Annuals Bulbs Plant propagation

When to Divide Dahlias: Fall or Spring?

Freshing dug dahlia showing multiple tuberous roots.

By Larry Hodgson

Question: (Re How to Lift and Store Tender Bulbs for Winter) Thanks for this information. I don’t have much experience with dahlias. Is each oval tuber hanging from the stalk a new plant? If I replanted each oval tuber separately, would I get more plants? Or do I just replant the entire clump next year? Thanks for your help!

Marla

Answer: Whether you divide dahlias or not depends on their state when you dig them up. If there are only a half dozen or so tuberous roots, there is no need to divide, but there probably will be in a year or so, as the plant grows. If there are a lot of tubers, it’s best to divide them. If the clump becomes too dense, that can negatively affect bloom.

As to whether you can divide dahlias in the fall, yes, you can do that … or you could do so in the spring. And it’s not quite as simple as just cutting a tuber free. Here’s why:

The tuber of a dahlia is a thickened root, called a tuberous root, a sort of a storage area for the starches, sugars and minerals that help the plant get through the winter storage to come. However, the eye (bud) that will give a new plant in spring doesn’t grow directly from the tuberous root: it’s not like a potato tuber that has eyes right on the tuber. With dahlias, the eye is found at the base of the stem just above: the crown. 

When you divide a dahlia, each tuber or cluster of tubers must have an eye: a small bud at the base of the stem. Photo: F. D. Richards, Flickr

Therefore, you can’t simply cut the tubers free: if you do, nothing will grow. (I once watched a garden commentator who clearly did not know dahlias do this on television: I cringed in horror as he cut off each root at its tip, thus killing it!) You need a tuber with a piece of stem attached and, furthermore, it has to be a piece of stem that bears an eye. Often, you’ll end up with two or three tubers (or even more) sharing just one eye. A tuber without an eye is said to be blind, as no plant will grow from it.

Dahlia tubers cut into clusters, each with an eye. Photo: freshcutky.co

Experienced growers do 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, a little bump at the base of the stem. 

For beginners, it might be best to wait until spring to divide the tubers. By then, the eyes will be well formed, even sprouting, so it will be very clear where you have to cut. 

The downside of waiting until spring to divide your dahlias that you may need to saw through the now 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. Not all dahlia stems are that hard to cut … but some are!

The intermediate method is to divide your tubers into clumps of two or three in the fall rather than individually, always, of course, including a section of stem. For example, you could cut the clump in half (cutting through the stem from above), then cut the half clump in half. That way, you almost always have at least one viable eye per section the plant can grow back from. 

As for winter storage, you can reread How to Lift and Store Tender Bulbs for Winter

Larry Hodgson is one of Canada’s best-known garden communicators. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He has written for many garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 60 other titles in English and French. He is a past president of the Garden Writers Association (now Garden Communicators International) and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. He resides in Quebec City, Quebec, Canada.

8 comments on “When to Divide Dahlias: Fall or Spring?

  1. Good info appreciated by all who love dahlias.

  2. Your link to your previous post on storing tender bulbs doesn’t work. When I click on the link, the page says “Expired nonce.”

  3. If the tubers are left in the ground for a week or so after they have been killed by frost it is much easier to see the eyes as they start to swell after the tops die.

    • Thanks for that tip! I usually divide in the spring and — as noted — it’s more difficult (and requires courage and determination, haha). Also, by spring I’ve forgotten how to do it (a full year has passed, after all) and have to research.

    • Excellent suggestion!

  4. I do it when I can. This is one of those climates in which we do not need to dig dahlias, but eventually do so for those that seem to get crowded. We can do it as soon as they die back, or as late as just prior to the resumption of growth. Yes, although we ‘can’ do it at any time within that range, we ‘should’ do it later than sooner, just so there is less time for them to rot before they resume growth. (The weather here does not stay cool enough to inhibit rot.) I divide dahlias by simple tearing them apart. I know it sounds harsh, but they naturally tear apart with the bud that they need. Of course, I do it carefully, and systematically, by pulling them upward toward the buds. Those that break away without buds, as well as bits that get cut with the shovel (oops) get buried too, and some of them grow. I never dig them up to see which ones grow and which ones do not.

  5. Thank you so much once again for answering my question. Your instructions are so helpful and the photos show me precisely what to do. I feel like I’m on my way to becoming a dahlia expert!

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