It’s not the cold temperatures that kill borderline hardy perennials in your garden, it’s the fluctuating temperatures. Plants prefer predictable stability when it comes to putting down roots. Once it gets cold, they can be perfectly fine with that. It just needs to stay that way with minimum fluctuations until spring.
If you live in northern Canada, as I do, this is an impossible scenario. I have seen a Chinook blow in and temperatures go from -30 C to +10 in a single day! During a warm spell in the winter our heaps of snow shrink to almost nothing before we get slapped with another round of cold.
We might celebrate the winter reprieve, but this can spell death to a perennial who doesn’t know whether to start sending up leaf buds, or tucking in for another couple months of cold.
When snow falls on a garden it is like throwing a lovely warm blanket over the bed. When the snow melts, the blanket is removed. If cold temperatures return before another dump of snow there the perennials are, in bed without a blanket. Not pleasant and not all perennials will survive.
Fortunately you can make sure whatever happens, the perennials have a blanket to insulate their roots. The answer comes in the form of mulch.
Once freezing temperatures settle into your garden you can help prevent fluctuations on the soil level by adding some kind of organic matter around your plant. It doesn’t really matter what you use; compost, wood chips, sawdust, leaves, straw, hay, even gravel…it all works. A depth of six inches (15 cm) to as much as a foot (30 cm) will do the job nicely.
Spreading the organic matter after freezing temperatures settle in – not before – will prevent critters with the nibbles from setting up their winter home in your lovely mulch.
In the spring you can begin to slowly remove the mulch until temperatures are done with dropping below the freezing mark.
If this sounds like too much bother, simply stick with perennials that are naturally hardy to your zone. There are lots to choose from.
However, if you like to push the zone envelope and plant things that are a zone or two above where you live, mulch could be your ace in the hole. Or on the hole, in this case.