Dahlias: To Dig or Not to Dig

11/06/202203/13/2023

field featuring 100's of dahlia varieties grown for cutting

Master Gardeners share their overwintering techniques

By Anne Hays, Skagit County WSU Master Gardener

Photo of writer — Anne Hays is a resident of La Conner and a Skagit County Master Gardener

Dahlias, first imported from Mexico and Central America, provide a rich and enthusiastic floral display in Pacific Northwest gardens. Continuous hybridization of Asteraceae has resulted in the development of a plethora of colors, flower forms, and sizes. The blossoms can be as tiny as the minion under 2 inches, or as large as the giant or dinner plate size at over 10 inches. Conventional wisdom incorporated in most PNW and national reference guides for Dahlia growers just about universally recommend digging Dahlia tubers in the fall and storing them in a cool dry place. The reference materials I consulted noted that while dahlias are considered hardy in our zones – digging and storing is their preferred recommendation for overwintering dahlias in our Zones 8-9.

I then looked out my window. My street garden is abundant with tall, healthy dahlias, and I didn’t dig them last winter. So, I decided to do a very unscientific survey of WSU Extension Skagit County Master Gardeners to see what their preferences and protocols for digging dahlias and overwintering them indoors.

Master Gardeners are a treasure trove of local gardening knowledge, underscoring the depth of their experience and knowledge and differing opinions. Their gardens represent a variety of unique “microclimates” found in our area. The following is a brief synopsis of replies I received with a notation on where their garden is located.

Karen from Camano has ten years’ experience raising dahlias. At first, she was a Dahlia digger, but now she leaves them in the ground after struggling with mold issues trying to store them. She cuts her 150 Dahlia stalks after the first frost, covers them with a layer of mulch and then with sword ferns and rocks to keep water from rotting the dahlias.

Among gardeners, selecting a favorite Dahlia is unique to each individual. Dahlias range in size from under 2” to over 10” and are classified into 18 categories by form. Photo by Anne Hayes

The Thomas Edison dahlia is a beautiful example of the deep vivid hues found among the fifteen different colors and color combinations recognized by the American Dahlia Society. Photo by Anne Hayes

Though the dahlia’s origins can be traced back to the high mountains of Central America, the plants thrive in the moist, moderate climate of the Pacific Northwest, blooming from midsummer until the first frost.

Jan from La Conner is a recent convert to the in-ground overwintering school of thought. Until last year she routinely dug her dahlias, wrapped them in newspaper and stored them in plastic. She left them in the ground last season and intends to again, though she plans to cover the planting area with cardboard and heavy mulch.

Rin from Anacortes describes her soil as sandy and has determined her tubers do best when left in the ground -- mostly because she hasn’t found an overwintering strategy that doesn’t result in mold issues.

Linda from Orcas Island digs her tubers, being careful not to damage or pierce the tubers. She then rinses the soil and lays them flat to dry for a day or two. She brushes the tubers – inspecting them for any signs of rot and places them in cardboard boxes using vermiculite. She places the top of the box on the tubers after barely moistening the vermiculite. She does not allow the tubers to touch and stores the box in her basement.

Allison on Samish Island digs her dahlias and stores them in cardboard with peat moss on top in a cool dry place in her garage.

Among all the WSU Extension Skagit County Master Gardeners I contacted, dividing dahlias is typically undertaken every three years in the spring. Many of the local Master Gardener’s caution that new Dahlia starts are “slug candy” and recommend careful slug containment and control at the start of a new Dahlia season. Dahlias need good soil, enriched with organic matter and some recommend bone meal. Planting depths vary from 6 inches to one foot, and many recommend placing stakes for larger varieties as you plant the tubers to avoid damaging the tubers.

I also asked each of the responders for their recommendations on their methods for placing cut dahlias in display vases. The consensus was to place them in water as soon as possible using a fresh cut and change the water frequently. Many mentioned using a floral preservative.

What was abundantly clear from the replies I received is the passionate enthusiasm among those in Dahlia growing communities! Color, abundance, ease of growing, all were mentioned as reasons to add dahlias to your garden. If you decide to include dahlias in your garden next spring, the WSU Extension Skagit County Master Gardeners are available at Plant Clinics throughout the county to offer guidance and answer questions. Please don’t hesitate to give us a call at 360-428-4270 to be directed to a Plant Clinic near you.

Resources:

Gardening in the Pacific Northwest, Carol Hall & Norman Hall, Timber Press, 2008 Ornamental

Bulbs, Corms & Tubers, A. R. Rees, C A B International, 1992

Sunset Western Garden Book, Sunset Publishing, Edited by Kathleen Norris Brenzel Gardening in the Pacific Northwest, Paul Bonine and Amy Campion, Timber Press

Questions about home gardening or becoming a master gardener, may be directed to: WSU Skagit County Extension Office, 11768 Westar Lane, Suite A, Burlington, WA 98233; by phone: 360-428-4270; or via the website: www.skagit.wsu.edu/mg

Washington State University Extension helps people develop leadership skills and use research-based knowledge to improve economic status and quality of life. Cooperating agencies: Washington State University, U.S. Department of Agriculture, and Skagit County. Extension programs and policies are available to all without discrimination. To request disability accommodations contact us at least ten days in advance.

Dahlias: To Dig or Not to Dig

Master Gardeners share their overwintering techniques

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