Digging and Dividing Dahlias

This is dahlia digging and dividing season. If you’ve ever wanted to know how to divide them like the experts, here is a rather long post on how some growers approach this challenge.

Digging, dividing and storing dahlias is a topic which comes up often not only at dahlia society meetings, but I get asked this question all the time – when  I am out and about, speaking on-the-road and in my inbox.

 I should preface this post with the disclaimer that there are far more qualified people out there who can provide advice on dividing dahlias, my preferred methods (both – the ‘right way’ as well as my ‘lazy-ass way’ work for me, so I will share both of them (Squirm dahlia society experts!).  Then again, most of them kind-of know what they are doing, so I would also highly suggest that you check to see if your local dahlia society has a page on the best method for your region.

Some growers especially those who raise many dahlias, suggest cutting stems down before frost, which is how I began dividing my dahlias this year. Always remember to save the tags so that things don’t get mixed up.

However, I will warn you since there is more than one way to dig and store a dahlia, few experts will agree on precisely what is considered the most perfect way. I started to augment my method this year after seeing a large dahlia farm showing images of how they store their dahlias, but I just found out that the wood-shaving they used have been causing problems, and that they are switching to vermiculite.

Dividing and storing dahlias is a topic which certainly raised heated disagreements even at dahlia society meetings, but usually everyone will err on ‘if it works for you, keep doing it. Good advice, but…..first, you will need to find that ideal method that ‘works for you’.

A search through your favorite content providers will result in a list of various methods – some as contradictory as “just wrap tubers in plastic wrap” (hey – they say it works!)  to “just dig ’em and store em’ in your cellar until spring” (basically, my ‘last-ass’ method – except I store mine in the greenhouse under a dry bench where it is cool).

All of these methods share some similar tips.

First, dahlias are a bit like potatoes, so the ideal storage conditions are usually dark, cool and slightly damp – just enough so that the tubers don’t sprout. Beyond this, things can get kooky.

Most of my clumps were loaded with tubers this year. Probably due to good fertilization with low nitrogen, and high phosphorus and potassium.

Look to the wild dahlia for hints.

Understanding where dahlias grow in the wild helps one understand what conditions to try to recreate when growing them in the home garden. Primarily dahlias grow in the more mountainous areas in Mexico in volcanic soil which quickly goes dry in the winter. Summer brings cool, rainy weather and then even cooler if not cold, yet dry winter. Trying to recreate this environment at home can be difficult. Difficult unless you live in an old farmhouse, that is.

This fact helped me understand a misconception I had made about dahlias – mainly that the require heat and humidity. Maybe my misunderstanding evolved from the very basic knowledge that dahlias come from Mexico and central America, but the truth is that dahlias grow in the cooler conditions found in the volcanic mountainous area of southern Mexico. No wonder they perform best in Oregon and Washington State – and not as well here in New England, that is until the cooler weather of September comes along.

Since we are talking about the proper storage of dahlias, their native land informs us as well. Dahlias like to be in a medium which is dry for the entire winter (volcanic soil in Mexico, but vermiculite in my cold cellar), Dry and cool are the conditions one wants to find, but also the addition of a medium in which they sleep is essential as well. One doesn’t want to store a dahlia out in the open, or in a sealed plastic bag. They need material around them, to help them breath and to  retail moisture, but they also need air so that they don’t rot.

Where most struggle however is with dividing, and proper winter storage.

So, why bother dividing dahlias at all?

  Well, while it’s true that in the wild no one is digging and dividing dahlia plants, the reason we divide them is to propagate the plant (sharing or selling the extras) and to plant out a smaller tuber in the spring which will produce a healthier, more robust plant.

but remember that modern dahlias  that we all know and love are nothing like the wild species, of which there are nearly 30. Today’s dahlias are ancestors dahlias which were first raised as a food crop by the Aztecs, and it is this reason that all modern dahlias have larger tubers than the wild species. Much like all of our carrots and potatoes, the dahlia has been genetically altered through selection.  Sorry.

Understanding Dahlia Tubers – they’re not exactly the same thing as potatoes (but close)

Just some basics here –  In the plant world, there two kinds of tubers.  Stem tubers and root tubers. Potatoes are stem tubers, and sweet potatoes are root tubers. No need to go into details beyond the basis facts that root tubers function differently (those old potatoes on your counter sprout all over, while root tubers sprout stems and roots from more defined real estate on the the tuber.

Dahlias are botanically root tubers. This means that they are more like sweet potatoes than regular white potatoes. In short, dahlias produce stems from buds only located on or near the tissues where the original plant stem was attached, and no where else. So if you break a nice, fat tuber off of your clump, and if it doesn’t have a bit of the old mature stem attached, it’s useless and will never sprout new grow. Toss it.

This one clump  – a variety which only produces a couple of tubers, is still fine. Not all plants will produce a massive clump like the AC Ben on the left, a dinner plate variety. This center clump came from the anemone flowered dahlia ‘Alpen Fury’.

It’s not always about size.

Sorry to disappoint you, but….yeah. Just sayin’.

Have you ever felt cheated when ordering a dahlia and you get a tiny little tuber? How about when you dig your clumps – are you only keeping the large sweet potato sized tubers and tossing all the others?  DOn’t over think this, but I have heard this over and over again, the best dahlias often grow from medium to smaller sized tubers (it depends on the variety), and sometimes the best come from cuttings which serious dahlia growers will take from their tubers which they start early under lights or in a greenhouse.

This giant tuber was too big, so I cut in half, with the top portion which was a large as a sweet potato saved to dry and scab over, and this bottom portion discarded.

I even know of one dahlia grower refers to those very large, fat dahlias tubers as ‘Lazy Tubers’. (sorry Donna!). Many pro’s will cut the larger tubers in half, discarding the bottom half or third, and allowing the tuber to dry before storage. I even saw this being done at a dahlia tuber demo a couple of weeks ago by dahlia expert Marjorie Schneer from Connecticut who’s been growing them for nearly 45 years.

The ‘mother tuber’ is obvious in this clump. It is older and not worth saving for another year.

Toss the ‘Mother Tuber’ from last year.

I hate to say this, but it is true. When dividing your clump, be sure to toss the old tuber ( with Swan Island, it’s the one with the name printed on it – you know!). This is referred to as  ‘the mother tuber’ by exhibitors.  She’s tired, and really won’t be worth saving, at least, not worth it if you have a handful of new, feisty young tubers to plant. You know – just like real life!

Don’t email me! Don’t be greedy and run a puppy mill. Hey – she’s done her job, she just needs a rest.

A washed clump of dahlia tubers ready for dividing. Can you see where the tubers attach to the main stem? Also, note the old ‘mother tuber’, which should be discarded.

Do I even have to divide my dahlias?

No, you could just replant the entire clump, but don’t expect a bigger plant. People divide dahlias mainly to get healthier plants for the next season, and a load of more tubers. It’s really about economics  and really – – the best part about dividing dahlias, is that single 9 dollar tuber that you set out in the spring, just produced  10 tubers.  Cha-ching. IF you are growing for cut flower or for show, one really needs a row of each variety, not just one plant of each. Oh – the same goes for those in the border – remember, the British set out 5 -10 of each variety together in the perennial border.

A freshly dug clump loaded with tubers. Sometimes I just keep my clumps intact like this until I am ready to divide later in the winter, but most people suggest doing it in the fall just after digging.

How to Divide Dahlias – First – The ‘Proper’ Way

Once your clump is carefully dug, wash it off with a hose and dry it off in the shade.  If there is any trick it’s that the tubers when first dug are crispy-fresh, and can easily be damaged, so dig and hand carefully. I’ve been taught to cut the stems just before or after frost, leaving about 5-6 inches which you can use as a handle (yet never try to pull them out of the ground this way). Dig with a pitch fork or shovel around the stem at some distance depending on the variety (perhaps 1.5 feet) so that you don’t pierce any tubers (I did ruin some this weekend just by guessing).

I place a new tag along with the entire clump and line them up on a shelf, in an old seed tray or in bread trays until they are dry, perhaps a few days.

Wash all soil off of tubers to reduce disease and insects if you are storing in the house or in material such as vermiculite. Allow clumps and tuber to dry first, by setting out of direct sun.

Decide if you are going to divide now or not

I should have said ‘don’t wash them off, if you are going to wait until spring to divide, but since that is the riskiest method, try to plan on dividing now.

If you are wondering why most books and dahlia grows advise one to wait until frost to kill the foliage, then wait a week or so to dig and divide, it’s because of this. There are little pimply-like buds which will emerge if the plant is stressed by frost, but these same buds will emerge if you cut the main plant down before frost, and may dahlia growers do this, especially if one have many plants.

The goal here is to dig and divide all of ones dahlias before a hard freeze, which can damage the tubers themselves. SO know how many you can handle before digging and dividing, and the last thing you want to deal with a hectic drive home when a hard freeze is forecasted.

If you are not going to divide in the fall, the entire clump could be stored in a cool, dry location but plan on keeping some soil around the tubers (or store them in a medium like vermiculite). I know, soil can contain worms (eew) or insects, so this could raise the risk of disease, but I have had no problems with this.

Before Cutting and Separating,  Manscape First

Begin by cutting off all unnecessary roots, rootlets and secondary tubers not attached to the stem. Also, look for damage at the intersection where a tuber attaches to the central stem. Often this becomes bent during digging and extraction, and if crushed, will only rot in storage.

Remove smaller feeder roots and the very this tubers. These are near the surface, and are useless.

With a sharp knife or secateurs, begin dividing the stem portion of the dahlia. Sometimes, one can easily cut off a section of the stem base with a tuber attached, at other times, the tubers may be too close together to allow a clean cut and one tuber may need to be sacrificed. Don’t feel bad, just do your best starting with the healthiest looking tubers.

A cut and some careful pulling to first separate clumps into smaller, more manageable pieces. Take care in tearing clumps this large, one can easily ruin a fine tuber which could get split or separated from the main stem tissue.

Then cut (carefully!)

Cutting off tubers with a bit of stem is more difficult than it looks. Stems are woody and hard this low near the ground and not crispy like celery as the stems are near the top, so be sure to have the sharpest knife possible, or use sharp clippers.

This stem portions is perfectly positioned so that three tubers can be saved, each with a portion of the set and tiny buds.

Cut tubers with stem ends like this are labeled with a Sharpie and are ready to be dried well for a few days (on the shady part of the potting bench in the greenhouse where they won’t freeze, or you can do this indoors), and then they are packaged in containers with wood shavings or vermiculite for the winter.

Be sure to keep varieties separated, especially before you label them. I always set aside and tore one variety each to a container – I use plastic shoe storage containers (with the wood shavings or vermiculite). Keep lids on loosely and set in a cool, dry place for the winter. A mouse trap can be handy.

How to store your dahlias for the winter

Knowing what dahlias in the wild experience with winter dryness when still in the ground – – one can begin to construct the ideal winter storage conditions based on what they can offer at home.  Look for offering cool, and relatively dry environment with slight moisture preservation provided by the medium they are stored in – enough so that the tubers don’t evaporate, but enough to keep them turgid. This can mean vermiculite which is dry, Perite, dry peat or wood shavings are also used by some.

Storing tubers in an air-tight container is not advised, as some air circulation is helpful – remember, these tubers are full of stored water, and one will want to avoid rot and decay. I know of members of the dahlia society who store bulbs in large 1 gallon Ziploc poly bags in large vermiculite, with the tops of the bags kept open, and I know of a dahlia nursery who stores bulbs in plastic shoe boxes in cedar wood shavings, with loose fitting lids in a cool potting shed that doesn’t freeze.

Do not store tubers in a refrigerator crisper, or in a warm, dry closet.  The gasses released by some fruit in a refrigerator will inhibit sprouting, and a warm closet will simply dry out the tubers. A frost-free unheated garage which does not freeze might work, if the tubers are stored in material where they can be dark, or a cardboard box with newspaper in a cool spot in the cellar might work in a modern home. In our 100 year old house, I have no shortage of winter storage areas, for even my bedroom closet would work! But I will most likely keep some under the bench in the greenhouse and the rest along a bench in our unheated dirt-floored cellar.

Tools that you may find useful.

a. Sharp, clean cutting tools ( and alcohol swipes to clean between cuts – really. Dahlias a highly prone to virus and they can spread faster than herpes. We had to ‘put down’ about 10 plants this year due to a window pane virus.
Also, you will need a sturdy surface on which to cut.
b. Hose, bucket and trays to wash tubers off with, and places to store tubers to dry.
c. A waterproof industrial Sharpie – not just water resistant, but Industrial- water proof. The ones with the red label – Amazon is best, or the source above – you most likely won’t find these at a Staples.
d. Storage containers in which to store tubers. Cardboard boxes, plastic bins or bulb crates depending on where you will store them.
c. Storage medium, ranging from wood shavings to dry peat, vermiculite or Perlite.

Dahlia tubers ready for storage, look at how many I got from just one clump! The smaller ones went into the trash, the rest will be added to our New England Dahlia Society tuber swap (later this month) and a secret members tuber sale in the spring.

The process outlined in steps as a refresh:

1. Assemble all your tools, and begin by cutting off the tops of your dahlias about 8 inches above the ground, and by carefully digging out as many clumps as you can handle in a day (about 10 for me).
2. You can dig your dahlia plants up before the frost kills them, (contrary to what many advise). Many expert growers do this to save time in the fall.
3. Dig carefully, and pull gently out of the ground, dahlia tubers must be attached to the main stem without damaging this connection, and it is very easy to bend and crush this connection thus damaging it while removing the plant from the soil.
4 If frost has killed your plants, then just cut off the main stems of your dahlia, but leave a 4-6 inch stem as a ‘handle’.
5. Wash off all the soil with hose  and set aside to dry – not in the full sun. Buds or eyes will become more visible in a few days, but will eventually disappear in a couple of weeks – this is your window when you can divide them, or you will need to wait until spring.
6. When ready to divide, use sharp clippers, secateurs or a very sharp knife and begin by giving your clumps a haircut. Remove small feeder roots and smaller roots first, and begin to identify the old mother tuber (which you will toss) and the better tubers worth saving. You will end up discarding many tubers. Be sure that portion of the original stem or have buds showing when dividing.
7. If storing in cellar and not washing off or dividing until spring, keep tuber clumps slightly in a medium (either the soil, or vermiculite). Rarely will tubers survive simple cleaned off and open to the dry, winter air indoors.
8. Some alternate storage methods include wrapping tubers in newspaper and then storing them in cardboard boxes in a cool location like a cold frost free cellar or garage.