Ill.: ngb.org

Every year, the National Garden Bureau, a non-profit organization promoting the pleasures of home gardening, selects one annual, one bulb, one perennial and one edible plant to celebrate. It’s a great way to discover a new plant or to learn a bit more about a plant you may already be growing.

Let’s look at the bulb chosen for 2019, the dahlia.

The Plant Behind the Name

There are some 42 species of Dahlia, most from Mexico (its center of diversity), where they generally grow in uplands and mountains. Some species are found elsewhere in Central America, even as far south as northern South America.

They’re in the Asteraceae family, known for its composite flowers, and thus related to such plants as the sunflower, the daisy and the zinnia. Each dahlia “flower” is therefore not a single bloom, but actually a compound bloom, an inflorescence*, composed of multiple small flowers called florets, typically fertile yellow florets forming a central disc surrounded by a circle of variously colored sterile ray florets. The latter are generally called petals by gardeners, but are not true petals.

*In the rest of this text, I’ll use the term “flower” to describe a dahlia inflorescence, according to customary usage.

The dahlia we know and love (we’ll call it the garden dahlia in this article) is a complex hybrid and its exact parentage is not fully understood. In fact, the plant’s exact botanical name remains “unresolved.” Long known as Dahlia variabilis, modern taxonomists tend to list it as D. × pinnata, the multiplication sign indicating it’s a hybrid species.

The garden dahlia is an octoploid, with 8 sets of chromosomes, whereas most plants have only two. This leads to an enormous multiplicity of genes and thus of possibilities in all the plant’s characteristics: height, leaf color and form, flower color and shape, etc. It’s therefore not surprising that there are some 57,000 cultivars of dahlia, with many new ones created each year. The garden dahlia can range in height from less than a foot (30 cm) to more than 8 feet (2.5 m) with flowers ranging from less than 2 inches (5 cm) in diameter to up to 1 foot (30 cm): the so-called “dinner plate dahlias.” Every flower color but true blue is possible.

Along with this great potential for variability comes a lot of genetic instability. Dahlias have many transposons, so-called “jumping genes,” DNA sequences that can change their position and thus change the plant’s appearance. Because of this, dahlias are renowned for their ability to mutate to new forms and to revert to previous forms. Not that this happens every day, but if your red dahlia suddenly switches to being a yellow one, you shouldn’t really be surprised.

The Name Behind the Plant

The dahlia was well-known to the native peoples of Mexico who grew it for its edible tubers and medicinal uses as well as its hollow stem, using in piping. Indeed, the Aztec name Cocoxochitl means “water pipe plant.”

Francisco Hernández, physician to Philip II, sent to Mexico in 1570 to study the country’s plants and animals, noticed the plant being commonly grown as a vegetable and had a companion, Francisco Dominguez, draw illustrations that were sent back to Spain. Curiously, all had double flowers, practically guaranteeing they were already carefully selected hybrids.

It was Vicente Cervantes, Director of the Botanical Garden at Mexico City, who first sent dahlias to Abbe Antonio José Cavanilles, Director of the Royal Gardens of Madrid. Whether he sent seeds or tubers is unknown, but most likely seeds, as they travel better. Oddly, all the first dahlias actually sent to Europe in 1789 had single flowers. Double flowers later showed up spontaenously in European gardens.

The plant was originally sent as a possible food source, but that aspect never caught on in Europe. Cavanilles instead saw the plant’s ornamental value. He named the plant Dahlia in honor of Swedish botanist Anders Dahl (1751-1789), calling the three varieties he received Dahlia pinnata for its pinnate foliage, D. rosea for its rose-purple flowers and D. coccinea for its scarlet color. These three plants, plus D. sambucifolia, sent over in 1804, are believed to be the main parents of today’s garden dahlia.

Cavanilles generously shared seeds and tubers with botanical gardens all over Europe and, as a result, by the early 1800s, dahlias were being widely planted in parks and gardens throughout Europe. Also, serious hybridization was going on, notably in Holland and England.

The royal courts of Europe too were infatuated by these new flowers. Josephine Bonaparte, wife of the French Emperor, was so enamored of dahlias she grew prize varieties in her garden at Malmaison, but refused to share the plants with other nobles. However, eventually a Polish count had one of her gardeners dig up over 100 dahlia plants, then absconded with them. Josephine was so enraged that she had all the remaining dahlias dug up and ground into mulch and, furthermore, banned the flower from her gardens.

By 1826, over 60 varieties were known and by 1841, one English dealer was offering more than 1,200 varieties. The first garden dahlias reached the United States in the early 1830s and hybridization began there as well. Today, dahlias are grown all over the world and there are many regional and national dahlia societies (national societies include the following: American Dahlia Society, Canadian Chrysanthemum and Dahlia Society, National Dahlia Society (United Kingdom) and Dahlia Society of Australia). And the dahlia is recognized in its native country where it was named Mexico’s national flower in 1963.

Recently, the idea of growing dahlias for their edible tubers has been revived and several hybridizers are working on developing varieties with especially large, prolific and tasty tubers. Maybe they’ll show up in supermarkets along with potatoes and onions some day!

So Much Variety

There is an astounding amount of variety among dahlias, although how they are classified varies a bit from one dahlia society to the next. Among the terms you’ll run into if you grow dahlias are the following:

Anemone: broad outer ray florets and a central “pincushion” of tubular disc florets, usually of a different color.

Ball: fully double flowers with blunt-tipped ray florets.

Cactus: highly rolled-up, very pointed ray florets, giving the appearance of cactus spines.

Collarette: semi-double with flat ray florets, a central yellow disc and, in between, shorter petaloid stamens forming a collar.

Decorative: fully double uniform flower with fairly flat ray florets (petals). Formal decorative flowers have rounded tips; the ray florets of informal decorative flowers have rolled tips.

Fimbriated: ray florets that are deeply notched.

Novelty or Miscellaneous: flowers that don’t fit into other classifications.

Orchid: single flower with tightly rolled ray florets.

Peony: semi-double with an open center and two or three layers of ray florets, looking much like a peony flower.

Pompon: like Ball, but with miniature flowers.

Semi-cactus: like a cactus flower, but with ray florets broader at the base.

Single: single flowers, i.e. a single row of ray florets surrounding a central disc.

Stellar: like a cactus flower, but curled so the reverse of the ray floret is showing.

Waterlily: double flowers with broad but fairly sparse ray florets looking overall like a waterlily blossom.

There are many more terms and definitions, but that should be enough to get you started with your dahlia collection.

Growing Dahlias

Dahlias are frost tender and should not be planted out until all risk of frost has passed and both the soil and the air have warmed up. One guideline is to plant at the same time as you would a tomato.

Plant them in full sun (partial shade is grudgingly accepted) in rich, well-drained, slightly acidic soil, in the garden or in containers. Plant the tubers with the crown just below the surface of the soil and water in. Give dahlias started in pots or trays similar treatment.

You can also start dahlia tubers indoors in pots, placing them in full sun or under lights under normal home temperatures, for a bit of an advance on the season (especially useful in short-season areas).

Dahlias are quite heavy feeders and will appreciate extra minerals through the summer, although they aren’t particular as to any specific formula. Probably the easiest thing to do is to apply an all-purpose slow-release fertilizer at planting to carry them through the summer.

Monitor soil moisture, especially if the local rainfall is less than 1 inch (2.5 cm) per week. In containers, dahlias will require more water because of their limited soil volume.

Taller types, especially those with large flowers, will need staking. Traditionally, you would hammer a simple wooden or bamboo stake as long as the plant is tall into the ground before you place the tuber into the planting hole. That way there is no risk of accidentally damaging the tubers. I find it simpler to place a tomato cage over the plant after planting.

Deadhead (remove faded flowers) to help maintain constant flowering. True enough, there are a few modern dahlias are self-cleaning, especially among dwarf and semi-dwarf bedding type dahlias. With them, no deadheading is necessary, the ideal situation for laidback gardeners, but sadly, they still remain a minority.

Many dwarf varieties are not influenced by day length and will bloom all summer. Taller dahlias tend to bloom from late summer into fall.

Overwintering Dahlias

In their native land, dahlias grow in mountainous areas with a dry, fairly cold winter and have learned to go dormant at that season. The top of the plant dies back while the tuberous roots are safe from the cold underground, then sprout again in spring when the weather warms up and rains return. In the garden, you have to maintain the same cycle.

Only in mild climates (hardiness zones 8 to 11) with fairly dry winters can you consider leaving dahlia tubers in the ground all winter. For aesthetic reasons, cut the tops back to near ground level when the plants stop blooming and the foliage dies back. Applying a good thick mulch is wise if there is any danger of frost.

Gardeners in colder climates or rainy mild ones will have to dig them up in the fall. Since the tubers really began to put on growth when days shorten, the longer you leave them in the ground without risking the ground freezing hard, the bigger the tubers will be and the better they’ll survive the winter. In many areas, you can wait until a killing frost has blackened the foliage before proceeding.

When you’re ready to do so, cut back the stems to about 4 to 8 inches (10 to 20 cm) in length, then dig up the tuberous roots using the stems as a handle. Shake as much soil free as possible, then rinse well with water to get most of the soil off. Spread the plants out and let them dry out for a few days in a garage or tool shed. Make sure you carefully label the each variety so there’ll be no confusion come spring.

Now store the tubers for the winter, ideally in a cardboard box, wooden crate or a well-aerated plastic container. Just barely cover them in vermiculite, sand, wood chips or peat, then place the containers under cool conditions (less than 50 °F/10 °C), but above freezing. If you store them at room temperatures, check frequently for dehydration

Check monthly for signs of rot and dehydration. Remove rotting tubers, of course. If any tubers start to become wrinkled, a sign they’re dehydrating, give them a quick spritz of water to plump them up again. By late winter, sprouts will be appearing from the crown and it will soon be time to start the tubers for a new growing season.

Multiplying Dahlias

There are several ways of multiplying dahlias, including from cuttings, tuber division and seed.

Stem Cuttings

The advantage to taking stem cuttings is that you can rapidly produce many plants of the same cultivar. Oddly enough, cutting grown plants usually come into bloom earlier than tuber-grown plants!

You can take cuttings at just about any season when the plant isn’t dormant as long as you remove any flowers or buds present on the stem. However, the best season is early in the year, when sprouts are just coming up from the tubers, as this will give you plants that will bloom the same summer. There’ll be several to many shoots per tuber, so little harm comes from harvesting a few. In fact, if your plant produces more than 5 shoots, you should remove the excess ones anyway.

When the stems have about 3 to 4 pairs of leaves, cut them off at the base and insert the cut end into moist potting soil, making sure at least one node is covered in mix. No rooting hormone is necessary. Cover with a clear plastic dome or bag and place at room temperature in moderate light (no sun at first, otherwise it will become too hot) or under lights. When you see new growth, a sign that stems have rooted, remove the covering and move to full sun, then harden off before planting out for the summer.

Dividing Tubers

This is the classical way of starting new dahlias. It can be done in fall, as you bring them in for the winter, or in spring, just before you plant them. The tuber is a thickened root and provides the energy for new growth, but new growth itself comes not from the roots, but from the base of the stem (its crown). Therefore, you can’t simply cut the tubers free: if you do, nothing will grow. You need a tuber with a piece of stem attached that bears an eye (bud). Often, you’ll end up with one eye and two or more attached tubers.

Experienced growers usually divide their tubers in the fall, when the stems are soft and easier to cut, but then, they’re able to recognize the beginning of an eye. For beginners, I suggest waiting until spring. By then, the eyes will be well formed, even sprouting, although you may need to saw through the hard stems with a serrated knife to separate the divisions or use a lopper (the long handles give you greater force); pruning shears may not be powerful enough.

Seed

Garden dahlias will not come true-to-type if you harvest their seed, therefore if you want to multiple a valuable variety, seeds are not the way to go. However, you can find commercially produced dahlia seed lines that are quite stable and will give excellent results. You just won’t have as wide a range of choice as with tuber-grown plants. Most dahlias grown from seed are dwarf or semi-dwarf varieties used as bedding plants, the idea being to produce lots of plants cheaply. They’re usually treated as annuals and tossed at the end of the growing season, but of course, you can recuperate the tubers and grow them again if you wish

Start the seed of bedding varieties indoors about 6 to 8 weeks before planting out (8 to 10 weeks for taller types). Sow the fine seeds very shallowly, about 1/8” (3 cm) deep in a lightly moistened seedling mix and cover with a clear plastic dome or bag. Place in moderate warmth (65 to 79 °F/18 to 26 °C) until germination, then remove the covering. Place in bright light to full sun or grow under lights (ideally, at 12 hours or more a day, as long days stimulate growth and flowering, while short days stimulate tuber growth and can lead to early dormancy), transplanting into individual pots when the leaves begin to touch. Always keep slightly moist. If seedlings seem slow to branch, pinch out the top pair of leaves to produce a bushier plant.

Acclimatize to outdoor conditions and plant out, as mentioned, at tomato planting season.

Cut Flowers

Dahlias make outstanding cut flowers. To make them last longer, harvest when the flower is open but not yet fully mature. Certainly avoid flowers still in a tight bud: they won’t open after cutting. Also, for maximum durability, prepare the flowers by plunging their stems into very hot tap water immediately after harvest, then letting them cool for an hour before you start arranging using fresh water. Of course, change the water every few days.

Pollinators

Dahlia flowers are highly attractive to a wide range of pollinators, including both native bees and honeybees. They prefer open-faced flowers, that is single and semi-double dahlias where the central disc of yellow florets is fully exposed. Double dahlias are not the best choices for pollinator gardens.

Protecting Your Plants From Pests

Small dahlia plants are susceptible to slug damage. It is a good idea to manually remove slugs or to protect them with commercial slug bait.

Japanese beetles seem to enjoy dahlia blooms just when they are ready for a bouquet. One of the best methods of control is to manually remove the beetles and drop them into a bucket of soapy water.

If other insects such as earwigs, thrips, or aphids become a problem, you might want to consider using an insecticidal soap or a commercial pesticide. Follow label directions carefully when using any pesticide

Deer problem? Dahlias are low on the deer’s list of favorite foods. While dahlias are not “deer-proof,” they are deer resistant.

Where to Find Dahlias?

Where not to find dahlias! You’ll find them everywhere that seeds or plants are sold: garden centers, farmers’ markets, supermarkets, etc. They’ll be in stores from late winter right through spring.

Here are some of the mail-order sources recommended by the National Garden Bureau.

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Dahlias: few other garden flowers have so much to offer!