A Love Affair with Dahlias

By Laverne Dahlin, Master Gardener

What is there not to love about dahlias. They are easy to grow, and provide great satisfaction with their eye-catching flower sizes, various shapes, and many colors (except true blue – a color that hybridizers are currently working on). They attract bees, butterflies, and hummingbirds, and are even deer resistant. Dahlias are a beautiful statement garden plant that will bloom mid-summer until the first frost and are exceptional cut flowers for arrangements. There are many species and cultivars so there is something for everyone.

There are over 40 species of dahlia with over 20,000 cultivars. Dahlias belong to the Asteraceae family. In the 16th century, dahlias grew wild along the hillside in parts of Mexico. The Spanish brought the flowers from Mexico to the Royal Botanic Gardens in Madrid towards the end of the eighteenth century, and Antonio Jose Cavanilles, a taxonomic botanist, named them in honour of Andreas Dahl (1751-1789), a Swedish scientist and environmentalist. Dahlias have now been in cultivation for over 200 years.

Dahlia tubers are readily available at garden centers for purchase, several vendors provide catalogues that prominently feature them, and large box stores sell a limited selection of cultivars. Initially, I purchased from some of these suppliers, but most of the tubers I planted last year were dug up and saved from previous years, with some being 4-5 years old. I prefer tubers that are a product of Holland as they have provided the greatest success in my gardens. Buying dahlias is an excellent investment if you have the space and time to save your tubers.

The American Dahlia Society (ADS) classifies dahlias according to their flower form, flower size and colour.
A photo illustration and description of each of the flower forms can be found on the ADS website. Some of my favorite dahlias, belong to the Formal Decorative, Informal Decorative and Ball cultivars that have grown well in a south exposure informal mixed garden. The Dinnerplate ‘Kevin Floodlight’, its blossom is classified as Formal Decorative form, has gorgeous golden-yellow 18-20 cm double blooms. ‘Tartan’, an Informal Decorative, has an eye-catching burgundy petal with contrasting white tips and 20 cm blooms. ‘Emperor’ is a dark purple and has 15 cm blooms. I also like to scatter the garden with the Informal Decorative cultivar ‘Café Au Lait’ which is an off-white with 20-25 cm blooms and is a favorite cut flower for weddings. I love the dramatic large size of these dahlias. Although not an official dahlia class the very large dahlias with blooms >20cm are often referred to as Dinnerplate dahlias. I am experimenting now with the intriguing Ball dahlia in yellow, purple, pink and white petals that are a perfect global shape, and will provide some interest and movement in the garden. Because these plants are mid-summer bloomers, I intersperse annuals for pops of color while patiently waiting for the explosion of blooms in July and August. This is truly nature at its best.

Dahlia ‘Kevin Floodlight’
Dahlia ‘Emperor’
Dahlia ‘Café au Lait’

First decide where you want to grow your dahlias – in a dedicated bed, along a fence or building, as a border, a mixed or dedicated garden or in containers. This will determine what cultivars to select as they vary in size. It is not recommended to plant the larger cultivars such as the Decorative forms as borders or in containers. Plant low growing dwarf size cultivars such as the Collarette dahlias that are flat with a small inner ring of flower petals around the center. The attractive flowers may be solid or bi-color and are generally small. There are Collarette varieties that have attractive darker foliage.

Collarette dahlia

To select tubers, they should be firm and sold in a medium that will prevent them from drying out. Clumps of two to three tubers are usually packaged by the vendors in peat moss or wood shavings. If you buy early, be sure to store them in a cool (50-60 degrees Fahrenheit, 10-15o C), dry and dark place, like a basement, until you are ready to plant.

Healthy Ball dahlia tubers.

To plant your dahlias first lay out the area with makers to provide enough space for the plants to have good air circulation – at least 16-24 inches is best for the large cultivars and 12-14 inches for the dwarf cultivars.

Dahlias require humus-rich well drained and slightly acidic soil with pH of 6.5 to 7. You can amend your soil with sand and compost to improve drainage. If it is too wet, the tubers will rot. A sunny location that will provide at least 5 to 8 hours of sun per day is required to grow healthy dahlias.

Should you desire earlier blooms, you can start your tubers indoors 6-8 weeks before transplanting outdoors after the last frost. It will take 3 to 4 weeks for the foliage to emerge. Dahlias are tender plants that do not like to be cold. Soil temperature should be 60 degrees Fahrenheit/15oC, before transplanting outside. The plants reach maturity and bloom about eight to ten weeks after planting. Traditionally I have planted the tubers outside the end of May in a sunny south exposure garden and I enjoy flowers by mid-summer.

Dahlia foliage emerges from the eyes at the top of tuber crown. Plant the crown 2 inches below the soil. Water thoroughly after planting.

To care for your dahlias, they like water and like to be fed. Be sure to use an organic fertilizer such as 5-10-10 or 10-20-20 every 3 to 4 weeks for best results. Deeply water 3-4 times weekly to keep the plantings moist but not wet. Containers will need daily watering and bi-weekly fertilizing. You can pinch shoot tips after 2 to 3 sets of leaves and two weeks later to encourage fullness. You can remove side buds to nourish the main flower stem should you want horticultural show-worthy flowers. Remove spent flowers, because the more you cut, the more they bloom. Several varieties will require staking.

Collarette dahlia in container.

Slugs and aphids are attracted to dahlias. Slugs can decimate young plants overnight. Placing crushed eggshells around the plant will keep slugs away from the stems of your plant. Slugs can be hand picked off, or a dish with beer placed strategically could attract the slugs more quickly. To control aphids, direct a strong as possible stream of water to the tops of the plants where aphids congregate. Frequently inspect your plans for these pests.

Dahlia tubers are Zone 8 and therefore not winter hardy in Manitoba. Leaving the leaves after the flowering will nourish the tuber for next year’s growth. Carefully dig up your tubers after the first frost darkens the leaves. Remove the top growth leaving two to three inches, shake off the soil, label your tuber, and then lay them in the sun (or in a garage for a few days if there is risk of frost) to dry. Store in a container with a medium of your choice. There is much debate on storage medium. Instead of peat moss, some gardeners opt to use vermiculite, wood chips (wood shavings used for small pets), or sand. Others wrap tubers in newspaper, or plastic wrap. There is no single answer for storge medium, experiment what will work for you. Lay the tubers in a single layer and cover with medium.

Store the complete crown, if possible, or divide it with 2-3 tubers. I find the eyes on the top of the crown are more noticeable after storage, so it is easier to divide in spring.

During winter storage inspect the tubers periodically to be sure they remain in good condition. If they appear overly dry spritz the medium with a spray bottle. If the tuber feels mushy, discard it before it infects the rest of the tubers.

Come spring check your stored tubers, separate the healthy and discard the wrinkled or rotten ones. You are now ready to start planting around May long weekend! You can plant the complete crown or divide them into 2-3 tubers per plant.

Should you want to see a spectacular display of various dahlias in full bloom, visit the English Gardens in Assiniboine Park in mid-summer. Last season they had several Ball, Decorative, and Cacti cultivars that may inspire you to grow your own dahlias.

Dahlia ‘Muskoka Sunset’
Dahlia ‘Park Princess’ (cactus form)

Photos by Laverne Dahlin, except Dahlia ‘Café au Lait’, Dahlia ‘Park Princess’ and Dahlia ‘Muskoka Sunset’ by Marilyn Dudek

Published: May 2023