All you need to know about dahlia tubers and dahlia bulbs

dahlia tubers
Dahlia tubers

What is a dahlia tuber?

Let’s start by clarifying the slightly misleading headline to this post and, in the process, clear up some important questions about dahlia tubers and bulbs.

First, there is no difference between what people commonly call dahlia tubers and dahlia bulbs. The important point here is that the weird potato-like structures that give us beautiful dahlia flowers are properly called dahlia tubers, or tuberous roots – not dahlia bulbs.

So, in answering the question ‘what do dahlia bulbs look like?’, we just need to point to the images of dahlia tubers on this page. This is because what people sometimes call dahlia bulbs are actually dahlia tubers.

In fact dahlia tubers are potato-like structures, because potatoes are themselves tubers – just edible ones.

And, it is by looking at the structure of the tubers that you can tell they are tubers, not bulbs. In the the video and article below I explain this which will help make clear.

In the video I show you all the key parts of the dahlia tuber and explain why they are important for growing and propagating dahlias

Tubers differ from bulbs in that they have no basal plate or scales. They are made of undifferentiated tissue. There are no layers or other obvious internal structures.

Dahlia tubers are the swollen roots that acts a store of energy for the plant. They sit under the surface and, with the feeder roots, they enable the plant to taken in, process and store water and nutrients from the soil.

Tubers have multiple growing points, called eyes. These originate from the crown of the tuber. It is from these that the dahlia stems grow.

Sometimes it is hard to tell whether a dahlia tuber has or will develop eyes, and which is the top or bottom. In that case, it’s best to plant the tuber on its side and see what happens.

Tubers don’t produce offsets but do get bigger each year, increasing the number of growing points.

What are dahlia bulbs?

As indicated above, technically there are no such things as dahlia bulbs. Bulbs – like those of tulips, daffodils or crocuses – consist of layers of what are fundamentally modified leaf structures (called scales) that surround the embryonic stem and flowers. As noted above, tubers in contrast are swollen roots, rather than leaf structures.

Bulbs also have a flat ‘bottom’ where roots attach (the ‘basal plate’). They go dormant in winter, having used their foliage and to take in and store the resources they need for growing the next year.

Typically, therefore, you can peel back the layers of a bulb (or cut one open) and see the mini structures inside.

Bulbs grow offsets, or mini-bulbs, that gradually increase in sized. This creates expanding clumps of plants.

So, instead of referring to dahlia bulbs, it is correct to refer to the underground parts of dahlias as dahlia tubers (and roots).

What does a dahlia tuber look like?

The picture below shows you exactly what a dahlia tuber looks like.

You can see all the parts of the dahlia tuber: the crown of the tuber, where the plants shoots stem from, the individual tubers themselves, and the feeder roots. At the top of the tuber structure are the dried stems from the previous year’s growth.

Dahlia tubers on my workbench showing key parts

In warmer climates where soil is not likely to get waterlogged, dahlia tubers can be left in the ground over winter. If you do this, cut the stems back to just above ground level and place a good layer of mulch over the soil around the stems.

Where temperatures are consistently below freezing or where soli may be waterlogged, it is best to lift your dahlia tubers at the end of the growing season and store them in a dry place.

Wait until the first frosts, because if you keep deadheading dahlias they will flower until then.

Then take a fork and loosen the soil around the plant, lifting it up as a whole. Clear away the loose soil from the tubers, taking care not to damage them.

Then, cut back the stems to about 6 inches. Turn the plant upside down and store them somewhere for a few days where the water can be allowed to drain our of the cut stems.

Once all the liquid has gone, brush away as much of the remaining soil as possible, brush with anti-fungal powder and store the tubers in a cool dry place. Use wooden crates or boxes, which allow air to flow around the tubers, so that they don’t rot.

If you take care of them in this way, you can plant out your dahlia tubers again in the spring, after the first frosts, and experience the beauty of these wonderful plants again for another year.

For a full and complete explanation of how to overwinter dahlia plants and tubers click here.

How to tell if dahlia tubers are dead

The truth is, it is not always easy to tell if dahlia tubers are dead, especially as there are really two different kinds of ‘dead’ in this context. Let me explain.

Firstly, a tuber is dead when there is no living material left within it. Typically this will be because it has dried out completely whilst in storage, because it has rotted in wet ground, or because it has succumbed to some kind of fungal disease.

A rotten tuber or one infected with fungal disease will be obvious, through the smell and/or visible signs of rot and disease.

Rotten tubers are soft to the touch, they may ooze liquid and have a distinctive sweet smell. The picture below shows the inside of a rotten dahlia tuber I found in my garden from a dahlia plant I tried to overwinter in the ground.

The inside of of a rotten dahlia tuber

Dried out tubers will be apparent if you take your dahlia tubers from storage and find that they are shrivelled and light in weight, and maybe hollow sounding if you knock your fingers on them.

But it is worth remembering that although one or more individual tubers may have died off, the whole bunch may not have done. So, it is often a good idea to remove the obviously dead sections, keeping the remainder in a suitable sandy medium to pot up in Spring, so that you can see if it will grow away then.

It is also worth trying this with any dry or shrivelled tubers, you are not sure about. Quite often, they will surprise you.

The other type of ‘dead’ dahlia tuber is one that has no piece of the tuber’s crown attached to it. The crown is the ring of material to which all the individual tubers attach. And, as you can see from the picture below, the crown contains the growing points from which the shoots of the dahlia plant emerge.

So, an individual tuber may be plump and composed of living material, but unless any material from the crown is attached to it, it will not be able to produce any shoots. It may produce roots, but in the end it dies off.

Often the individual tubers that break off from the group when you are unpacking them are like this. I usually pot them up anyway just in case, but more often than not I am disappointed.

Crown of dahlia tuber
Crown of dahlia tuber

Why do dahlia tubers rot?

Dahlia tubers rot in a few different circumstances. If your dahlia tubers have rotted, it will likely be for one or more of the following reasons:

  • If dahlias are left in waterlogged ground for any kind of extended period, the tubers are likely to rot. This is why you need to make sure you grow dahlias in soil that can drain freely. You can amend the soil for better drainage by adding grit and/or organic matter. Dahlias can be grown successfully in raised beds because raised beds drain better than natural garden border
  • If dahlia tubers get damaged, for example by a garden fork or spade when being lifted, then they can very easily get infected with some kind of fungal disease. This will also cause the tubers to rot.
  • Dahlia tubers will also rot if they are stored overwinter in damp or humid conditions, or in a material (such as potting mix) that is itself too damp.

Will damaged dahlia tubers grow?

If you have a number of tubers all attached to the central crown and some are damaged, you should removed the damaged ones. If you do this, you should have no problem with getting the rest of the tubers to grow.

If you are trying to grow a dahlia from an individual damaged tuber, you may not be successful. It probably depends what the damage is.

If there is rot in the single tuber, then it is probably best to dispose of it.

If there is damage caused by an implement, you can try to save the dahlia tuber by cutting out the damaged section and dusting it with an anti-fungal powder. Then leave the tuber for several days so that the cut can harden over and ‘heal’ before you pot in on.

How many dahlias grow from one tuber?

A dahlia tuber will grow shoots from all of the eyes that develop on the crown of the plant. In most cases, you can expect 3 or 4 shoots per plant. Obviously, there will be more shoots from more mature tubers, where there is a larger crown area.

If you pinch out the shoots when they are 18 inches or so tall, this will encourage the shoots to branch. The more branches the plant develops, the more flowers your dahlia plant will develop – as long as you continue to remove the spent flowers when they are finished.

If you divide the tubers (see details below), you will get more dahlia plants.

How long do dahlia tubers last?

In theory, if well looked after, dahlia tubers can last indefinitely.

Dahlia tubers multiply each growing season. They should therefore be divided from time to time to prevent them becoming too congested.

When you divide the tubers, you are creating a new (potential) plant. In due course, you will be able to divide the tubers of that dahlia plant, and so on through the generations of plants. This is why dahlia tubers can be said to last indefinitely.

You can divide dahlia tubers easily if you lift your dahlias for overwintering. In that case, you should divide them before waking your dahlias up for planting in the spring.

If you leave your dahlia tubers in the ground over winter, then you should dig them up to divide them every two or three years before they start re-growing in spring.

When you divide dahlias, make sure that each individual tuber or group or tubers has a portion of the tuber crown attached to it, since that it where the new shoots grow from.

However, dahlia tubers will not last indefinitely in storage. They need to be kept dry, but not parched, and they need to be covered a suitable material to keep them from drying out completely. For this you can use wood shavings, perlite, dry potting soil or newspaper.

You will probably experience some tuber losses if your store enough dahlia tubers over enough winters, which is why it is a good idea to divide your tubers to increase your stocks.

How you grow your dahlias affects how long the tubers will last

As Kristine Albrecht from Santa Cruz Dahlias makes clear, how you grow your dahlia plants throughout the season greatly influences the growth of tubers.

This is because dahlia tubers fulfil different functions at different times in the dahlia’s growth cycle. Initially, before any leaves develop, tubers act as the primary source of nutrients for young dahlia plants. However, as the leaves develop and begin to photosynthesise, the leave become the primary nutrient source, and tubers shift to becoming storage organs, benefitting from the sugars and amino acids that the leaves produce.

However, once flowers start to bloom, they have a strong demand for nutrients, especially when they are pollinated. Since flowers and seeds require significant amounts of nutrients, that leaves fewer resources for tuber growth.

Therefore, if you want to ensure that dahlia tubers develop sufficiently well to perform in succeeding years (and provide divisions), you need to actively take care of dahlias when they are budding and flowering. Proper care, such as disbudding, cutting blooms and deadheading, can reduce the demand on the plant and lead to the growth of more substantial tubers.

Thus, neglecting flower care can result in poor tuber production. The care given to plants specifically grown for tuber production greatly affects their yield, though differences between different dahlia varieties also play a part.

If you think about this in terms of evolutionary importance, it makes sense that plants prioritise seed production over tuber production. Seeds contribute to genetic diversity, whilst tubers are clones and do not progress the species in the same way.

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Dahlia Dalina® Maxi Tampico PBR

 out of stock
Thompson-morgan.com
as of June 22, 2024 1:19 am

Carpet your garden in rich colour and keep the weeds at bay with our perennial creeping phlox collection. This hardy rockery plant has many uses ? underplant trees, shrubs and tall perennials, soften path edging or cover awkward slopes and banks. Height: 15cm (6). Spread: 50cm (20).

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Dahlia 'Dalaya Yogi

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1 x Dahlia Purple Haze - 9cm Pot by The Secret Gardening Club

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Secretgardeningclub.co.uk
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Dahlia tubers are ideal for propagating, for pot growing, or for planting out directly once all risks of frost are past.

1 x Dahlia x Hybrida Mystic Sparkler - 9cm Pot by The Secret Gardening Club

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Perennial from Tuberous root. Deer resistant. Showy star-shaped flowers are held above dark purple foliage. Makes a good border, patio and container plant. Attracts butterflies & hummingbirds. Bloom time: summer-fall. Bloom color: hot flamingo pink w/ warm yellow halos & dark central discs. Height: 24-30 inches. Exposure: sun/part shade.

1 x Dahlia Mystic Enchantment - 9cm Pot by The Secret Gardening Club

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Dahlias are old-fashioned favourites for the flower garden. Plants are usually treated as tender perennials, the tubers lifted in the fall and stored indoors for the winter. This selection from the Mystic™ series, features stunning purple-black foliage with a great show of starry single orange-red flowers with a gold-brown eye. Midsized habit. Excellent in containers and in the border. Be on the lookout for aphids and spider mites, and control these as soon as noticed. Terrific for cutting.

1 x Dahlia x Hybrida Mystic Dreamer (Pbr) - 9cm Pot by The Secret Gardening Club

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Secretgardeningclub.co.uk
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Fashionable filigreed deep mahogany to black foliage topped with pale pink-white blooms with a strong magenta stripe on the individual petals.


More on Growing Dahlias

You can also get much more guidance on growing dahlias in these posts:

Love Dahlias?I've written the book on them

Check out my comprehensive step by step guide, with plain language explanations and ultra-useful images and illustrations. This is for you if you love dahlias and want to the best out of the dahlias you grow.

7 thoughts on “All you need to know about dahlia tubers and dahlia bulbs”

    • Hi Cheryl, yes you can certainly leave tubers in where conditions are appropriate. Tubers need to be lifted to protect them from damage by frost or waterlogging. Some frost (i.e. down to max -5 degrees celcius) can be tolerated, but not over a prolonged period or when the ground is sodden. If tubers are left in the ground and exposed to prolonged waterlogging, they are likely to rot.

      Reply
  1. I was ordering dahlia tubers last night and saw dahlia seeds available. Seeds? Is this a thing? If so, do they take alot longer to establish than a tuber? Also, if that was the case would it be worthwhile to start in a greenhouse during winter to get a head start? Need lots of them blooming by August for a backyard wedding

    Reply
    • Hi Anna, yep, seeds are definitely a thing. They often come in mixed packets, so you’re not entirely sure what your getting. If you go down this route, Check the descriptions well to make sure you are going to end up with kind of plants you want.

      The good thing about growing from seed is you can get a lot of plants, much more cheaply than through tubers.

      Most should flower in the first year, but check the details for the seeds you are looking at. But, you are right, start them off in the greenhouse and grow them on until they can be planted out after the last frosts.

      Hope that helps. Good luck with the wedding.

      Reply
  2. Hi. I have lots of Dahlia tubers that I haven’t put in the ground yet. I’ve been sick and believe I don’t have time as I should have had them in by May. They are currently in bags that identify the flower. How should I store them over the winter, so I can plant next year.? Or if I put them in the ground will they have enough time to flower. I live in Washington State.
    Thank you

    Reply

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