I recently made an amazing discovery. It seems that several dahlias survived the winter outside in my garden. Dahlias do not survive freezing temperatures. This past winter, it got down to minus 4 degrees at my house, so what could account for this phenomena? When I mentioned this to a friend, she asked if they were near the foundation of the house.
Many an unlikely plant will come through winter where just enough heat escapes the house to keep the soil slightly warmer. But, no, these dahlias were some distance from the house. It’s tempting to imagine that a new strain of hardy dahlias has appeared in my garden, but these were planted last season. Natural selection hasn’t had time to work its magic on this group.
It probably didn’t hurt that I’d covered this bed with nearly a foot of leaves last fall. It’s not just how cold the air temperature gets that determines a plant’s hardiness. With herbaceous perennials (plants that die back to their roots, leaving only a ground- hugging rosette of foliage above ground), it’s the soil temperature that makes all the difference.
The soil where my dahlias grew, buried under a thick blanket of mulch, did not get cold enough to kill their tubers.
Perhaps you’ve noticed something similar in your garden. A plant you purchased as an annual is still in the garden the following spring. Occasionally a plant self sows in exactly the same location its parent vacated, giving the false impression that it came back. Seeds are generally much hardier than the plant itself. It’s a kind of insurance for the infrequent cold snap that can hit even tropical regions.
For as long as I’ve lived in Colorado, every year a few snapdragons and dusty miller survive winter. With these it is a function of microclimates, a place in the garden where conditions are vastly different from those that prevail. Snapdragons generally endure in a microclimate that doesn’t dry out completely over the winter. Dusty miller, on the other hand, often comes back in whisky barrels where the potting soil is fast draining and bone-dry all winter long.
We’re seeing a lot more of this kind of thing. Many zone 6 plants are becoming reliably hardy in our zone 4 and 5 gardens. It partly is natural selection at work. When we plant tens of thousands of one particular species each year, it isn’t at all surprising that a few might carry a slight genetic mutation allowing them to tolerate more cold than their normal range.
None of this explains my dahlias’ survival. Almost every gardener I’ve talked to has come to the same conclusion: Our winters are not as cold as they once were. The potential peril of global warming may be frightening, but every gardener I know is secretly enjoying the side benefit of overwintering plants that never before survived Colorado’s winter chill.
Marcia Tatroe is a garden writer and lecturer. E-mail her at rltaurora@aol.com.
