How to Get Rid of Mold on Dahlia Tubers During Storage

How to get rid of MOLD on dahlia tubers, mold on dahlia tubers, dahlia tubers for sale from small flower farm

Updated January 15, 2024

My first few years of learning how to store dahlia tubers were a struggle. I’ll admit that I didn’t understand dahlias at all, or what it took to store dahlia tubers properly, until a few years into flower farming. Overwinter storage of dahlia tubers has been the biggest learning curve. Flash forward through many dahlia storage blunders and I quickly went from from 0 to 600 tubers in two years! You can, too.

I don’t have a heated barn or any outdoor buildings with controlled temperatures to keep the tubers at their preferred 45-50 degrees F, so I store dahlia tubers in my basement. An outdoor building would be ideal (lugging heavy bins of dahlia tubers down narrow stairs, along with the earthworms and wolf spiders that inevitably hitchhike their way in, is dirty work). With no other choice than my basement, I make storing dahlias work well.

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fuzzy mold on dahlia tubers

One problem that can occur when storing dahlia tubers in a basement is overly damp or humid conditions that can cause fuzzy mold to grow on the dahlia tubers. Mold on dahlia tubers will cause them to rot very quickly.

How do you know if your dahlia tubers are rotted beyond saving? If your dahlia tubers are squishy and soft, and running juice out of them when you squeeze them, then they are completely rotted and will need to be thrown in the compost pile. It happens, and learning how to properly store your dahlia tubers will help prevent rot in the future.

A note on humidity levels. You need 80-90% humidity when storing dahlia tubers so that they don’t dry out, but too much humidity can also be bad because it will be too wet and encourage mold. There is a fine line between the two, and you can only learn what works best in your storage space by simply doing it, being very observant, and taking good notes for next year. Every single person’s storage situation is different and therefore, you cannot completely replicate what someone else is doing, myself included. You must learn what works well in the space and conditions you have. For instance, there have been years where I’ve run a dehumidifier in my basement, but now that I have my storage conditions under control, I no longer run it. I’ve come to learn that storing dahlia tubers in wood chips is most effective because the wood chips suck up extra moisture in the container while also keeping the dahlia tubers insulated with enough moisture to prevent them from drying out.

It is also important to run a HEPA Air Purifier in your dahlia tuber storage space, to filter out any existing mold spores.

Anyway, this one year early on in my flower farming journey, I noticed white mold growing on my dahlia tubers (bad mold, not the good mycorrhizal fungi). By taking quick action, I was actually able to save all of my dahlia tubers from entirely rotting. Here is what I did…

timeline of storing dahlia tubers and getting rid of mold on dahlias

1) I dug my dahlia tubers at the end of October (I’m in Zone 6b where we had an early frost that year). I did not wash the soil off the tubers (I still don’t wash my tubers — in my opinion, this is waste of water and time). The soil helps insulate the tubers to help them from drying out.

2) I stored the dahlia tubers in my basement, in boxes (big mistake, plastic bins are best because boxes evaporate moisture) near the basement boiler until I had adequate time to properly pack them. In hindsight, this was really dumb and you should never do this. Do not bring unpacked/uninsulated dahlia tubers into your house or they are likely to dry out quickly.

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3) Not surpisingly, two weeks later, I realized the tubers were drying out due to the heat of the boiler (imagine that) so I moved them to a tiny insulated room in my basement that is slightly cooler. I sprayed the dahlia tubers with water (another mistake!) because I was worried about them drying out from lack of insulation. At this point, I still had not properly packed them with wood shavings (I had a lot going on in November and clearly my priorities weren’t straightr), and simply left them sitting in an open box in this cold room.

4) A few weeks later (at this point, it is December), I realized the insulated room was too humid, and combined with spraying the dahlias with water, the dahlia tubers were now all covered in a layer of white fuzzy mold.

5) I hauled all the tubers from the basement to my backyard to deal with the mold outdoors (thankfully we were having a 40-degree winter heat wave). I mixed 1 cup of regular bleach in a 5-gallon bucket of water and dunked each tuber into the bucket to kill the mold spores. I dunked each tuber for at least 10 seconds and shook them around in the water to make sure all the nooks & crannies were bleached.

6) I laid the wet tubers all out on a tarp in groups to be packed into tubs. After a bit of time, once the tubers were mostly dry, I packed them all in large plastic tubs with peat moss (Update: I no longer use peat moss due to the negative environmental impact this product has on natural peat bogs. I now use wood shavings to pack my dahlia tubers, which can be purchased in bags here). Layering the tubers with the peat moss, I made sure none were touching. I read online that cinnamon supposedly helps with controlling fungus, so I did an experiment and put cinnamon on only some of the tubers. In my very limited experiment, I decided that there was no apparent difference between the dahlia tubers with cinnamon and the non-cinnamoned tubers so I won’t be adding cinnamon to my dahlia tubers again. Sulfur also helps with preventing fungus, but I have not tried using sulfur just yet.

7) Finally, I moved all the tuber & peat-filled tubs to the basement corner, far away from the boiler and not in the too-humid, insulated room. I put tops on the tubs with a small crack to let extra moisture escape.

8) I then set up a HEPA Air Purifier to help remove mold spores from the air (a HEPA filter is also good to use as a preventative before you even have a mold issue).

9) I brought the tubers up from the basement on April 14th (four months later). Only three tubers were too dried out to be saved, and they were small to begin with, so it was to be expected. I am happy and proud to say I saved all of my dahlia tubers from desiccation (drying out) and rotting by eliminating the white mold on the tubers using household bleach.

what i do differently now to prevent mold from growing on dahlia tubers

Once dug up, the dahlia tubers are immediately packed in wood pine shavings to absorb extra moisture. I still do not wash my dahlia tubers (which will actually encourage mold to grow, if you pack them when they are too wet).
When splitting tubers, I make sure my tools are sanitized. Read about why tool sanitation when dividing dahlia tubers is so important.
I regularly check on the plastic storage bins to look for condensation on the top of the bin. If there are visible water droplets, I take the entire lid off to let the extra water evaporate out of the bin.
Since storing dahlia tubers in wood shavings, I’ve not experienced tuber rot (except for mother tubers, which are susceptible to both rotting and drying out in less than ideal conditions).

using bleach to kill mold on dahlia tubers

Using household bleach worked for the surface mold issue on my dahlia tubers. I’ve seen other people write about how household bleach does not work on mold for reasons of actual science, but I am telling you, it worked for me and helped save nearly 100 dahlia tubers from imminent death. To be clear, had my dahlia tubers been more moldy to the point of rotting on the inside, I do not think the bleach would have worked.

I would never recommend using bleach as a preventative for mold (we should eliminate the use of chemicals wherever we can, but if you are going to use a preventative, then use Sulfur). If you have mold growing on the outside of your dahlia tubers and are desperate to get rid of it, I would at least try using bleach. I used 1 cup of bleach in a 5 gallon bucket of water and soaked each tuber for 10+ seconds. Remember you must let the tubers dry out before repacking them. If you are pulling moldy dahlia tubers that have already been packed in wood pine shavings (or whatever your packing material is), I would definitely replace the pine shavings with new material, and use a new container or sanitize the container, so you don’t reinfect the tubers with the same mold spores.

I hope this is helpful for those of you growing and storing dahlias on a larger scale, or even if you are only storing a few! I’ve made many mistakes storing dahlia tubers, and I hope this shed some light on preventing and dealing with mold and rot issues.