Frustrated by Overwintering Culls and Volunteers? Blame Potatoes’ Wild Roots

Frustrated by Overwintering Culls and Volunteers? Blame Potatoes’ Wild Roots

Photo credit: Zauber Kartoffel

It sometimes seems that culls and volunteers survive even winter conditions, while your desired crop needs coddling. Taking a look at how feral potatoes behave will give you insights that will help you get the results you need for your crop.

Daughter tubers of wild potatoes, cousins of the ones we grow, are “stored” by simply overwintering in the soil underneath the parent plant. Volunteer potatoes in commercial production do essentially the same thing.

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Cull potatoes have a slightly different survival strategy, but they belong in this group as well.

Really, about all that’s required for tubers to get through the winter is that they avoid being frozen.

How low does the temperature have to be to freeze and kill a potato tuber? The generally accepted temperature is 28°F, though this number may be a degree or two lower for some varieties. In addition to temperature dropping to the tuber freezing point, several hours of exposure to this temperature will be required.

Because of this, in-ground winter storage is beyond risky. So why do so many volunteers survive? If tubers are surviving, it means the soil temperatures didn’t get below 28°F for any appreciable length of time. And early snows can add an insulating layer.

Depth also is important, though you’d be surprised at how many tubers survive with only 4 inches of soil covering them. As you would expect, tubers deeper in the soil have a better chance of surviving than those near the top.

Metabolism doesn’t stop in stored potato tubers, and a pile of them can produce a considerable amount of heat. In the case of many storages, this “heat of metabolism” is enough to keep the entire facility at the desired temperature without any additional source of heat.

The same principle applies to a potato cull pile: Heat of metabolism can be adequate to allow some tubers to overwinter, especially if the pile is deep enough. To be sure, tubers on the outer portion of a cull pile, those directly exposed to the elements, will freeze solid. In contrast, those in the very center of the pile become starved for oxygen and begin to break down.

During this breakdown, even more heat is generated by bacterial soft rot activity in the warm, anaerobic conditions in the center of the pile.

This is exactly the same process that creates hot spots in storage. Between the frozen outer barrier and the soft, juicy center of the cull pile lies a sort of goldilocks zone where tubers can make it through the winter to sprout another day.

Can we manage feral potatoes? Unfortunately, little can be done about volunteers until after they emerge, and even then they can be a real challenge.

Culls, on the other hand, are much easier to deal with. If you want all of them to freeze, you need to spread them out. The general recommendation is for culls to be spread no more than 6 inches deep.