You are likely already familiar with several members of the dahlia genus since they are popular as garden plants and ornamental flowers. Thousands of cultivars of dahlia have been bred for their vibrantly colored flowers which bloom in a variety of different sizes and shapes throughout the summer months.
I. Appearance and Characteristics
Dahlia is a genus of bushy, tuberous, herbaceous perennial plants native to Mexico and Central America. As a member of the Asteraceae (synonym name: Compositae) family of dicotyledonous plants, its relatives include the sunflower, daisy, chrysanthemum, and zinnia. There are 49 species of dahlia, with flowers in almost every hue (except blue), with hybrids commonly grown as garden plants.
Dahlias were known to the Aztecs until their Spanish conquest, after which the plants were brought to Europe. The (high in sugar) tubers of some varieties are of value to humans.
Dahlias are perennial plants with tuberous roots, though they are grown as annuals in some regions with cold winters. While some have herbaceous stems, others have stems which lignify in the absence of secondary tissue and resprout following winter dormancy, allowing further seasons of growth. As a member of the Asteraceae, the dahlia has a flower head that is actually a composite (hence the older name Compositae) with both central disc florets and surrounding ray florets. Each floret is a flower in its own right, but is often incorrectly described as a petal. The modern name Asteraceae refers to the appearance of a star with surrounding rays.
The stems are leafy, ranging in height from as low as 30 centimeters (12 inches) to more than 1.8–2.4 meters (6–8 feet). Flower forms are variable, with one head per stem; these can be as small as 5 cm (2 in) in diameter or up to 30 cm (1 ft) (“dinner plate”). The majority of species do not produce scented flowers. Like most plants that do not attract pollinating insects through scent, they are brightly colored, displaying most hues, with the exception of blue.
The great variety in species results from garden dahlias being octoploids, having eight sets of homologous chromosomes. In addition, dahlias also contain many transposons—genetic pieces that move from place to place upon an allele—which contributes to their manifesting such great diversity.
Dahlias grow naturally in climates that do not experience frost (the tubers are hardy to USDA Zone 8), consequently they are not adapted to withstand sub-zero temperatures. However, their tuberous nature enables them to survive periods of dormancy, and this characteristic means that gardeners in temperate climates with frosts can grow dahlias successfully, provided the tubers are lifted from the ground and stored in cool yet frost-free conditions during the winter. Planting the tubers quite deep (10–15 cm or 4–6 in) also provides some protection. When in active growth, modern dahlia hybrids perform most successfully in well-watered yet free-draining soils, in situations receiving plenty of sunlight. Taller cultivars usually require some form of staking as they grow, and all garden dahlias need deadheading regularly, once flowering commences.
Though dahlias are not highly toxic to pets, you should keep these blooms away from them because they can cause unpleasant symptoms such as vomiting and diarrhoea. If you intend to plant your dahlias in the garden or keep them in a vase, consider where you will place them if you have a curious pet at home.
II. How to Grow and Care
Sunlight
Dahlias need full to partial sun. Full sun encourages upright plants that need less staking, but these plants will still flower in part shade. If planted in shaded areas, the foliage tends to look more green than burgundy on dark-leaved cultivars. Partial shade, especially during the afternoon hours, is a good idea in hot climates.
Temperature and Humidity
Dahlias are native to the mountains of southern Mexico and Central America, where the summers are warm but not excessively hot. They are not frost-hardy, and below zone 8, they can only be grown as annuals. Dahlias tolerate humidity.
Watering
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.
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.
Fertilizing
Because you want your dahlias to bloom lushly over an extended period, regularly fertilize them with a bloom-boosting fertilizer that is low in nitrogen and high in phosphorus. For frequency, follow the product directions but stop fertilizing the plants towards the end of August so the tubers can prepare for dormancy.
Planting is a good time to incorporate an organic fertilizer with an N-P-K ratio of 5-10-15, 5-10-10, 10-20-20, or 0-20-20. The higher middle number, phosphorous, assists with bloom production, while the third number, potassium, helps root development. Any fertilizer recommended for vegetables can be used for dahlias.
Planting Instructions
Dahlias can be planted directly in the ground in late spring when the ground has warmed to at least 60°F, and all danger of frost has passed.
To get a head start on the growing season, start them indoors in early spring. Plant the bare-root tubers in well-draining potting soil about six weeks before the last frost. Place the pots in a warm, sunny window. Keep the soil evenly moist but not wet to avoid rot. Once the foliage emerges and the danger of frost has passed, transplant them into the ground.
Dig a 6- to 8-inch-deep hole for each tuber and add some compost or bonemeal. Place the tuber in the hole with the growing points (the “eye” that looks like a potato sprout) facing up. Loosely cover the tuber with 2 to 3 inches of soil. Don’t water it immediately; wait for the sprouts to poke through the soil. As the sprouts grow, gradually add more soil until the hole is filled.
The spacing depends on the variety. Smaller types are fine with 1 foot between them, but larger dahlias should be spaced at least 2 feet apart.
Most dahlias need staking. To prevent injury to the tubers, put the support in the ground before planting the dahlias.
Pruning
To encourage a bushier growth habit, pinch off the top 3 to 4 inches of the center branch when the plants are about 1 foot tall. For fewer but larger flowers, also remove the two smaller side buds next to the central bud in each flower cluster. Deadheading the flowers extends the bloom season.
Propagation
Gardeners can propagate dahlias through stem cuttings, division, or seed. Both stem cuttings and divisions produce plants identical to the parent. Those grown from seed harvested from a dahlia are not identical to the parent.
- Stem cuttings
In spring to early summer, cut 4- to 6-inch-long shoots with at least three sets of leaves from healthy stems. Leave two sets of leaves at the top of the cutting and remove the others. If the remaining leaves are longer than 2 inches, trim them to half their size. Fill 4-inch pots with moist sterile potting medium and create a hole in the center with a pencil-sized object.
Dip the end of the cutting into the rooting hormone and put it in the pot, pressing the planting medium around the stem. Place the pots in a warm place under a grow light where they receive at least 14 hours of light daily. Mist them occasionally until you see new growth.
You can also take cuttings from tubers as they grow in the garden. As shoots appear above ground, cut them at the base and handle them as stem cuttings. Leave the tuber in place to produce additional growth.
- Division
Divide dahlia tubers at planting time by cutting the tubers into several pieces, each containing a piece of the main stem and an eye. It is easier to do this if you wait until the eyes sprout in storage. Place each division in a small pot with moist potting soil or sterile potting medium and keep them in a warm place until new growth appears.
- Seed
Sow dahlia seeds indoors six to eight weeks before the last frost date in spring. Because they have a low germination rate, sandwich the seeds between two wet paper towels. Place the paper towels with seeds in a plastic bag and put them away somewhere warm for three to five days; then start checking daily to see if they have started germinating. Carefully place any germinated seeds in a seed tray with seed-starting mix and cover them lightly with a dusting of the planting medium.
Put the seed tray under grow lights. The seeds must be kept warm. Continue to check the paper towels each day to see if more seeds have germinated. They don’t all germinate at the same time. After a couple of weeks, discard any remaining ungerminated seeds and the paper towels. When the seedlings are large enough, transplant them into individual pots. Wait until after the weather warms in spring to move them outside, hardening them off first.
Potting and Repotting
Dahlias, especially dwarf-size types, grow equally well in the ground or in containers. Plant them in pots at least 16 inches in diameter and 12 inches deep so that you can gradually add soil to the tubers as they start growing.
Make sure the container has large drain holes and use a combination of well-draining potting mix and compost.
Like all potted plants, they will need more watering than dahlias in the landscape, as well as more frequent fertilizer applications as the fertilizer washes out of the pot.
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.
Pests and Diseases
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.
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.
III. Uses and Benefits
- Tubers were used in pre-Spanish Mexico due to their high fructose and inulin content.
- Aztecs used petals and tubers to treat infected grazes, rashes, and skin cracks.
- The skin of tubers is high in antibiotic compounds.
- The crushed and mashed petals are used to relieve stings and insect bites.
- To relieve tired feet, soak the petals in a foot soak.
- The flowers are used to treat epilepsy by the Aztecs.
- Dahlias became religiously associated with the resurrection of Jesus during the Victorian era. They were also thought to be a symbol of the everlasting life promised to faithful followers.
- The Dahlia flower has a completely different meaning in Thai culture than it does in the rest of the world. While the flowers thrive as a perennial there because of the warm temperatures, many people avoid giving them as gifts because of their symbolism.
- Dahlia flowers are a popular wedding bouquet addition for modern brides who want colour options that other, more traditional options do not provide.
IV. Harvesting and Storage
Dahlias are beautiful in the garden, but don’t be afraid to snip a few for a vase; cutting dahlia flowers for bouquets actually encourages the plant to produce more flowers. Be sure to deadhead any spent blooms, too.
The best time to cut flowers, including dahlias, is in the cool of the morning. Snip horizontally with clean pruners or scissors, taking stems long enough for your bouquet. Choose flowers that are open or nearly open because the buds will not open after they’re cut. You should cut as much length as you need for your bouquet, but try to make the cut just above a set of leaf nodes and side buds. New shoots will grow from those nodes.
After you’ve harvested dahlia flowers, make a fresh horizontal cut at the bottom of the stem and place the cut ends in about 2 to 3 inches of very hot (not quite boiling) water. Leave the stems in the water for at least one hour. This hot-water treatment conditions the stems so that the blooms last four to six days.
After the cut stems have been conditioned, strip off all the leaves that would be below the water line in your vase. This is true for all flower arrangements, not just dahlias. When leaves stay underwater, they decay and release bacteria that shorten the vase life of the flowers. Change the water in the vase every two or three days, adding a floral preservative to help the blooms last longer. Dahlias can be combined with other cut flowers to create beautiful effects, but dahlia arrangements are spectacular on their own.
