Checking dalia tubers in storage over winter

Bob Dluzen
The Detroit News

This week, I checked my dahlias that I dug and put into storage last fall. I save some every year to plant the following spring. 

While the fleshy roots on dahlias are technically tubers and not actual bulbs, they are often referred to as bulbs or even just dahlia roots. They have long been categorized as “summer-flowering bulbs” by horticulturists as opposed to spring flowering bulbs, such as tulips or daffodils.

There are many different ways to store dahlia bulbs over winter and any one method can be successful if three storage conditions are met.

The first condition is that the tubers are in sound condition when they are put away. Cut, broken, scraped, punctured or otherwise damaged tubers are the most likely ones to spoil in storage. This is because that sort of damage allows fungi or bacteria to enter the tuber and begin rot.

The possibility of pathogens infecting tubers is one reason why some gardeners don’t cut their roots apart before storing. Commercial growers and many gardeners do separate them beforehand however.

Next, there must be the right amount of moisture in the storage medium. Too much water will encourage rotting of the tubers. On the other hand, if the medium is too dry, the tubers will begin to shrivel, and if enough moisture is lost, they’ll die. 

The third condition is the bulbs must be kept dormant by storing them in a cool area. Temperatures ranging from a low of just above the freezing point to a high of around 50 degrees are ideal. If kept too cold for any length of time, they will be damaged from the cold. Warm temperatures will cause them to break dormancy and encourage sprouting. 

When dahlias are inspected in midwinter, we look at all three conditions and make sure they are still being met. 

I have my tubers stored in a large paper compost bag, the kind used for the disposal of leaves, with sawdust as my storage medium. The paper bag slows the rate of evaporation of water from my storage medium while allowing a little bit of oxygen exchange for the dormant roots. 

My storage is nothing fancy.  I use paper composting bags that fit perfectly into a plastic laundry hamper.

Before opening my storage container to look at the bulbs, I like to lift the entire container. It gives me a clue if there was an appreciable amount of water lost. If it feels abnormally light, I’d know right away there was a problem with excessive drying. It felt just about right to me. 

How would one know if the weight was right? Try to remember what the weight of it was when you put it away last fall.

The tubers are covered with a few inches of coarse sawdust.

Upon opening them up for a visual inspection, I saw no signs of rot or mold starting to set in. Also, there was not one whiff of mold, only the scent of damp sawdust. 

The top layer of tubers were a little dry and the tubers looked like they lost some moisture, but not enough to do any permanent damage. I added some water to the storage medium as I replaced the bulbs. That will help rehydrate them. 

The dahlia tubers are in fine shape. Damp sawdust sticks to the surface keeping the tubers hydrated.

The temperature in my storage area is in the safe zone. The top of the storage container is about 50 degrees and at the bottom, on the floor it’s around 46 degrees.

Keep a thermometer with your stored bulbs so temperatures can be easily monitored.

All three criteria for optimum storage, for the most part,  are being met. The only small problem was the drying out of the sawdust near the top. Since I was able to catch it early, I was able to mitigate any potential damage. 

Even though you might store your dahlias differently than mine, it’s still time to peek in on them and make sure they are still in good condition.