Propagating Dahlias Using Pot Tubers

By Steve K. Lloyd

Copyright © 2024 All Rights Reserved

Greetings once again, dahlia nutters! Today we are taking a trip across the pond for a lesson in dahlia propagation courtesy of our friends in Great Britain, where they have been breeding, growing and propagating show-winning dahlias for more than 200 years.

In the UK, dahlia enthusiasts are a bit more adventurous than many hobby growers in the United States, and many are keen to box up their field tubers in late winter to start them on for making cuttings. 

Pot tubers—also called sink pots or (in the olden days) “pot-roots”—are a common way of multiplying dahlias stock for trading, selling, and for over-wintering prized dahlias through the gray British winters. They are starting to catch on in this country as well, but there remains a bit of confusion about what they are and how they can be used.

So slip on your Wellies, grab a jumper, and let's head out to our glasshouse for a look at propagating dahlias with pot tubers!

Dahlia growers with a season or two of experience know that there are three primary means of propagating dahlias: From seed, from vegetative cuttings, and from tuber division. Because dahlias do not grow true to type from seed, growing dahlia from cuttings or tubers are the only ways to create an exact duplicate of the parent plant.

In the United States, the most common way to grow dahlias is to plant individual tubers which have been separated from a tuber clump grown the previous season.

Due partly to a long tradition of growing dahlias in small plots and community garden allotments, British dahlia growers have long adopted an efficient and space-saving method of growing dahlias in small 4-6" pots, which are sunk into the ground and grown for an entire season. 

This method induces the formation of pot-bound roots and tubers, resulting in a compact tuber clump that carries all the same genetic information as field tubers, which have been grown in the open through the previous season and are typically much larger.

Although enterprising American dahlia growers are beginning to sell pot tubers, in this country they are primarily seen in the bagged displays at garden centers, big-box retailers and grocery stores. 

These tubers are grown and packaged in Holland, which leads the world in production of flowers as an agricultural product. To achieve this volume, Dutch dahlias tubers are grown in massive fields where they are harvested by machine, cleaned and sorted on conveyor belts, then packaged and shipped to the US for distribution and sale.

This process results in a great deal of breakage and damage to the fragile dahlia tubers. In addition, Dutch-grown dahlia tubers are well known to carry a higher incidence of bacterial diseases than hand-cultivated domestic dahlias.

Flourishing in northern England since 1921, Halls of Heddon sells dahlias and other flowers across the British Isles. They do not sell individual dahlia tubers at all, but instead market hundreds of dahlia varieties as both pot tubers and mini plants, which are rooted cuttings produced annually in their expansive greenhouse.

Pots tubers can be planted directly in the garden and grown into full-sized dahlias, but they excel at producing shoots that can be taken as cuttings. Even a small, pot-sized dahlia tuber clump has multiple eyes which will produce growth, meaning that a pot tuber will grow more harvestable cuttings than even the more robust single tuber.

Sink pots are used by noted American dahlia breeder Kristine Albrecht as a space-efficient way of producing a large number of seed-grown dahlias in a small space. Kristine is the author of two excellent dahlia books, and generously shares her dahlia knowledge in an informative series of videos on social media.

Because she is creating hybrids, only a small percentage of the dahlias Kristine grows from seeds have characteristics she's looking for. To find out, she needs to grow all her plants to the size where they produce a bloom for her to evaluate.

If she were to plant each of her seed-grown dahlias with a normal 12-24" spacing, Kristine’s garden would have room for just a fraction of the plants she can grow in sink pots. The dahlias that meet her breeding goals are left in their small pots to produce a compact tuber clump, which she then uses to make cuttings and further propagate that hybrid dahlia.

Gareth Rowlands is a British research geneticist and dahlia expert. In his book "The Gardener's Guide to Growing Dahlias", Dr. Rowlands writes that many growers find dahlia tubers grown in small pots produce healthier tubers, with higher vitality, than field tubers. He also explains that many dahlia varieties are known as poor tuber producers, and often fail to make tubers that can be successfully stored over the winter. Growing these in sink pots and saving the miniature clumps, he writes, "may be the only way to ensure stock for the following season."

English dahlia grower and exhibitor Ian Sutherland agrees. He writes on social media, "I do make some pot tubers of some varieties that struggle to make a field tuber... they are my security so I do not lose the varieties."

But why make cuttings at all? Why not simply plant dahlia tubers—whether individual field tubers or pot-grown clumps—and call it good?

Keith Hammett is a botanist and dahlia breeder in New Zealand. Dr. Hammett has more than 50 years experience hybridizing and growing dahlias—he is the originator of the beautiful, dark-foliage 'Mystic' series of dahlias, among others. 

He has explained that dahlias grown from cuttings are healthier than ones that originated from tubers, for several reasons. One is that the soil nutrients have a more direct plant to the cutting-grown dahlia, since they begin life without tubers that water and nutrients must travel through on the way to the plant.

In addition, tubers that have been stored since the previous season are already a year old, compared to cuttings which consist entirely of brand-new plant material. Just as a human body accumulates aches and pains as it ages, plants too can exhibit genetic or systemic failures the older they get. 

A new dahlia cutting is primed to deliver peak growth, healthy foliage, excellent flower form and brilliant bloom color throughout its first season.

Dahlia exhibitor Ian Sutherland agrees. "If like me you grow for show, in the main a single struck cutting will produce good strong plants with better more vibrant blooms than from a tuber."

Whether you are growing dahlias for exhibition, to produce a bountiful stream of fresh-cut blooms, or to create a colorful garden oasis abuzz with pollinators, don't be afraid to try growing from dahlia pot tubers. Learn the difference between store-bought tuber clumps and those produced by small local growers.

If you're interested in experimenting with propagating dahlias from cuttings, or if you'd like to learn more about growing dahlias from seed, check out the articles to those topics linked below. Happy growing!