Garden Guy: Dahlias for a late season experience

By Bruce Bennett

Yes, it is March and still too early to plant many gardeners’ favorite flowering tuber, the Dahlia, despite what may currently be on sale at Costco and our area box stores. 

It is a ‘Look at ME!’ perennial that can stand as a specimen plant or be massed around taller shrubs and serve as a bright spot in a mixed border. 

What it can’t stand is the cold of our winters when the plant reduces down to a brown heap in the garden. So, why discuss it now – in late winter? 

The reply is an easy one: Tubers showed up in multiple booths at the recent NW Flower and Garden Show and will be on sale at the annual Puget Sound Dahlia Association Sale later this month. More about this event at the end of the column.

What is there not to love about dahlias?
Photo by PugetSoundDahlias.com

What is there not to love about dahlias?

These blousy beauties are easy to grow and provide great color spots in the landscape. They have many eye-catching flower sizes, shapes and colors. 

 After all, there are some 40 species and over 20,000 varieties of these blooming sweethearts in the horticultural world. 

Consequently, a gardener should have no problem finding the right combination of height, color and shape for the garden focal point that needs a bit of pizzazz. 

Dahlias are an attractive floral statement whether planted in the garden or in a patio container that will bloom from mid-summer through the first heavy frost. Also, as cut flower arrangements, they will brighten a kitchen table in early autumn.

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? The location will determine what cultivars to select as they vary greatly in size. It is not recommended to plant the larger cultivars forms as borders or in containers. Low growing dwarf size cultivars such as the Collarette dahlias will be excellent in porch and balcony containers.

When selecting tubers, look for firm and large ones with multiple eyes. If you buy early, like at the Flower and Garden Show, box stores or the Puget Sound Dahlia Association’s tuber sale, store them in a cool (50-60 degrees F.), dry and dark place. The basement or near the house-adjacent wall of the garage should do nicely.

Tubers. Photo by PugetSoundDahlias.com

Dig a planting hole larger than the tuber. 

Incorporate compost and a sprinkle of bone meal. Plant the tubers four to six inches deep and laid horizontally with the ‘eyes’ pointing up (as much as is possible). 

When planting your dahlias, place large cultivars two feet apart in holes that are four to six inches deep. Around one foot apart is good for the smaller varieties. 

Doing this will provide good airflow and reduce fungal issues (like Powdery Mildew). 

Dahlias prefer loamy, well-drained soil and adding compost and a bit of bone meal will improve drainage and feed the tuber roots. If the area 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. Soil temperature should be pushing 50 degrees F before planting. 

Expect the plants to reach maturity and bloom in about eight to ten weeks. 

I tend to plant the tubers outdoors at the end of May in a sunny south exposure garden. For each tuber, dig a hole 4 – 6 inches deep and plant the eyes 2 inches below the soil. This is also a good time to add support stakes. You can see the tubers and won’t pierce them with the stakes. 

Water-in thoroughly after planting. Your dahlia foliage will emerge from the eyes at the top of tuber crown. They will appreciate supplemental water and an organic fertilizer, something like 5-10-10 or 10-20-20 once a month. Depending on air temperatures, your plants may need to be watered three or so times a week. Container-grown plants may need to be watered daily. Think about pinching plant tips after 2 to 3 sets of leaves have developed and two weeks later to encourage fullness. Removing spent flowers will encourage more blooms over a longer period of time.

Dahlias’ worst enemies tend to be slugs and aphids. Slugs can decimate young plants overnight. Sprinkle your favorite slug and snail treatment in the planting bed or, get really organic, and simply hand-pick them off the plants. To control aphids, direct a strong stream of water at the infected leaves where the aphids are doing their dastardly deeds.

Photo by PugetSoundDahlias.com

Dahlia tubers are USDA Zone 8 – 10 hardy plants and, in our part of the world, there is a perennial end-of-season discussion about them: leave or dig. 

If you are an optimistic gardener, leave the tubers in the ground and cover the area with tree or fern leaves or mulch. If there are dahlias that you really love, then digging some or all of them makes the most sense. 

Carefully dig up your tubers after the first frost kills the leaves. Cut the stalk down to around 3”, shake of any extra soil and, if dealing with multiple varieties, label your tubers. Then, lay them in the sun (or protected spot if frost is in the offing) to dry. Store them in a container with a bedding medium of your choice. 

For a number of ecological reasons, don’t use peat moss. Instead, opt for vermiculite, wood chips (wood shavings used for small pets), or sand. Other options include wrapping the tubers in newspaper or plastic wrap. There is no single answer for the medium. Experiment and find what works for you. During winter storage, inspect the tubers periodically to be sure they remain in good condition. If they appear to be drying out, spritz the medium with a spray bottle. If a tuber feels mushy, discard it before it infects the rest of the tubers.

Photo by PugetSoundDahlias.com

Store the complete crown, if possible, or divide it into smaller groupings. The eyes on the top of the crown will be more noticeable after storage. So. It’s probably easier to divide in spring. Come planting time, check the tubers one last time. Put aside the healthy and discard the wrinkled or rotten ones. 

You are now ready to start planting around May. You can plant the whole crown or divide it into 2-3 tuber plants. Do enjoy these brilliant color spots throughout your summer/autumn planting beds. They are soooooo easy to grow and worth bringing color to the yard and into your home. Happy gardening!

The Puget Sound Dahlia Association Tuber Sale will be held this March 22 - 23, 2024 from 10:00am – 2:00pm, at the Bellevue Botanical Garden, 12001 Main Street, Bellevue, WA. 

Annual show at Sky Nursery
Photo by PugetSoundDahlias.com

The annual regional Dahlia Exhibition is the largest such dahlia program in the country and is scheduled to be held August 10 - 11, 2024 at Sky Nursery, 18528 Aurora Ave N, Shoreline WA. Look at the Exhibition’s flowers this August and purchase the tubers next March.

Further Reading:

• Albrecht, Kristine & Sprinsock. Brion, DAHLIAS: Seed to Bloom. 2023. Independent Publications: Santa Cruz, CA.
• McClaren, Bill. Encyclopedia of Dahlias. 2009. Timber Press: Portland, OR,
• Rowlands, Gareth. The Gardener’s Guide To Growing Dahlias. 2003. Timber Press: Portland, OR.

Garden Guy Bruce Bennett

Contributing columnist, Bruce Bennett, is a WSU Certified Master Gardener, Certified Professional Horticulturist, garden designer and lecturer. 

If you have questions concerning this article, have a gardening question to ask concerning your own landscape or want to suggest a topic for a future column, contact him at gardenguy4u@gmail.com

Read his previous columns here