needs.

As you discover some of the tricks on how to grow dahlias, you’ll find the plants do well when you spend a bit of time through the summer months deadheading and pinching them back.35 Cut the flowers back to the main stem to stimulate the growth of longer stems. The longer stems are better for cutting and displaying in your home.

If you're intent on producing large show blooms, you'll want to remove the outer two buds of the three that develop. Although this reduces the number of blooms, the ones that develop will be larger.36

Cutting or pinching the plants may help promote a shorter, bushier plant, according to Swan Island Dahlias.37 You can accomplish this by either pinching off or cutting the center shoot slightly higher than the third set of leaves. Also, if you expect any plants to reach 3 feet or higher, staked them to support the stems and flowers.38

How to Overwinter Dahlias in Cold Climates

The type of weather your dahlia withstands depends on the cultivar, but most are grown as perennials in hardiness zones 8 to 11.39 Although they originated in Mexico and Central America, hot climates can be challenging for current cultivars because they need additional water.40

As the winter season approaches, you may need to make some accommodations so they overwinter successfully. Outside of hardiness zones 8 to 11, dahlias are grown as annuals. After the first fall frost blackens the greenery, cut each plant down to 4 inches above the soil. Carefully dig around the plants and pull up tubers without damaging them.41

Keep them out of direct sunlight and in a frost-free area to dry for a few days. Once the tubers have dried, carefully remove any excess soil and keep about 2 inches of the stem.42 Store the tubers in a ventilated box or in a basket filled with sand, peat moss or vermiculite that is slightly moist.

Place the container in a cool, dry area where temperatures stay above 45 but not more than 55 degrees F. Check them from time to time to make sure they’re not rotting or drying out. If they start to shrivel, mist them with water. If you find rot, trim that portion so it doesn't spread.43

In spring, separate the healthy areas from the parent and plant as described above. Each tuber you use must have at least one piece of crown attached to it or an eye; otherwise it won’t develop. The eye looks like a little pink bump at the base of the stem.44

If you live in zones 8 to 11, the tubers can be left in the ground to overwinter, covered with a deep layer of dry mulch after cutting the stems 2 inches above the ground.45

Keep the Pests Away from Your Dahlia Flowers

Deer don’t particularly care for dahlias, so there’s a good chance they’ll leave your flowers alone.46 Dahlias are prone to fungal diseases, though, so keep the greenery as dry as possible when watering and the plants well-spaced for good air circulation to reduce the potential for fungal growth.47

When it comes to bugs, the biggest pests are slugs when the plants are young and tender.48 You can get rid of them naturally by setting up a beer trap.49 Bury a shallow container until the rim is at ground height. Put about an inch of beer at the bottom. The slugs are attracted to the liquid and will drown when they fall in. Clean it out every morning.

If you drink coffee, use coffee grounds to deter slugs and feed your plants as they decompose.50 You can mix egg shells with the coffee grounds as well.

Spider mites are another pest that can attack your dahlia plants. These are not true insects but, rather, relatives of spiders and scorpions. They live in colonies on the undersides of leaves and are more common in places that are hot and dry.51

Address this problem by pruning the leaves or stems of infected parts of the plants and washing the tops and bottoms of the leaves with a strong stream of water.52 Do this in the morning to allow the foliage to dry out during the day and reduce the chance for fungal growth. Another option is to bring in natural predators such as ladybugs and lacewings.

Aphids are another bug that ladybugs like. Aphids can also be dislodged from the plant with a strong stream of water. Cut infected parts away and discard them. If pruned parts are infested with insects or mold, don’t put them in your compost pile.53

Long-Lasting, Cut Dahlias Brighten Your Home

Dahlia plant blooms look beautiful in both your garden and in your home or office. They make wonderful cut flowers, and the more you cut, the more blooms the plant produces.54 The best time to cut your flowers is early in the morning, placing them directly into a bucket of cool water.

Remove the bottom leaves from the stems so they don't rot in the water or take up any nutrients from the plant. Be sure to place the stems in a vase. Put the vase in a cool spot away from direct sunlight and change the water every day. If you take care of your flowers this way, the blooms should last a week or a little longer.55

To preserve your flowers, consider drying them.56 Dahlias, zinnias and marigolds with thick heads or delicate blooms respond best to a drying agent such as borax or rice. Spread the drying agent in an airtight container — preferably glass — about an inch deep. Don’t use wood or cardboard because they draw moisture.

Remove most of the foliage and place the blooms on the top. Next, cover the blooms with more drying agent and seal the container. Put it in a cool, dark place for about a week. Gently remove the agent and the dried flowers and spray them with a fixative from a craft store or hairspray.57

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