Mild winter is allowing canna roots to overwinter outdoors in Michigan gardens

This winter has been so mild for so long that the ground never really froze. What little freezing that did occur was just a shallow frozen crust over the surface of the soil that usually quickly thawed. That meant that what ever was below the surface was never exposed to freezing temperatures.

I was curious about this, so a few days ago I got out my garden fork and walked out to the garden. Last fall we left a few canna roots in a flower bed after digging as many as we needed for storing and planting this spring. I often leave any extras out in the garden and let them freeze in place. Then later in the spring, till them in. They rot down pretty quickly after being frozen and add some organic matter to the soil.

After digging up a few of those extra canna roots I discovered a majority of them had survived the winter. This is in late February. The roots appeared plump and healthy and looked like they were all set to begin growing as soon as the weather warmed up.

This is interesting since cannas are basically a tropical plants that are hardy only up to USDA hardiness zone 8, while we are in a much colder zone 6 here in southeastern Michigan.

In the past, I’ve had cannas survive Michigan winters but those plants were always in a bed that was up against a foundation wall of a heated structure. There was enough heat radiating through the foundation wall to keep the soil at a moderate temperature for a couple of feet away from the building.

The cannas this year are out in a flower bed that’s nowhere near any structure, much less a heated building. Plus the beds are bare without any mulch covering them.

Because it is so dense, soil all by itself has a protecting influence. It is much more resistant to temperature fluctuations than the surrounding air. While air temperature can typically vary 10, 20 or more degrees in one day, the nearby soil temperature changes much more slowly, especially the deeper you go.

Many plants can survive colder temperatures than they would otherwise if they slowly become used to the cold. This seems to be the case with this year’s cannas. Looking back at the fall weather conditions, the canna tops were frozen back with the first hard frost but the roots remained intact. After that the weather stayed comparatively milder than normal.

I would never rely on keeping canna roots in place in the garden as a way to keep cannas from one year to the next in our area. We could have just as well had a cold snap. Several weeks back, there was a brief concern of a potential polar vortex settling in. That would have been cold enough to freeze the soil deep enough to obliterate the roots.

This week I dug up a few roots just to see how well they’ll perform in regular storage for the rest of the winter. The rest I’ll continue leave in the beds as a part of this unintentional experiment. I’ll keep a close eye on the roots and let you know they turn out.